Publications

Absorption features in stellar atmospheres are often used to calibrate photocentric velocities for the kinematic analysis of further spectral lines. The Li feature at ∼6708 Å is commonly used, especially in the case of young stellar objects, for which it is one of the strongest absorption lines. However, this complex line comprises two isotope fine-structure doublets. We empirically measured the wavelength of this Li feature in a sample of young stars from the PENELLOPE/VLT programme (using X-shooter, UVES, and ESPRESSO data) as well as HARPS data. For 51 targets, we fit 314 individual spectra using the STAR-MELT package, resulting in 241 accurately fitted Li features given the automated goodness-of-fit threshold. We find the mean air wavelength to be 6707.856 Å, with a standard error of 0.002 Å (0.09 km s⁻¹), and a weighted standard deviation of 0.026 Å (1.16 km s⁻¹). The observed spread in measured positions spans 0.145 Å, or 6.5 km s⁻¹, which is higher by up to a factor of six than the typically reported velocity errors for high-resolution studies. We also find a correlation between the effective temperature of the star and the wavelength of the central absorption. We discuss that exclusively using this Li feature as a reference for photocentric velocity in young stars might introduce a systematic positive offset in wavelength to measurements of further spectral lines. If outflow tracing forbidden lines, such as [O I] 6300 Å, is more blueshifted than previously thought, this then favours a disc wind as the origin for this emission in young stars.

We report results of a spectropolarimetric and photometric monitoring of the weak-line T Tauri star LkCa 4 within the SPIRou Legacy Survey large programme, based on data collected with SPIRou at the Canada-France-Hawaii Telescope and the TESS space probe between October 2021 and January 2022. We applied Zeeman-Doppler Imaging to our spectropolarimetric and photometric data to recover a surface brightness distribution compatible with TESS photometry, as well as the large-scale magnetic topology of the star. As expected from the difference in wavelength between near-infrared and optical data, the recovered surface brightness distribution is less contrasted than the previously published one based on ESPaDOnS data, but still features mid-latitude dark and bright spots. The large-scale magnetic field is consistent in shape and strength with the one derived previously, with a poloidal component resembling a 2.2 kG dipole and a toroidal component reaching 1.4 kG and encircling the star at the equator. Our new data confirm that the surface differential rotation of LkCa 4 is about 10 times weaker than that of the Sun, and significantly different from zero. Using our brightness reconstruction and Gaussian Process Regression, we were able to filter the radial velocity activity jitter down to a precision of 0.45 and 0.38 kms−1 (from an amplitude of 6.10 kms−1), respectively, yielding again no evidence for a close-in massive planet orbiting the star.

Context. FU Orionis (FUor) and EX Lupi (EXor) type objects represent two small but rather spectacular groups of low-mass, young, eruptive stars. In both cases, outbursts of several magnitudes are observed, which are attributed to enhanced mass accretion from the circumstellar disc onto the central protostar. Although these objects are well studied at optical and near-infrared wavelengths, their host molecular environments are poorly explored because of the scarcity of systematic molecular line observations.
Aims: We aim to carry out the first dedicated survey of the molecular environments of a large sample of FUors and EXors, observing a total of 51 sources, including some Gaia alerts, to study the ammonia (NH₃) emission in their host environments.
Methods: We observed the ammonia (J, K) = (1,1), (2,2), and (3,3) inversion transitions at ~23.7 GHz in position-switching mode using the Effelsberg 100-m radio telescope. For 19 of the 51 sources in our sample, we derived H₂ column densities and dust temperatures using archival Herschel/SPIRE data at 250 µm, 300 µm, and 500 µm.
Results: We detected the NH₃ (1,1) transition toward 28 sources and the (2,2) transition toward 12 sources, while the (3,3) transition was detected towards only two sources in our sample. We find kinetic temperatures between ~12 K and 21 K, ammonia column densities from 5.2 × 10¹³ cm⁻² to 3.2 × 10¹⁵ cm⁻², and fractional ammonia abundances with respect to H₂ from 4.7 × 10⁻⁹ to 1.5 × 10⁻⁷. These results are comparable to those found in infrared dark clouds (IRDCs). Our kinematic analysis suggests that most of the eruptive stars in our sample reside in rather quiescent (sonic or transonic) host environments.
Conclusions: Our NH₃ observations and analysis of the SPIRE dust-based H₂ column density maps confirm the presence of dense material towards seven sources in our sample; additional sources might also harbour dense gas based on their NH₂ (2,2) detections, potentially indicating an earlier phase than originally classified. Based on our results, we suggest that observations targeting additional molecular lines would help to refine the evolutionary classification of eruptive stars.

EX Lup is a low-mass pre-main-sequence star that occasionally shows accretion-related outbursts. Here, we present JWST/MIRI medium-resolution spectroscopy obtained for EX Lup 14 yr after its powerful outburst. EX Lup is now in quiescence and displays a Class II spectrum. We detect a forest of emission lines from molecules previously identified in infrared spectra of classical T Tauri disks: H₂O, OH, H₂, HCN, C₂H₂, and CO₂. The detection of organic molecules demonstrates that they are back after disappearing during the large outburst. Spectral lines from water and OH are for the first time deblended and will provide a much-improved characterization of their distribution and density in the inner disk. The spectrum also shows broad emission bands from warm, submicron-size amorphous silicate grains at 10 and 18 μm. During the outburst, in 2008, crystalline forsterite grains were annealed in the inner disk within 1 au, but their spectral signatures in the 10 μm silicate band later disappeared. With JWST we rediscovered these crystals via their 19.0, 20.0, and 23.5 μm emission, the strength of which implies that the particles are at ~3 au from the star. This suggests that crystalline grains formed in 2008 were transported outwards and now approach the water snowline, where they may be incorporated into planetesimals. Containing several key tracers of planetesimal and planet formation, EX Lup is an ideal laboratory to study the effects of variable luminosity on the planet-forming material and may provide an explanation for the observed high crystalline fraction in solar system comets.

The FU Orionis-type objects (FUors) are low-mass pre-main-sequence objects that go through a short-lived phase (~100 yr) of increased mass accretion rate (from 10^-8 to 10^-4 M _⊙ yr^-1). These eruptive young stars are in the early stages of stellar evolution and thus still deeply embedded in a massive envelope that feeds material to the circumstellar disk that is then accreted onto the star. Some FUors drive molecular outflows, i.e., low-velocity wide-angle magnetohydrodynamical winds, that inject energy and momentum back to the surrounding envelopes and help clear the material surrounding the young star. Here we present a ¹²CO (3-2), ¹³CO (3-2), and ¹²CO (4-3) survey of 20 FUor-type eruptive young stars observed with APEX. We use our ¹³CO (3-2) observations to measure the masses of the envelopes surrounding each FUor and find an agreement with the FUor evolutionary trend found from the 10 μm silicate feature. We find outflows in 11 FUors, calculate their masses and other kinematic properties, and compare these with those of outflows found around quiescent young stellar objects gathered from the literature. This comparison indicates that outflows in FUors are more massive than outflows in quiescent sources, and that FUor outflows have a higher-ratio outflow mass with respect to the envelope than the quiescent sample, indicating that the eruptive young stars have lower star-forming efficiencies. Finally, we find that the outflow forces in FUors are similar to those of quiescent young stellar objects, indicating that their accretion histories are similar or that the FUor outflows have lower velocities.

Classical T Tauri stars (CTTSs) are young stellar objects that accrete materials from their accretion disc influenced by their strong magnetic field. The magnetic pressure truncates the disc at a few stellar radii and forces the material to leave the disc plane and fall onto the stellar surface by following the magnetic field lines. However, this global scheme may be disturbed by the presence of a companion interacting gravitationally with the accreting component. This work is aiming to study the accretion and the magnetic field of the tight eccentric binary DQ Tau, composed of two equal-mass (~ 0.6 M_⊙) CTTSs interacting at different orbital phases. We investigated the variability of the system using a high-resolution spectroscopic and spectropolarimetric monitoring performed with ESPaDOnS at the CFHT. We provide the first ever magnetic field analysis of this system, the Zeeman-Doppler imaging revealed a stronger magnetic field for the secondary than the primary (1.2 and 0.5 kG, respectively), but the small-scale fields analysed through Zeeman intensification yielded similar strengths (about 2.5 kG). The magnetic field topology and strengths are compatible with the accretion processes on CTTSs. Both components of this system are accreting, with a change of the main accretor during the orbital motion. In addition, the system displays a strong enhancement of the mass accretion rate at periastron and apastron. We also discovered, for the first time in this system, the apsidal motion of the orbital ellipse.

T Tauri stars produce broad Lyα emission lines that contribute ~88% of the total UV flux incident on the inner circumstellar disks. Lyα photons are generated at the accretion shocks and in the protostellar chromospheres and must travel through accretion flows, winds, and jets, the protoplanetary disks, and the interstellar medium before reaching the observer. This trajectory produces asymmetric, double-peaked features that carry kinematic and opacity signatures of the disk environments. To understand the link between the evolution of Lyα emission lines and the disks themselves, we model HST-COS spectra from targets included in Data Release 3 of the Hubble UV Legacy Library of Young Stars as Essential Standards program. We find that resonant scattering in a simple spherical expanding shell is able to reproduce the high-velocity emission line wings, providing estimates of the average velocities within the bulk intervening H I. The model velocities are significantly correlated with the K-band veiling, indicating a turnover from Lyα profiles absorbed by outflowing winds to emission lines suppressed by accretion flows as the hot inner disk is depleted. Just 30% of targets in our sample have profiles with redshifted absorption from accretion flows, many of which have resolved dust gaps. At this stage, Lyα photons may no longer intersect with disk winds along the path to the observer. Our results point to a significant evolution of Lyα irradiation within the gas disks over time, which may lead to chemical differences that are observable with ALMA and JWST.

Stars collect most of their mass during the protostellar stage, yet the accretion luminosity and stellar parameters, which are needed to compute the mass accretion rate, are poorly constrained for the youngest sources. The aim of this work is to fill this gap, computing the stellar properties and the accretion rates for a large sample of Class I protostars located in nearby (<500 pc) star-forming regions and analyzing their interplay. We used a self-consistent method to provide accretion and stellar parameters by modeling the spectral energy distribution and using veiling information from near-IR observations when possible. We calculated accretion and stellar properties for the first time for 50 young stars. We focused our analysis on the 39 confirmed protostars, finding that their mass accretion rate varies between ~10^-8 and ~10^-4 M _⊙ yr^-1 in a stellar mass range between ~0.1 and 3 M _⊙. We find systematically larger mass accretion rates for our Class I sample than for Class II objects. Although the mass accretion rate we found is high, it still suggests that either stars collect most of their mass before the Class I stage, or eruptive accretion is needed during the overall protostellar phase. Indeed, our results suggest that for a large number of protostars the disk can be unstable, which can result in accretion bursts and disk fragmentation in the past or in the future.

In order to build up a legacy spectroscopy survey of accreting low-mass pre-main sequence stars, synchronized HST and ESO/VLT observational campaigns, named ULLYSES and PENELLOPE, respectively, were carried out in the Orion star forming region in late 2020. The spectroscopic program was supported by an extensive multi-band photometric monitoring at different ground-based observatories, including our domestic telescopes at Konkoly Observatory, Hungary, as well as by simultaneous TESS observations. Here we report on our ongoing analysis aiming at characterizing the accretion process of the targeted objects based on multi-filter photometry. We observed BVr'i' light curves with an approximately daily cadence, and derived the brightness and color evolution of 13 T Tauri-type stars. We developed a novel method to estimate the accretion rate from simultaneous optical data, utilizing a grid of slab modeling. Using the high time resolution TESS light curves as temporal variability templates, we interpolated the BVr'i' multicolor data sets to a time resolution of 10 min. We also produced accretion rate curves with this cadence, and could define the exact variability state of the stars at the precise epochs of the spectroscopy. We analysed the amplitudes and timescales present in the accretion rate curves, and proposed to connect the different characteristic timescales with physical mechanisms. The photometry-based results on the accretion process will be later combined with HST and VLT spectroscopy of the same sample. The results, together with the similar T Tauri samples from other star forming regions, will make ULLYSES/PENELLOPE one of the highest impact study of the stellar accretion process of today.

The research of star formation is one of the most popular area of modern astrophysics. Nowadays, we can use ground-based and space-borne observatories to study Young Stellar Objects in different states of their evolution, their time-domain behaviour related to accretion process, and the connection between the stars and their circumstellar environment. The Chamaeleon I molecular cloud complex is one of the most famous star-forming regions, located at a distance of 190 pc. In our study we use photometric data to analyse the multi-wavelength variability of eight T Tauri stars (CR Cha, CT Cha, Glass I, VW Cha, VZ Cha, WW Cha, WX Cha and XX Cha) in Cha I. Our aim is to determine the disk's thermal response on changing irradiation by the central source, with special emphasis on the little known far-infrared flux variations of the outer disk. For our analysis, we performed a 4-epoch weekly monitoring of the 8 stars using the CTIO 1.3m optical – near-IR telescope, the VLT/ISAAC L- and M-band camera, and the Herschel Space Observatory at 70 and 160 micrometer. We looked for far-infrared flux changes and their correlation with the simultaneous variation of the stellar luminosity. Here, we present the first results, and perform a simply analytical modeling of the expected far-infrared variability using a two-layer disk model. The project provides some first hints on how the stellar radiation influences the large outer regions of the circumstellar disk.

Classical T Tauri stars are low-mass objects that are still accreting material from the surrounding circumstellar disk. The accretion process is essential in the formation of Sun-like stars and in setting the properties of the disk at the time when planet formation occurs. We constructed a complex data set in order to examine the accretion process of VW Cha, a classical T Tauri multiple system, with the aim of studying the physical origin of the photometric and spectroscopic variability of the system. The TESS space telescope observed VW Cha between 2019 April 22 and June 19, and we complemented these data with contemporaneous ground-based I _CJHK-band photometric measurements. In addition, we obtained high-resolution optical spectra with the VLT/ESPRESSO and the 2.2 m/FEROS instruments. Analyzing these data, we found that the TESS light curve shows photometric variations on timescales from minutes to weeks with a peak-to-peak amplitude of ~0.8 mag. The near-infrared light curves follow the shape of the optical measurements; however, the peak-to-peak amplitudes are slightly increasing toward the longer wavelengths. We took spectra in both fainter and brighter photometric states of the system, allowing us to examine the origin of a photometric brightening event. Our results show that this brightening event can be explained by increased accretion. In addition, our spectroscopic data also suggest that the primary component of VW Cha is a spectroscopic binary, as was proposed in earlier works.

Gaia19fct is one of the Gaia-alerted eruptive young stars that has undergone several brightening events. We conducted monitoring observations using multifilter optical and near-infrared photometry, as well as near-infrared spectroscopy, to understand the physical properties of Gaia19fct and investigate whether it fits into the historically defined two classes. We present the analyses of light curves, color variations, spectral lines, and CO modeling. The light curves show at least five brightening events since 2015, and the multifilter color evolutions are mostly gray. The gray evolution indicates that bursts are triggered by mechanisms other than extinction. Our near-infrared spectra exhibit both absorption and emission lines and show time variability throughout our observations. We found lower rotational velocity and lower temperature from the near-infrared atomic absorption lines than from the optical lines, suggesting that Gaia19fct has a Keplerian rotating disk. The CO overtone features show a superposition of absorption and emission components, which is unlike other young stellar objects. We modeled the CO lines, and the result suggests that the emission and absorption components are formed in different regions. We found that although Gaia19fct exhibits characteristics of both types of eruptive young stars, FU Orionis-type objects and EX Lupi-type objects, it shows more similarity with EXors in general.

The coevolution of T Tauri stars and their surrounding protoplanetary disks dictates the timescales of planet formation. In this paper, we present magnetospheric accretion and inner disk wall model fits to near-UV (NUV) to near-IR (NIR) spectra of nine classical T Tauri stars in Orion OB1b as part of the Outflows and Disks around Young Stars: Synergies for the Exploration of ULLYSES Spectra (ODYSSEUS) survey. Using NUV-optical spectra from the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) Director's Discretionary Program and optical-NIR spectra from the PENELLOPE VLT Large Programme, we find that the accretion rates of these targets are relatively high for the region's intermediate age of 5.0 Myr; rates are in the range of (0.5-17.2) × 10^-8 M _☉ yr^-1, with a median value of 1.2 × 10^-8 M _☉ yr^-1. The NIR excesses can be fit with 1200-1800 K inner disk walls located at 0.05-0.10 au from the host stars. We discuss the significance of the choice in extinction law, as the measured accretion rate depends strongly on the adopted extinction value. This analysis will be extended to the complete sample of T Tauri stars being observed through ULLYSES to characterize accretion and inner disks in star-forming regions of different ages and stellar populations.

Context. Gaia-CRF3 is the celestial reference frame for positions and proper motions in the third release of data from the Gaia mission, Gaia DR3 (and for the early third release, Gaia EDR3, which contains identical astrometric results). The reference frame is defined by the positions and proper motions at epoch 2016.0 for a specific set of extragalactic sources in the (E)DR3 catalogue.
Aims: We describe the construction of Gaia-CRF3 and its properties in terms of the distributions in magnitude, colour, and astrometric quality.
Methods: Compact extragalactic sources in Gaia DR3 were identified by positional cross-matching with 17 external catalogues of quasi-stellar objects (QSO) and active galactic nuclei (AGN), followed by astrometric filtering designed to remove stellar contaminants. Selecting a clean sample was favoured over including a higher number of extragalactic sources. For the final sample, the random and systematic errors in the proper motions are analysed, as well as the radio-optical offsets in position for sources in the third realisation of the International Celestial Reference Frame (ICRF3).
Results: Gaia-CRF3 comprises about 1.6 million QSO-like sources, of which 1.2 million have five-parameter astrometric solutions in Gaia DR3 and 0.4 million have six-parameter solutions. The sources span the magnitude range G = 13-21 with a peak density at 20.6 mag, at which the typical positional uncertainty is about 1 mas. The proper motions show systematic errors on the level of 12 µas yr^-1 on angular scales greater than 15 deg. For the 3142 optical counterparts of ICRF3 sources in the S/X frequency bands, the median offset from the radio positions is about 0.5 mas, but it exceeds 4 mas in either coordinate for 127 sources. We outline the future of Gaia-CRF in the next Gaia data releases. Appendices give further details on the external catalogues used, how to extract information about the Gaia-CRF3 sources, potential (Galactic) confusion sources, and the estimation of the spin and orientation of an astrometric solution.

Light curves of young star systems show photometric variability due to different kinematic and physical processes. One of the main contributors to the photometric variability is the changing mass accretion rate, which regulates the interplay between the forming young star and the protoplanetary disk. We collected high-resolution spectroscopy in eight different epochs, as well as ground-based and space-borne multiepoch optical and infrared photometry of WX Cha, an M0 binary system, with an almost edge-on disk (i = 87°) in the Chamaeleon I star-forming region. Spectroscopic observations cover 72 days, the ground-based optical monitoring covers 42 days while space-borne TESS photometry extends for 56 days. The multiwavelength light curves exhibit quasi-periodic variability of 0.35-0.53 mag in the near-infrared, and of 1.3 mag in the g band. We studied the variability of selected emission lines that trace the accretion, computed the accretion luminosity and the mass accretion rate using empirical relations, and obtained values between L _acc ~ 1.6 L _⊙ − 3.2 L _⊙ and M˙acc ∼ 3.31×10^-7 M⊙/yr^-1 − 7.76×10^-7 M _⊙ yr^-1. Our results show that WX Cha is accreting at a rate larger than what is typical for T Tauri stars in the same star-forming region with the same stellar parameters. We theorize that this is due to the higher disk mass of WX Cha than what is usual for stars with similar stellar mass and to the binary nature of the system. Daily changes in the accretion luminosity and in the extinction can explain the photometric variability.

Young stars show variability on different time-scales from hours to decades, with a range of amplitudes. We studied two young stars, which triggered the Gaia Science Alerts system due to brightenings on a time-scale of a year. Gaia20bwa brightened by about half a magnitude, whereas Gaia20fgx brightened by about two and half magnitudes. We analysed the Gaia light curves, additional photometry, and spectra taken with the Telescopio Nazionale Galileo and the Gran Telescopio Canarias. Several emission lines were detected towards Gaia20bwa, including hydrogen lines from H α to H δ, Pa β, Br γ, and lines of Ca II, O I, and Na I. The H α and Br γ lines were detected towards Gaia20fgx in emission in its bright state, with additional CO lines in absorption, and the Pa β line with an inverse P Cygni profile during its fading. Based on the Br γ lines, the accretion rate was (2.4−3.1)×10^-8M⊙ yr^-1 for Gaia20bwa and (4.5−6.6)×10^-8M⊙ yr^-1 for Gaia20fgx during their bright state. The accretion rate of Gaia20fgx dropped by almost a factor of 10 on a time-scale of half a year. The accretion parameters of both stars were found to be similar to those of classical T Tauri stars, lower than those of young eruptive stars. However, the amplitude and time-scale of these brightenings place these stars to a region of the parameter space, which is rarely populated by young stars. This suggests a new class of young stars, which produce outbursts on a time-scale similar to young eruptive stars, but with smaller amplitudes.

Evidence of a relation between the mass accretion rate and the disk mass is established for young, Class II pre-main-sequence stars. This observational result opened an avenue to test theoretical models and constrain the initial conditions of disk formation, fundamental in the understanding of the emergence of planetary systems. However, it is becoming clear that planet formation starts even before the Class II stage, in disks around Class 0 and I protostars. We show for the first time evidence for a correlation between the mass accretion rate and the disk mass for a large sample of Class I young stars located in nearby (<500 pc) star-forming regions. We fit our sample, finding that the Class I object relation has a slope flatter than Class II stars, and the former have higher mass accretion rates and disk masses. The results are put in context of disk evolution models.

Historically, FU Orionis-type stars are low-mass, pre-main-sequence stars. The members of this class experience powerful accretion outbursts and remain in an enhanced accretion state for decades or centuries. V1515 Cyg, a classical FUor, started brightening in the 1940s and reached its peak brightness in the late 1970s. Following a sudden decrease in brightness, it stayed in a minimum state for a few months, then started brightening for several years. We present the results of our ground-based photometric monitoring complemented with optical/near-infrared spectroscopic monitoring. Our light curves show a long-term fading with strong variability on weekly and monthly timescales. The optical spectra show P Cygni profiles and broad blueshifted absorption lines, common properties of FUors. However, V1515 Cyg lacks the P Cygni profile in the Ca II 8498 Å line, a part of the Ca infrared triplet, formed by an outflowing wind, suggesting that the absorbing gas in the wind is optically thin. The newly obtained near-infrared spectrum shows the strengthening of the CO bandhead and the FeH molecular band, indicating that the disk has become cooler since the last spectroscopic observation in 2015. The current luminosity of the accretion disk dropped from the peak value of 138 L _⊙ to about 45 L _⊙, suggesting that the long-term fading is also partly caused by the dropping of the accretion rate.

Exploring the properties of exoplanets near or inside the radius valley provides insight on the transition from the rocky super-Earths to the larger, hydrogen-rich atmosphere mini-Neptunes. Here, we report the discovery of TOI-1452b, a transiting super-Earth (R _p = 1.67 ± 0.07 R _⊕) in an 11.1 day temperate orbit (T _eq = 326 ± 7 K) around the primary member (H = 10.0, T _eff = 3185 ± 50 K) of a nearby visual-binary M dwarf. The transits were first detected by the Transiting Exoplanet Survey Satellite, then successfully isolated between the two 3.″2 companions with ground-based photometry from the Observatoire du Mont-Mégantic and MuSCAT3. The planetary nature of TOI-1452b was established through high-precision velocimetry with the near-infrared SPIRou spectropolarimeter as part of the ongoing SPIRou Legacy Survey. The measured planetary mass (4.8 ± 1.3 M _⊕) and inferred bulk density ( 5.6−1.6+1.8 g cm^-3) is suggestive of a rocky core surrounded by a volatile-rich envelope. More quantitatively, the mass and radius of TOI-1452b, combined with the stellar abundance of refractory elements (Fe, Mg, and Si) measured by SPIRou, is consistent with a core-mass fraction of 18% ± 6% and a water-mass fraction of 22−13+21 %. The water world candidate TOI-1452b is a prime target for future atmospheric characterization with JWST, featuring a transmission spectroscopy metric similar to other well-known temperate small planets such as LHS 1140b and K2-18 b. The system is located near Webb's northern continuous viewing zone, implying that is can be followed at almost any moment of the year.

The processes regulating protoplanetary disk evolution are constrained by studying how mass accretion rates scale with stellar and disk properties. The spread in these relations can be used as a constraint to the models of disk evolution, but only if the impact of accretion variability is correctly accounted for. While the effect of variability might be substantial in the embedded phases of star formation, it is often considered limited at later stages. Here we report on the observed large variation in the accretion rate for one target, XX Cha, and we discuss the impact on population studies of classical T Tauri stars. The mass accretion rate determined by fitting the UV-to-near-infrared spectrum in recent X-shooter observations is compared with the one measured with the same instrument 11 years before. XX Cha displays an accretion variability of almost 2 dex between 2010 and 2021. Although the timescales on which this variability happens are uncertain, XX Cha displays an extreme accretion variability for a classical T Tauri star. If such behavior is common among classical T Tauri stars, possibly on longer timescales than previously probed, it could be relevant for discussing the disk evolution models constrained by the observed spread in accretion rates. Finally, we remark that previous studies of accretion variability based on spectral lines may have underestimated the variability of some targets.

Aims: We studied the accretion disk of the archetypal eruptive young star FU Orionis with the use of mid-infrared interferometry, which enabled us to resolve the innermost regions of the disk down to a spatial resolution of 3 milliarcseconds (mas) in the L band, that is, within 1 au of the protostar.
Methods: We used the interferometric instrument MATISSE/VLTI to obtain observations of FU Ori's disk in the L, M, and N bands with multiple baseline configurations. We also obtained contemporaneous photometry in the optical (UBVRIr'i'; SAAO and Konkoly Observatory) and near-infrared (JHK_s; NOT). Our results were compared with radiative transfer simulations modeled by RADMC-3D.
Results: The disk of FU Orionis is marginally resolved with MATISSE, suggesting that the region emitting in the thermal infrared is rather compact. An upper limit of ~1.3 ± 0.1 mas (in L) can be given for the diameter of the disk region probed in the L band, corresponding to 0.5 au at the adopted Gaia EDR3 distance. This represents the hot, gaseous region of the accretion disk. The N-band data indicate that the dusty passive disk is silicate-rich. Only the innermost region of said dusty disk is found to emit strongly in the N band, and it is resolved at an angular size of ~5 mas, which translates to a diameter of about 2 au. The observations therefore place stringent constraints for the outer radius of the inner accretion disk. Dust radiative transfer simulations with RADMC-3D provide adequate fits to the spectral energy distribution from the optical to the submillimeter and to the interferometric observables when opting for an accretion rate M ~ 2 × 10⁻⁵ M_⊙ yr⁻¹ and assuming M_* = 0.6 M_⊙, Most importantly, the hot inner accretion disk's outer radius can be fixed at 0.3 au. The outer radius of the dusty disk is placed at 100 au, based on constraints from scattered-light images in the literature. The dust mass contained in the disk is 2.4 × 10⁻⁴ M_⊙, and for a typical gas-to-dust ratio of 100, the total mass in the disk is approximately 0.02 M_⊙. We did not find any evidence for a nearby companion in the current interferometric data, and we tentatively explored the case of disk misalignment. For the latter, our modeling results suggest that the disk orientation is similar to that found in previous imaging studies by ALMA. Should there be an asymmetry in the very compact, inner accretion disk, this might be resolved at even smaller spatial scales (≤1 mas).

DI Cha A is K0-type pre-main-sequence star, the brightest component of a quadruple stellar system. Here we report on a detailed study of this star based on archival VLTI/MIDI and VLTI/PIONIER infrared interferometric observations, as well as optical-infrared photometric monitoring from ground-based and space-born instruments. We determined the structure of the circumstellar disk by fitting simultaneously the interferometric visibilities and the spectral energy distribution, using both analytical models and the radiative transfer code RADMC-3D. The modeling revealed that the radial density distribution of the disk appears to have a gap between 0.21 and 3.0 au. The inner ring, whose inner size coincides with the sublimation radius, is devoid of small, submicrometer-sized dust grains. The inner edge of the outer disk features a puffed-up rim, typically seen in intermediate-mass stars. Grain growth, although less progressed, was also detected in the outer disk. The inner ring is variable at mid-infrared wavelengths on both daily and annual timescales, while the star stays remarkably constant in the optical, pointing to geometrical or accretion changes in the disk as possible explanations for the flux variations.

To investigate the feasibility of ancillary target observations with ESA's ARIEL mission, we compiled a list of potentially interesting young stars: FUors, systems harbouring extreme debris discs and a larger sample of young stellar objects showing strong near/mid-infrared excess. These objects can be observed as additional targets in the waiting times between the scheduled exoplanet transit and occultation observations. After analyzing the schedule for ARIEL an algorithm was constructed to find the optimal target to be observed in each gap. The selection was mainly based on the slew and stabilization time needed to observe the selected YSO, but it also incorporated the scientific importance of the targets and whether they have already been sufficiently measured. After acquiring an adequately large sample of simulation data, it was concluded that approximately 99.2% of the available - at least one hour long - gaps could be used effectively. With an average slewing and stabilization time of about 16.7 minutes between scheduled exoplanet transits and ancillary targets, this corresponds to an additional 2881 ± 56 hours of active data gathering. When this additional time is used to observe our selected 200 ancillary targets, a typical signal-to-noise ratio of ∼1 0⁴ can be achieved along the whole spectral window covered by ARIEL.

Context. Classical T Tauri stars are pre-main-sequence stars that are surrounded by a circumstellar disk from which they accrete material. The mass accretion process is essential in the formation of Sun-like stars. Although often described with simple and static models, the accretion process is inherently time variable.
Aims: We examine the accretion process of the low-mass young stellar object CR Cha on a wide range of timescales from minutes to a decade by analyzing both photometric and spectroscopic observations from 2006, 2018, and 2019.
Methods: We carried out a period analysis of the light curves of CR Cha from the TESS mission and the ASAS-SN and the ASAS-3 databases. We studied the color variations of the system using I, J, H, K-band photometry obtained contemporaneously with the TESS observing window. We analyzed the amplitude, timescale, and the morphology of the accretion tracers found in a series of high-resolution spectra obtained in 2006 with the AAT/UCLES, in 2018 with the HARPS, and in 2019 with the ESPRESSO and the FEROS spectrographs.
Results: All photometric data reveal periodic variations compatible with a 2.327-day rotational period. In addition, the ASAS-SN and ASAS-3 data indicate a long-term brightening by 0.2 mag between 2001 and 2008, and a slightly lower brightening than 0.1 mag in the 2015-2018 period. The near-infrared photometry indicates a short-term brightening trend during the observations in 2019. The corresponding color variations can be explained either by a changing accretion rate or changes in the inner disk structure. The Hα line profile variability suggests that the amplitude variations of the central peak, likely due to accretion, are most significant on daily or hourly timescales. On yearly timescales, the line morphology also changes significantly.
Conclusions: The photometric variability shows that the period of about 2.3 days is stable in the system over decades. Our results show that the amplitude of the variations in the Hα emission increases on timescales from hours to days or weeks, after which it remains similar even at decadal timescales. On the other hand, we found significant morphological variations on yearly or decadal timescales, indicating that the different physical mechanisms responsible for the line profile changes, such as accretion or wind, are present to varying degrees at different times.

This work is part of a multi-wavelength program to study the effects of X-ray/UV/optical stellar radiation on the chemistry of the circumbinary disk around the young high-eccentricity binary DQ Tau. ALMA observations for near/around 2021 December 5 periastron were postponed due to bad weather, but supporting Swift-XRT-UVOT TOO observations were successful. These Swift observations along with previous X-ray-optical-mm data show that DQ Tau keeps exhibiting powerful flares near periastron, offering a unique laboratory for studies of flare effects on the gas-phase ion chemistry in protoplanetary disks.

We report on optical and near-infrared light curves of the young eruptive star EX Lupi, which has shown significant brightening in the past month, indicating a new accretion burst.

Mass accretion from the circumstellar disk onto the protostar is a fundamental process during star formation. Measuring the mass accretion rate is particularly challenging for stars belonging to binary systems, because it is often difficult to discriminate which component is accreting. DQ Tau is an almost equal-mass spectroscopic binary system where the components orbit each other every 15.8 days. The system is known to display pulsed accretion, i.e., the periodic modulation of the accretion by the components on eccentric orbit. We present multi-epoch ESO/Very Large Telescope X-Shooter observations of DQ Tau, with the aim of determining which component of this system is the main accreting source. We use the absorption lines in the spectra to determine the radial velocity of the two components, and measure the continuum veiling as a function of wavelength and time. We fit the observed spectra with nonaccreting templates to correct for the photospheric and chromospheric contribution. In the corrected spectra, we study in detail the profiles of the emission lines and calculate mass accretion rates for the system as a function of orbital phase. In accordance with previous findings, we detect elevated accretion close to periastron. We measure the accretion rate as varying between 10^-8.5 and 10^-7.3 M _⊙ yr^-1. The emission line profiles suggest that both stars are actively accreting, and the dominant accretor is not always the same component, varying in a few orbits.

We present follow-up photometric and spectroscopic observations, and subsequent analysis of Gaia20eae. This source triggered photometric alerts during 2020 after showing a ~3 mag increase in its brightness. Its Gaia Alert light curve showed the shape of a typical eruptive young star. We carried out observations to confirm Gaia20eae as an eruptive young star and classify it. Its pre-outburst spectral energy distribution shows that Gaia20eae is a moderately embedded Class II object with L _bol = 7.22 L _⊙. The color-color and color-magnitude diagrams indicate that the evolution in the light curve is mostly gray. Multiple epochs of the Hα line profile suggest an evolution of the accretion rate and winds. The near-infrared spectra display several emission lines, a feature typical of EX Lupi-type (EXor) eruptive young stars. We estimated the mass accretion rate during the dimming phase to be M˙ = 3-8 × 10^-7 M _⊙ yr^-1, higher than typical T Tauri stars of similar mass and comparable to other EXors. We conclude Gaia20eae is a new EXor-type candidate.

The Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) Director's Discretionary Program of low-mass pre-main-sequence stars, coupled with forthcoming data from Atacama Large Millimeter/submillimeter Array and James Webb Space Telescope, will provide the foundation to revolutionize our understanding of the relationship between young stars and their protoplanetary disks. A comprehensive evaluation of the physics of disk evolution and planet formation requires understanding the intricate relationships between mass accretion, mass outflow, and disk structure. Here we describe the Outflows and Disks around Young Stars: Synergies for the Exploration of ULLYSES Spectra (ODYSSEUS) Survey and present initial results of the classical T Tauri Star CVSO 109 in Orion OB1b as a demonstration of the science that will result from the survey. ODYSSEUS will analyze the ULLYSES spectral database, ensuring a uniform and systematic approach in order to (1) measure how the accretion flow depends on the accretion rate and magnetic structures, (2) determine where winds and jets are launched and how mass-loss rates compare with accretion, and (3) establish the influence of FUV radiation on the chemistry of the warm inner regions of planet-forming disks. ODYSSEUS will also acquire and provide contemporaneous observations at X-ray, optical, near-IR, and millimeter wavelengths to enhance the impact of the ULLYSES data. Our goal is to provide a consistent framework to accurately measure the level and evolution of mass accretion in protoplanetary disks, the properties and magnitudes of inner-disk mass loss, and the influence of UV radiation fields that determine ionization levels and drive disk chemistry.

Aims: Luminosity bursts in young FU Orionis-type stars warm up the surrounding disks of gas and dust, thus inflicting changes on their morphological and chemical composition. In this work, we aim at studying the effects that such bursts may have on the spatial distribution of dust grain sizes and the corresponding spectral index in protoplanetary disks.
Methods: We use the numerical hydrodynamics code FEOSAD, which simulates the co-evolution of gas, dust, and volatiles in a protoplanetary disk, taking dust growth and back reaction on gas into account. The dependence of the maximum dust size on the water ice mantles is explicitly considered. The burst is initialized by increasing the luminosity of the central star to 100-300 L_⊙ for a time period of 100 yr.
Results: The water snowline shifts during the burst to a larger distance, resulting in the drop of the maximum dust size interior to the snowline position because of more efficient fragmentation of bare grains. After the burst, the water snowline shifts quickly back to its preburst location followed by renewed dust growth. The timescale of dust regrowth after the burst depends on the radial distance so that the dust grains at smaller distances reach the preburst values faster than the dust grains at larger distances. As a result, a broad peak in the radial distribution of the spectral index in the millimeter dust emission develops at ≈10 au, which shifts further out as the disk evolves and dust grains regrow to preburst values at progressively larger distances. This feature is most pronounced in evolved axisymmetric disks rather than in young gravitationally unstable counterparts, although young disks may still be good candidates if gravitational instability is suppressed. We confirmed our earlier conclusion that spiral arms do not act as strong dust accumulators because of the Stokes number dropping below 0.01 within the arms, but this trend may change in low-turbulence disks.
Conclusions: We argue that, depending on the burst strength and disk conditions, a broad peak in the radial distribution of the spectral index can last for up to several thousand years after the burst has ended and can be used to infer past bursts in otherwise quiescent protostars. The detection of a similar peak in the disk around V883 Ori, an FU Orionis-type star with an unknown eruption date, suggests that such features may be common in the post-outburst objects.

Close encounters between young stellar objects in star-forming clusters are expected to markedly perturb circumstellar disks. Such events are witnessed in numerical simulations of star formation, but few direct observations of ongoing encounters have been made. Here we report sub-0.1″-resolution Atacama Large Millimeter/Submillimeter Array and Jansky Very Large Array observations towards the million-year-old binary protostar Z Canis Majoris in dust continuum and molecular line emission. A point source ~4,700 au from the binary has been discovered at both millimetre and centimetre wavelengths. It is located along the extension of a ~2,000 au streamer structure previously found in scattered light imaging, whose counterpart in dust and gas emission is also newly identified. Comparison with simulations shows signposts of a rare flyby event in action. Z CMa is a `double burster', as both binary components undergo accretion outbursts, which may be facilitated by perturbations to the host disk by flybys.

We report results of a spectropolarimetric and photometric monitoring of the weak-line T Tauri star V410 Tau based on data collected mostly with SPIRou, the near-infrared (NIR) spectropolarimeter recently installed at the Canada-France-Hawaii Telescope, as part of the SPIRou Legacy Survey large programme, and with TESS between October and December 2019. Using Zeeman-Doppler Imaging (ZDI), we obtained the first maps of photospheric brightness and large-scale magnetic field at the surface of this young star derived from NIR spectropolarimetric data. For the first time, ZDI is also simultaneously applied to high-resolution spectropolarimetric data and very-high-precision photometry. V410 Tau hosts both dark and bright surface features and magnetic regions similar to those previously imaged with ZDI from optical data, except for the absence of a prominent dark polar spot. The brightness distribution is significantly less contrasted than its optical equivalent, as expected from the difference in wavelength. The large-scale magnetic field (∼410 G), found to be mainly poloidal, features a dipole of ∼390 G, again compatible with previous studies at optical wavelengths. NIR data yield a surface differential rotation slightly weaker than that estimated in the optical at previous epochs. Finally, we measured the radial velocity of the star and filtered out the stellar activity jitter using both ZDI and Gaussian Process Regression down to a precision of ∼0.15∼ and 0.08 km/s RMS, respectively, confirming the previously published upper limit on the mass of a potential close-in massive planet around V410 Tau.

The eruptive young star V899 Mon shows characteristics of both FUors and EXors. It reached a peak brightness in 2010, then briefly faded in 2011, followed by a second outburst. We conducted multifilter optical photometric monitoring, as well as optical and near-infrared spectroscopic observations, of V899 Mon. The light curves and color-magnitude diagrams show that V899 Mon has been gradually fading after its second outburst peak in 2018, but smaller accretion bursts are still happening. Our spectroscopic observations taken with Gemini/IGRINS and VLT/MUSE show a number of emission lines, unlike during the outbursting stage. We used the emission line fluxes to estimate the accretion rate and found that it has significantly decreased compared to the outbursting stage. The mass-loss rate is also weakening. Our 2D spectroastrometric analysis of emission lines recovered jet and disk emission of V899 Mon. We found that the emission from permitted metallic lines and the CO bandheads can be modeled well with a disk in Keplerian rotation, which also gives a tight constraint for the dynamical stellar mass of 2 M _⊙. After a discussion of the physical changes that led to the changes in the observed properties of V899 Mon, we suggest that this object is finishing its second outburst.

We present the results of our study of the close pre-main sequence spectroscopic binary CVSO 104 in Ori OB1, based on data obtained within the PENELLOPE legacy program. We derive, for the first time, the orbital elements of the system and the stellar parameters of the two components. The system is composed of two early M-type stars and has an orbital period of about five days and a mass ratio of 0.92, but contrary to expectations, it does not appear to have a tertiary companion. Both components have been (quasi-)synchronized, but the orbit is still very eccentric. The spectral energy distribution clearly displays a significant infrared excess that is compatible with a circumbinary disk. The analysis of He I and Balmer line profiles, after the removal of the composite photospheric spectrum, reveals that both components are accreting at a similar level. We also observe excess emission in Hα and Hβ, which appears redshifted or blueshifted by more than 100 km s⁻¹ with respect to the mass center of the system, depending on the orbital phase. This additional emission could be connected with accretion structures, such as funnels of matter from the circumbinary disk. We also analyze the optical companion located at about 2.″4 from the spectroscopic binary. This companion, which we named CVSO 104 B, turns out to be a background Sun-like star that is not physically associated with the PMS system and does not belong to Ori OB1.

The FU Orionis-type objects (FUors) are low-mass pre-main-sequence stars undergoing a temporary but significant increase of mass accretion rate from the circumstellar disk onto the protostar. It is not yet clear what triggers the accretion bursts and whether the disks of FUors are in any way different from the disks of nonbursting young stellar objects. Motivated by this, we conducted a 1.3 mm continuum survey of 10 FUors and FUor-like objects with ALMA, using both the 7 m array and the 12 m array in two different configurations to recover emission at the widest possible range of spatial scales. We detected all targeted sources and several nearby objects as well. To constrain the disk structure, we fit the data with models of increasing complexity from 2D Gaussian to radiative transfer, enabling comparison with other samples modeled in a similar way. The radiative transfer modeling gives disk masses that are significantly larger than what is obtained from the measured millimeter fluxes assuming optically thin emission, suggesting that the FUor disks are optically thick at this wavelength. In comparison with samples of regular class II and class I objects, the disks of FUors are typically a factor of 2.9-4.4 more massive and a factor of 1.5-4.7 smaller in size. A significant fraction of them (65%-70%) may be gravitationally unstable.

Among the low-mass pre-main sequence stars, a small group called FU Orionis-type objects (FUors) are notable for undergoing powerful accretion outbursts. V1057 Cyg, a classical example of an FUor, went into outburst around 1969-1970, after which it faded rapidly, making it the fastest-fading FUor known. Around 1995, a more rapid increase in fading occurred. Since that time, strong photometric modulations have been present. We present nearly 10 yr of source monitoring at Piszkéstető Observatory, complemented with optical/NIR photometry and spectroscopy from the Nordic Optical Telescope, Bohyunsan Optical Astronomy Observatory, Transiting Exoplanet Survey Satellite, and Stratospheric Observatory for Infrared Astronomy. Our light curves show continuation of significant quasi-periodic variability in brightness over the past decade. Our spectroscopic observations show strong wind features, shell features, and forbidden emission lines. All of these spectral lines vary with time. We also report the first detection of [S II], [N II], and [O III] lines in the star.

Young stellar objects are surrounded by a circumstellar disk, from which material is falling onto the stellar surface. According to the magnetospheric accretion model, the stellar magnetic field truncates the disk at the distance of a few stellar radii and channels the disk material onto the star. Although usually described with simple and static models, this accretion process is inherently time variable, therefore our aim is to characterize the accretion process in time and study the accretion diagnostic parameters. Here, we present a complex study of VW Cha, a low-mass young star. We combined the TESS observations with contemporaneous ground-based IJHK-band photometry, and multi-epoch optical spectroscopic observations obtained by the VLT/ESPRESSO and the 2.2m/FEROS spectrographs. Besides uncovering the periodic light variations that can be attributed to the stellar rotation, the uninterrupted TESS observations allow us to examine the shorter timescale fluctuations probably due to accretion variability. Using the additional observations, we identify accretion tracers and study where they form, measure the accretion rate, and determine the distribution and the kinematics of the accreting material.

V555 Ori is a T Tauri star, whose 1.5 mag brightening was published as a Gaia science alert in 2017. We carried out optical and near-infrared (NIR) photometric, and optical spectroscopic observations to understand the light variations. The light curves show that V555 Ori was faint before 2017, entered a high state for about a year, and returned to the faint state by mid-2018. In addition to the long-term flux evolution, quasi-periodic brightness oscillations were also evident, with a period of about 5 d. At optical wavelengths both the long-term and short-term variations exhibited colourless changes, while in the NIR they were consistent with changing extinction. We explain the brightness variations as the consequence of changing extinction. The object has a low accretion rate whose variation in itself would not be enough to reproduce the optical flux changes. This behaviour makes V555 Ori similar to the pre-main sequence star AA Tau, where the light changes are interpreted as periodic eclipses of the star by a rotating inner disc warp. The brightness maximum of V555 Ori was a moderately obscured (A_V = 2.3 mag) state, while the extinction in the low state was A_V = 6.4 mag. We found that while the Gaia alert hinted at an accretion burst, V555 Ori is a standard dipper, similar to the prototype AA Tau. However, unlike in AA Tau, the periodic behaviour was also detectable in the faint phase, implying that the inner disc warp remained stable in both the high and low states of the system.

The evolution of young stars and disks is driven by the interplay of several processes, notably the accretion and ejection of material. These processes, critical to correctly describe the conditions of planet formation, are best probed spectroscopically. Between 2020 and 2022, about 500orbits of the Hubble Space Telescope (HST) are being devoted in to the ULLYSES public survey of about 70 low-mass (M_⋆ ≤ 2 M_⊙) young (age < 10 Myr) stars at UV wavelengths. Here, we present the PENELLOPE Large Program carried out with the ESO Very Large Telescope (VLT) with the aim of acquiring, contemporaneously to the HST, optical ESPRESSO/UVES high-resolution spectra for the purpose of investigating the kinematics of the emitting gas, along with UV-to-NIR X-shooter medium-resolution flux-calibrated spectra to provide the fundamental parameters that HST data alone cannot provide, such as extinction and stellar properties. The data obtained by PENELLOPE have no proprietary time and the fully reduced spectra are being made available to the whole community. Here, we describe the data and the first scientific analysis of the accretion properties for the sample of 13 targets located in the Orion OB1 association and in the σ-Orionis cluster, observed in November-December 2020. We find that the accretion rates are in line with those observed previously in similarly young star-forming regions, with a variability on a timescale of days (≲3). The comparison of the fits to the continuum excess emission obtained with a slab model on the X-shooter spectra and the HST/STIS spectra shows a shortcoming in the X-shooter estimates of ≲10%, which is well within the assumed uncertainty. Its origin can be either due to an erroneous UV extinction curve or to the simplicity of the modeling and, thus, this question will form the basis of the investigation undertaken over the course of the PENELLOPE program. The combined ULLYSES and PENELLOPE data will be key in attaining a better understanding of the accretion and ejection mechanisms in young stars.

Young stellar objects are still surrounded by a circumstellar disk, from which material is accreting onto the stellar surface. This mass accretion process is essential in the formation and evolution of Sun-like stars. Although usually described with simple and static models, the accretion process is inherently time variable. Here, we present a multi-epoch analysis of CR Cha, a low-mass young stellar object with the age of 1-3 Myr. The aim of our work is to characterize the geometry and variability of the accretion process over a wide range of timescales in several accretion tracers using high-cadence photometric and high-resolution spectroscopic data. We analyse high-precision optical photometry obtained by the TESS space telescope, contemporaneous IJHK-band photometric measurements obtained by the SMARTS telescope, and combine these with high-resolution spectroscopic monitoring with the VLT/ESPRESSO and the 2.2m/FEROS spectrographs. Our extensive dataset allows us to examine the amplitude, timescale and pattern of variability in tracers which carry information on the distribution and the kinematics of the accreting material, the density structure of the inner disk, stellar activity, and the presence of outflows or jets. Where we have contemporaneous photometry and spectroscopy we link the photometric variability to spectroscopic variations. We complemented our data with spectropolarimetric observations from earlier years, which allow us to examine the role the large scale stellar magnetic field plays in the accretion process and to study spectroscopic variations over decadal timescales.

A protoplanetary disk (PPD) typically forms a dead zone near its midplane at a distance of a few astronomical units from the central protostar. Accretion through such a magnetically layered disk can be intrinsically unstable and has been associated with episodic outbursts in young stellar objects. We present the first investigation into the effects of a low-metallicity environment on the structure of the dead zone, as well as the resulting outbursting behavior of the PPD. We conducted global numerical hydrodynamic simulations of PPD formation and evolution in the thin-disk limit. The consequences of metallicity were considered via its effects on the gas and dust opacity of the disk, the thickness of the magnetically active surface layer, and the temperature of the prestellar cloud core. We show that the metal-poor disks accumulate much more mass in the innermost regions as compared to the solar-metallicity counterparts. The duration of the outbursting phase also varies with metallicity; the low-metallicity disks showed more powerful luminosity eruptions with a shorter burst phase, which was confined mostly to the early, embedded stages of the disk evolution. The lowest-metallicity disks with the higher cloud core temperature showed the most significant differences. The occurrence of outbursts was relatively rare in the disks around low-mass stars, and this was especially true at the lowest metallicities. We conclude that the metal content of the disk environment can have profound effects on both the disk structure and evolution in terms of episodic accretion.

Extreme outbursts in young stars may be a common stage of pre-main-sequence stellar evolution. These outbursts, caused by enhanced accretion and accompanied by increased luminosity, can also strongly impact the evolution of the circumstellar environment. We present Atacama Large Millimeter Array (ALMA) and Very Large Array observations of EX Lupi, a prototypical outburst system, at 100, 45, and 15 GHz. We use these data, along with archival ALMA 232 GHz data, to fit radiative transfer models to EX Lupi's circumstellar disk in its quiescent state following the extreme outburst in 2008. The best-fit models show a compact disk with a characteristic dust radius of 45 au and a total mass of 0.01 M_⊙. Our modeling suggests grain growth to sizes of at least 3 mm in the disk, possibly spurred by the recent outburst, and an ice line that has migrated inward to 0.2-0.3 au post-outburst. At 15 GHz, we detected significant emission over the expected thermal disk emission which we attribute primarily to stellar (gyro)synchrotron and free-free disk emission. Altogether, these results highlight what may be a common impact of outbursts on the circumstellar dust.

We present new ALMA 233 GHz continuum observations of the FU Orionis Object HBC722. With these data we detect HBC722 at millimeter wavelengths for the first time, use this detection to calculate a circumstellar disk mass of 0.024 M_⊙, and discuss implications for the burst triggering mechanism.

Context. EX Lup is a well-studied T Tauri star that represents the prototype of young eruptive stars known as EXors. They are characterized by repetitive outbursts that are due to enhanced accretion from the circumstellar disk onto the star. In this paper, we analyze new adaptive optics imaging and spectroscopic observations to study EX Lup and its circumstellar environment in near-infrared in its quiescent phase.
Aims: We aim to provide a comprehensive understanding of the circumstellar environment around EX Lup in quiescence, building upon the vast store of data provided by the literature.
Methods: We observed EX Lup in quiescence with the high contrast imager SPHERE/IRDIS in the dual-beam polarimetric imaging mode to resolve the circumstellar environment in near-infrared scattered light. We complemented the data with earlier SINFONI spectroscopy, which was also taken in quiescence.
Results: We resolve, for the first time in scattered light, a compact feature around EX Lup azimuthally extending from ~280° to ~360° and radially extending from ~0.3'' to ~0.55'' in the plane of the disk. We explore two different scenarios for the detected emission. The first one accounts for the emission as coming from the brightened walls of the cavity excavated by the outflow whose presence was suggested by ALMA observations in the J = 3-2 line of ¹²CO. The second attributes the emission to an inclined disk. In this latter case, we detect, for the first time, a more extended circumstellar disk in scattered light, which shows that a region between ~10 and ~30 au is depleted of μm-size grains. We compare the J-, H-, and K-band spectra obtained with SINFONI in quiescence with the spectra taken during the outburst, showing that all the emission lines result from the episodic accretion event.
Conclusions: Based on the morphology analysis, we favor the scenario that assumes the scattered light is coming from a circumstellar disk rather than the outflow around EX Lup. We determine the origin of the observed feature as either coming from a continuous circumstellar disk with a cavity, from the illuminated wall of the outer disk, or from a shadowed disk. Moreover, we discuss the potential origins of the depleted region of μm-size grains, exploring the possibility that a sub-stellar companion may be the source of this feature.

We present optical-infrared photometric and spectroscopic observations of Gaia 18dvy, located in the Cygnus OB3 association at a distance of 1.88 kpc. Gaia 18dvy was noted by the Gaia alerts system when its light curve exhibited a ≳4 mag rise in 2018-2019. The brightening was also observable at mid-infared wavelengths. The infrared colors of Gaia 18dvy became bluer as the outburst progressed. Its optical and near-infrared spectroscopic characteristics in the outburst phase are consistent with those of bona fide FU Orionis-type young eruptive stars. The progenitor of the outburst is probably a low-mass K-type star with an optical extinction of ∼3 mag. A radiative transfer modeling of the circumstellar structure, based on the quiescent spectral energy distribution, indicates a disk with a mass of 4 × 10^-3 M_⊙. Our simple accretion disk modeling implies that the accretion rate had been exponentially increasing for more than 3 yr until mid-2019, when it reached a peak value of 6.9 × 10^-6 M_⊙ yr^-1. In many respects, Gaia 18dvy is similar to the FU Ori-type object HBC 722.

FU Orionis objects (FUors) are rapidly accreting, pre-main-sequence objects that are known to exhibit large outbursts at optical and near-infrared wavelengths, with post-eruption, small-scale photometric variability superimposed on longer-term trends. In contrast, little is known about the variability of FUors at longer wavelengths. To explore this further, we observed six FUor objects using the NOrthern Extended Millimeter Array (NOEMA) and for a subset of three objects we obtained coordinated observations with NOEMA and the Lowell Discovery Telescope. In combination with previously published NOEMA observations from 2014, our 2017 observations of V1735 Cyg provide the first detection of variability in an FUor object at 2.7 mm. In the absence of significant optical variability, we discount the possibility that the millimeter flux density changed as a result of irradiation from the central disk. In addition, a change in the dust mass due to infall is highly unlikely. A plausible explanation for the change in 2.7 mm flux density is variability in free-free emission due to changes in the object's jet/wind. Thus, it may be that free-free emission in some FUor objects is significant at ∼3 mm and must be considered when deriving disk masses in order to help constrain the mechanism responsible for triggering FUor outbursts.

FU Orionis-type stars are young stellar objects showing large outbursts due to highly enhanced accretion from the circumstellar disk onto the protostar. FU Orionis-type object (FUor) outbursts happen in a wide variety of sources from the very embedded ones to those with almost no sign of extended emission beyond the disk. The subsequent eruptions might gradually clear up the obscuring envelope material and drive the protostar on its way to become a disk-only T Tauri star. We used the Very Large Telescope (VLT)/VLT spectrometer and imager for the mid-infrared (VISIR) to obtain the first spectra that cover the 8-13 μm mid-infrared wavelength range in low resolution of five recently discovered FUors. Four objects from our sample show the 10 μm silicate feature in emission. We study the shape and strength of the silicate feature in these objects and find that they mostly contain large amorphous grains, suggesting that large grains are typically not settled to the midplane in FUor disks. This is a general characteristic of FUors, as opposed to regular T Tauri-type stars whose disks display anything from pristine small grains to significant grain growth. We classify our targets by determining whether the silicate feature is in emission or in absorption, and confront them with the evolutionary scenarios on the dispersal of the envelopes around young stars. In our sample, all Class II objects exhibit silicate emission, while for Class I objects, the appearance of the feature in emission or absorption depends on the viewing angle with respect to the outflow cavity. This highlights the importance of geometric effects when interpreting the silicate feature.

Turbulence is a crucial factor in many models of planet formation, but it has only been directly constrained among a small number of planet-forming disks. Building on the upper limits on turbulence placed in disks around HD 163296 and TW Hya, we present ALMA CO J = 2-1 line observations at ∼0"3 (20-50 au) resolution and 80 ms^-1 channel spacing of the disks around DM Tau, MWC 480, and V4046 Sgr. Using parametric models of disk structure, we robustly detect nonthermal gas motions around DM Tau of between 0.25c_s and 0.33c_s, with the range dominated by systematic effects, making this one of the only systems with directly measured nonzero turbulence. Using the same methodology, we place stringent upper limits on the nonthermal gas motion around MWC 480 (<0.08c_s) and V4046 Sgr (<0.12c_s). The preponderance of upper limits in this small sample and the modest turbulence levels consistent with dust studies suggest that weak turbulence (α ≲ 10^-3) may be a common, albeit not universal, feature of planet-forming disks. We explore the particular physical conditions around DM Tau that could lead this system to be more turbulent than the others.

While accreting through a circumstellar disk, young stellar objects are observed to undergo sudden and powerful accretion events known as FUor or EXor outbursts. Although such episodic accretion is considered to be an integral part of the star formation process, the triggers and mechanisms behind them remain uncertain. We conducted global numerical hydrodynamics simulations of protoplanetary disk formation and evolution in the thin-disk limit, assuming both magnetically layered and fully magnetorotational instability (MRI)-active disk structure. In this paper, we characterize the nature of the outbursts occurring in these simulations. The instability in the dead zone of a typical layered disk results in "MRI outbursts." We explore their progression and their dependence on the layered disk parameters as well as cloud core mass. The simulations of fully MRI-active disks showed an instability analogous to the classical thermal instability. This instability manifested at two temperatures—above approximately 1400 K and 3500 K—due to the steep dependence of Rosseland opacity on the temperature. The origin of these thermally unstable regions is related to the bump in opacity resulting from molecular absorption by water vapor and may be viewed as a novel mechanism behind some of the shorter duration accretion events. Although we demonstrated local thermal instability in the disk, more investigations are needed to confirm that a large-scale global instability will ensue. We conclude that the magnetic structure of a disk, its composition, as well as the stellar mass, can significantly affect the nature of episodic accretion in young stellar objects.

Gas has been detected in a number of debris disks. It is likely secondary, I.e., produced by colliding solids. Here, we report ALMA Band 8 observations of neutral carbon in the CO-rich debris disk around the 15-30 Myr old A-type star HD 32297. We find that C⁰ is located in a ring at ∼110 au with an FWHM of ∼80 au and has a mass of (3.5 ± 0.2) × 10^-3 M_⊕. Naively, such a surprisingly small mass can be accumulated from CO photodissociation in a time as short as ∼10⁴ yr. We develop a simple model for gas production and destruction in this system, properly accounting for CO self-shielding and shielding by neutral carbon, and introducing a removal mechanism for carbon gas. We find that the most likely scenario to explain both C⁰ and CO observations is one where the carbon gas is rapidly removed on a timescale of order a thousand years and the system maintains a very high CO production rate of ∼15 M_⊕ Myr^-1, much higher than the rate of dust grind-down. We propose a possible scenario to meet these peculiar conditions: the capture of carbon onto dust grains, followed by rapid CO re-formation and rerelease. In steady state, CO would continuously be recycled, producing a CO-rich gas ring that shows no appreciable spreading over time. This picture might be extended to explain other gas-rich debris disks.

While the Sun is a quiet and well-balanced star now, during its first few million years it possessed a strong magnetic field and actively accreted material from its circumstellar environment. Theoretical models predict that under certain circumstances the interaction of a strongly magnetic star and its circumstellar disk may lead to short bursts of increased accretion onto the star (D'Angelo & Spruit 2012). Examples for this phenomenon may be the members of a group of young eruptive stars called EXors. Their prototype, EX Lup, had its historically largest outburst in 2008. Spectroscopic evidence suggests that the mass accretion proceeds through the same magnetospheric accretion channels both in quiescence and in outburst but with different mass flux (Sicilia-Aguilar et al. 2012). To characterize for the first time EX Lup's magnetic field, we obtained spectropolarimetric monitoring for it with the CFHT/ESPaDOnS. We detected strong, poloidal magnetic field with a prominent cool polar cap and an accretion spot above it. We compared our results with numerical simulations, in order to check the applicability of the d'Angelo & Spruit model as an explanation of EX Lup's accretion outbursts. If EX Lup is a good proxy for the proto-Sun, similar magnetic field-disk interactions and outbursts might have happened during the early evolution of the Solar System as well.

FU Orionis-type objects (FUors) are young low-mass stars undergoing powerful accretion outbursts. The increased accretion is often accompanied by collimated jets and energetic, large-scale molecular outflows. The extra heating during the outburst may also induce detectable geometrical, chemical, and mineralogical changes in the circumstellar material, affecting possible planet formation around these objects. V346 Nor is a southern FUor with peculiar spectral characteristics. Decades after the beginning of its outburst, it unexpectedly underwent a fading event around 2010 due to a decrease in the mass accretion rate onto the star by at least two orders of magnitude. Here we present optical and near-infrared photometry and spectroscopy obtained after the minimum. Our light curves show a gradual re-brightening of V346 Nor, with its K_s-band brightness only 1.5 mag below the outburst brightness level. Our Very Large Telescope (VLT)/XSHOOTER spectroscopic observations display several strong forbidden emission lines toward the source from various metals and molecular hydrogen, suggesting the launch of a new jet. Our N-band spectrum obtained with VLT/VISIR outlines a deeper silicate absorption feature than before, indicating that the geometry of the circumstellar medium has changed in the post-outburst period compared to peak brightness.

The early evolution of Sun-like stars may be interspersed by energetic FU Orionis (FUor) type accretion outbursts. We analysed eight years of photometric and spectroscopic variability of V582 Aur, a bona fide FUor, in outburst. While the accretion rate derived from near-infrared measurements was constant, radical brightness changes occurred due to dust clumps crossing the line of sight. The brightness minima resemble the variability patterns of the UXor phenomenon. Orbiting density enhancements or short-lived clumps moving in and out of the line-of-sight may explain these observations. Our message is that during FUor outbursts the inner disk is a dynamically active place, affecting the initial conditions for planet formation.

Young stellar objects often show photometric variability, which is well examined at optical wavelengths, but more and more infrared data are also available. The wavelength dependence of the variability carries information on the physical cause of the changing brightness. Here, we examine seven T Tauri-type stars known for their large amplitude variability selected from the Campaign 13 field of the Kepler K2 mission. We complemented the K2 light curves by multifilter optical monitoring observations made with the 90 cm Schmidt telescope of Konkoly Observatory, and by 3.6 and 4.5 μm infrared photometry with a 20 hours cadence using the Spitzer Space Telescope. We found that the wavelength dependence of the observed variability is not consistent with changing interstellar extinction. We suggest that the brightness changes are due to variable accretion, causing a variable illumination of the inner disk.

Our aim is to present a new and so far most complete catalog of optically selected young stars. The basis of this work is an extensive literature search for young stars in all the known nearby (< 2 kpc) star forming regions, included in the Handbook of Star Forming Regions [4, 5], and in 67 additional catalogs. We collected data on known young, pre-main-sequence stars detected in optical bands. The catalog contains the celestial coordinates, object names, names of the enclosing star forming region, identification methods, distances, and other information (e.g., references, binarity) for 15208 young stellar objects. It is already in use by the Gaia Photometric Science Alerts Team to identify variable young stars in the Gaia data. Our catalog was cross-correlated with the Gaia DR2 and we obtained flux and distance estimations for 86% of the stars.

The origin of optical-infrared variability in young, intermediate mass Herbig Ae/Be stars is linked to their circumstellar disk. Therefore, variability could serve as a diagnostic tool to constrain the structure and dynamics of the (inner) disk. Here we discuss this diagnostic potential, and report some preliminary results from our coordinated BV RIJHK_s and Spitzer monitoring observations of nine Herbig Ae stars. We aim to understand the response of the inner disks thermal emission on the changing stellar irradiation, and to separate it from UX Orionis-type fading events, which also provide information on the disk. This project is a pilot study for the era of time domain astronomy of young stars, opened by Kepler K2, Gaia, ASAS-SN, TESS, Spitzer, WISE, and JWST.

A long-standing problem of the general paradigm of low-mass star formation is the "luminosity problem": protostars are less luminous than theoretically predicted. One possible solution is that the accretion process is episodic. FU Orionis-type stars (FUors) are thought to be the visible examples for objects in the high accretion state and it is still debated what physical mechanism triggers the phenomenon. For many of these objects their disk properties are still largely unknown so we conducted a deep, high spatial resolution (down to 20 au) ALMA Band 6 (1.3 mm) dust continuum survey of a sub-sample of known FUors. Here we present preliminary results of our survey, including the mass, size and spectral slope of each disk.

Luminosity outbursts of the FU Ori type stars, which have a magnitude of ~100 L_☉ and last for decades, may affect chemical composition of the surrounding protoplanetary disk. Using astrochemical modelling we analyse the changes induced by the outburst and search for species sensitive to the luminosity rise. Some changes in the disk molecular composition appear not only during the outburst itself but can also retain for decades after the end of the outburst. We analyse main chemical processes responsible for these effects and assess timescales at which chemically inert species return to the pre-outburst abundances.

We conducted global hydrodynamic simulations of protoplanetary disk evolution with an adaptive Shakura-Sunyaev α prescription to represent the layered disk structure, and starting with the collapse phase of the molecular cloud. With the canonical values of model parameters, self-consistent dead zones formed at the scale of a few au. The instabilities associated with the dead zone and corresponding outbursts, similar to FUor eruptions, were also observed in the simulations.

Having disk-to-star accretion rates on the order of 10^-4 M_☉/yr, FU Orionis-type stars (FUors) are thought to be the visible examples for episodic accretion. FUors are often surrounded by massive envelopes, which replenish the disk material and enable the disk to produce accretion outbursts. We observed the FUor-type star V346 Nor with ALMA at 1.3 mm continuum and in different CO rotational lines. We mapped the density and velocity structure of its envelope and analyzed the results using channel maps, position-velocity diagrams, and spectro-astrometric methods. We discovered a pseudo-disk and a Keplerian disk around a 0.1 M_☉ central star. We determined an infall rate from the envelope onto the disk of 6×10^-6 M_☉ /yr, a factor of few higher than the quiescent accretion rate from the disk onto the star. This hints for a mismatch between the infall and accretion rates as the cause of the eruption.

DQ Tau is a young low-mass spectroscopic binary, consisting of two almost equal-mass stars on a 15.8 day period surrounded by a circumbinary disk. We analyzed DQ Tau's light curves obtained by Kepler K2, the Spitzer Space Telescope, and ground-based facilities. We observed variability phenomena, including rotational modulation by stellar spots, energetic stellar flares, brightening events around periastron due to increased accretion, and short dips due to temporary circumstellar obscuration. The study on DQ Tau will help in discovering and understanding the formation and evolution of other real-world examples of "Tatooine-like" systems. This is especially important because more and more evidence points to the possibility that all Sun-like stars were born in binary or multiple systems that broke up later due to dynamical interactions.

While the Sun is nowadays a quiet and well-balanced star, in its first few million years it might have been often out of temper, like those young low-mass stars which episodically undergo unpredictable outbursts. The prototype of one of the two classes of young erupting stars, EX Lupi, had its historically largest outburst in 2008. It brightened by a factor of 30 for six months, due to elevated accretion from the circumstellar disk on to the star. Our group observed the system during the outburst, and discovered the crystallisation of amorphous silicate grains in the inner disk by the heat of the outburst. Our mid-infrared monitoring of the freshly produced crystals revealed that their emission in the inner disk quickly dropped already within a year after the outburst. Here we report on new observations of the 10 µm silicate feature, obtained with the MIDI and VISIR instruments at Paranal Observatory, which demonstrate that within five years practically all forsterite disappeared from the inner disk. We attempt to model this process by an expanding wind that transports the crystals from the terrestrial zone to outer disk regions where comets are supposed to form. Since the eruptions of EX Lup are recurrent, we speculate that the early Sun also experienced similar brightenings, and the forming planetary system might have incorporated some of the mineralogical and chemical yields provided by the outbursts. EX Lup, as a proxy for the proto-Sun, may be a telltale object to understand the origin of molecules and minerals we routinely encounter on Earth.

We present our results from a mid-infrared interferometric survey targeted at the planet-forming region in the circumstellar disks around low- and intermediate-mass young stars. Our sample consists of 82 objects, including T Tauri stars, Herbig Ae stars, and young eruptive stars. Our main results are: 1) Disks around T Tauri stars are similar to those around Herbig Ae stars, but are relatively more extended once we account for stellar luminosity. 2) From the distribution of the sizes of the mid-infrared emitting region we find that inner dusty disk holes may be present in roughly half of the sample. 3) Our analysis of the silicate spectral feature reveals that the dust in the inner ~1 au region of disks is generally more processed than that in the outer regions. 4) The dust in the disks of T Tauri stars typically show weaker silicate emission in the N band spectrum, compared to Herbig Ae stars, which may indicate a general difference in the disk structure. Our data products are available at VizieR, and at the following web page: http://konkoly.hu/MIDI_atlas.

The earliest phases of star formation are characterised by intense mass accretion from the circumstellar disk to the central star. One group of young stellar objects, the FU Orionis-type stars exhibit accretion rate peaks accompanied by bright eruptions. The occurance of these outbursts might solve the luminosity problem of protostars, play a key role in accumulating the final star mass, and have a significant effect on the parameters of the envelope and the disk. In the framework of the Structured Accretion Disks ERC project, we are conducting a systematic investigation of these sources with millimeter interferometry to examine whether they represent normal young stars in exceptional times or they are unusual objects. Our results show that FU Orionis-type stars can be similar to both Class I and Class II systems and may be in a special evolutionary phase between the two classes with their infall-driven episodic eruptions being the main driving force of the transition.

The Herbig Ae star HD 169142 is known to have a gaseous disk with a large inner hole, and also a photometrically variable inner dust component in the sub-astronomical-unit region. Following up on our previous analysis, we further studied the temporal evolution of inner dust around HD 169142, which may provide information on the evolution from late-stage protoplanetary disks to debris disks. We used near-infrared interferometric observations obtained with the Very Large Telescope Interferometer/PIONIER to constrain the dust distribution at three epochs spanning six years. We also studied the photometric variability of HD 169142 using our optical─infrared observations and archival data. Our results indicate that a dust ring at ∼0.3 au formed some time between 2013 and 2018, and then faded (but did not completely disappear) by 2019. The short-term variability resembles that observed in extreme debris disks, and is likely related to short-lived dust of secondary origin, though variable shadowing from the inner ring could be an alternative interpretation. If confirmed, this is the first direct detection of secondary dust production inside a protoplanetary disk.

EX Lup is the prototype of a class of pre-main-sequence eruptive stars defined by their repetitive outbursts lasting several months. In 2008 January─September EX Lup underwent its historically largest outburst, brightening by about 4 mag in visual light. In previous studies we discovered ongoing silicate crystal formation in the inner disk during the outburst, but also noticed that the measured crystallinity fraction started decreasing after the source returned to the quiescent phase. Here we present new observations of the 10 μm silicate feature, obtained with the MIDI and VISIR instruments at Paranal Observatory. The observations demonstrate that within five years practically all crystalline forsterite disappeared from the surface of the inner disk. We reconstruct this process by presenting a series of parametric axisymmetric radiative transfer models of an expanding dust cloud that transports the crystals from the terrestrial zone to outer disk regions where comets are supposed to form. It is possible the early Sun also experienced similar flare-ups, and the forming planetesimals might have incorporated crystalline silicate material produced by such outbursts. Finally, we discuss how far the location of the dust cloud could be constrained by future James Webb Space Telescope observations.

We present ALMA observations of ¹²CO, ¹³CO, and C¹⁸O J = 2-1 lines and the 230 GHz continuum for the FU Ori-type object (FUor) V900 Mon (d ̃ 1.5 kpc), for which the accretion burst was triggered between 1953 and 2009. We identified CO emission associated with a molecular bipolar outflow extending up to an ̃10⁴ au scale and a rotating molecular envelope extending over >10⁴ au. The interaction with the hot energetic FUor wind, which was observed using optical spectroscopy, appears limited to a region within ̃400 au of the star. The envelope mass and collimation of the extended CO outflow suggest that the progenitor of this FUor is a low-mass Class I young stellar object (YSO). These parameters for V900 Mon, another FUor, and a few FUor-like stars are consistent with the idea that FUor outbursts are associated with normal YSOs. The continuum emission is marginally resolved in our observations with a 0.″2 × 0.″15 (̃300 × 225 au) beam, and a Gaussian model provides a deconvolved FWHM of ̃90 au. The emission is presumably associated with a dusty circumstellar disk, plus a possible contribution from a wind or wind cavity close to the star. The warm compact nature of the disk continuum emission could be explained with viscous heating of the disk, while gravitational fragmentation in the outer disk and/or a combination of grain growth and their inward drift may also contribute to its compact nature.

We report new Atacama Large Millimeter/submillimeter Array Band 3 (86-100 GHz; ̃80 mas angular resolution) and Band 4 (146-160 GHz; ̃50 mas angular resolution) observations of the dust continuum emission toward the archetypal and ongoing accretion burst young stellar object FU Ori, which simultaneously covered its companion, FU Ori S. In addition, we present near-infrared (2-2.45 μm) observations of FU Ori taken with the General Relativity Analysis via VLT InTerferometrY (GRAVITY; ̃1 mas angular resolution) instrument on the Very Large Telescope Interferometer (VLTI). We find that the emission in both FU Ori and FU Ori S at (sub)millimeter and near-infrared bands is dominated by structures inward of ̃10 au radii. We detected closure phases close to zero from FU Ori with VLTI/GRAVITY, which indicate the source is approximately centrally symmetric and therefore is likely viewed nearly face-on. Our simple model to fit the GRAVITY data shows that the inner 0.4 au radii of the FU Ori disk has a triangular spectral shape at 2-2.45 μm, which is consistent with the H₂O and CO absorption features in a \dot{M} ̃ 10^-4 M _☉ yr^-1, viscously heated accretion disk. At larger (̃0.4-10 au) radii, our analysis shows that viscous heating may also explain the observed (sub)millimeter and centimeter spectral energy distribution when we assume a constant, ̃10^-4 M _☉ yr^-1 mass inflow rate in this region. This explains how the inner 0.4 au disk is replenished with mass at a modest rate, such that it neither depletes nor accumulates significant masses over its short dynamic timescale. Finally, we tentatively detect evidence of vertical dust settling in the inner 10 au of the FU Ori disk, but confirmation requires more complete spectral sampling in the centimeter bands.

L1551 IRS 5 is a FUor-like object located in the Taurus star-forming region. We present Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm continuum observations using a wide range of baselines. The observations recovered the two circumstellar disks comprising the system and, for the first time, resolved the circumbinary ring. We determined the geometry and estimated lower mass limits for the circumstellar disks using simple models. We calculated lower limits for the total mass of both circumstellar disks. After subtracting the two circumstellar disk models from the image, the residuals show a clearly resolved circumbinary ring. Using a radiative transfer model, we show that geometrical effects can explain some of the brightness asymmetries found in the ring. The remaining features are interpreted as enhancements in the dust density.

Global numerical simulations of protoplanetary disk formation and evolution were conducted in the thin-disk limit, where the model included a magnetically layered disk structure, a self-consistent treatment for the infall from cloud core, and the smallest possible inner computational boundary. We compared the evolution of a layered disk with a fully magnetically active disk. We also studied how the evolution depends on the parameters of the layered disk model—the MRI triggering temperature and active layer thickness—as well as the mass of the prestellar cloud core. With the canonical values of parameters a dead zone formed within the inner ≈15 au region of the magnetically layered disk. The dead zone was not a uniform structure, and long-lived, axisymmetric, gaseous rings ubiquitously formed within this region owing to the action of viscous torques. The rings showed a remarkable contrast in the disk environment as compared to a fully magnetically active disk and were characterized by high surface density and low effective viscosity. Multiple gaseous rings could form simultaneously in the dead zone region, which were highly dynamical and showed complex, time-dependent behavior such as inward migration, vortices, gravitational instability, and large-scale spiral waves. An increase in MRI triggering temperature had only marginal effects, while changes in active layer thickness and the initial cloud core mass had significant effects on the structure and evolution of the inner disk. Dust with large fragmentation barrier could be trapped in the rings, which may play a key role in planet formation.

The second Gaia Data Release (DR2) contains astrometric and photometric data for more than 1.6 billion objects with mean Gaia G magnitude <20.7, including many Young Stellar Objects (YSOs) in different evolutionary stages. In order to explore the YSO population of the Milky Way, we combined the Gaia DR2 data base with Wide-field Infrared Survey Explorer (WISE) and Planck measurements and made an all-sky probabilistic catalogue of YSOs using machine learning techniques, such as Support Vector Machines, Random Forests, or Neural Networks. Our input catalogue contains 103 million objects from the DR2xAllWISE cross-match table. We classified each object into four main classes: YSOs, extragalactic objects, main-sequence stars, and evolved stars. At a 90 per cent probability threshold, we identified 1 129 295 YSO candidates. To demonstrate the quality and potential of our YSO catalogue, here we present two applications of it. (1) We explore the 3D structure of the Orion A star-forming complex and show that the spatial distribution of the YSOs classified by our procedure is in agreement with recent results from the literature. (2) We use our catalogue to classify published Gaia Science Alerts. As Gaia measures the sources at multiple epochs, it can efficiently discover transient events, including sudden brightness changes of YSOs caused by dynamic processes of their circumstellar disc. However, in many cases the physical nature of the published alert sources are not known. A cross-check with our new catalogue shows that about 30 per cent more of the published Gaia alerts can most likely be attributed to YSO activity. The catalogue can be also useful to identify YSOs among future Gaia alerts.

We studied the disc of the unclassified B[e] star HD 50138 in order to explore its structure and to find indications for the evolutionary status of this system, whether it is a young Herbig Be or a post-main-sequence star. Using high spatial resolution interferometric measurements from MIDI instrument (N-band) on the Very Large Telescope Interferometer, we analysed the disc size, the time-variability of the disc's thermal emission, and the spectral shape of the 10 μm silicate feature. By fitting simple disc models, we determined the inclination and the mid-infrared size of the disc, confirming earlier results based on a lower number of observations. We searched for mid-infrared temporal variability of different regions of the disc, and concluded that its morphology is not experiencing significant changes over the observed epochs. We characterized the mid-infrared silicate feature by determining the feature amplitude and the 11.3/9.8 μm flux ratio. The latter parameter is a good indicator of the grain size. The shape of the feature suggests the presence of crystalline silicate grains in the disc. The interferometric data revealed a strong radial trend in the mineralogy: while the disc's innermost region seems to be dominated by forsterite grains, at intermediate radii both forsterite and enstatite may be present. The outer disc may predominantly contain amorphous silicate particles. A comparison of the observed spectral shape with that of a sample of intermediate-mass stars (supergiants, Herbig Ae/Be stars, unclassified B[e] stars) implied that the evolutionary state of HD 50138 cannot be unambiguously decided from mid-IR spectroscopy.

We discuss how a spectroscopic and imaging survey of protoplanetary disks with Extremely Large Telescopes will enhance our understanding of planet formation by (1) measuring the physical and chemical conditions in disks using infrared spectroscopy and (2) studying planet-disk interactions using imaging and spectro-astrometry.

The accretion-driven outbursts of young FU Orionis-type stars may be a common stage of pre-main-sequence evolution and can have a significant impact on the circumstellar environment as it pertains to the growth of solids and eventually planets. This episodic accretion is thought to be sustained by additional gas infalling from the circumstellar envelope and disk. We present APEX observations of the CO gas in the envelope around V883 Orionis, a young outbursting star. The observations mapped the ¹²CO(4-3), ¹²CO(3-2), and ¹³CO(3-2) lines with the FLASH⁺ instrument and the ¹²CO(6-5) line with the SEPIA instrument. We detected high signal-to-noise emission extending out to radii >10,000 au and calculated integrated fluxes of 1100 Jy km s^-1 for ¹²CO(6-5), 2400 Jy km s^-1 for ¹²CO(4-3), 1600 Jy km s^-1 for ¹²CO(3-2), and 450 Jy km s^-1 for ¹³CO(3-2). We used the thermochemical code PRODIMO to test several models and find the data are best described by an envelope structure with M _env ≈ 0.2-0.4 M _☉ and a mass-infall rate of {\dot{M}}_\inf =1{--}2× {10}^-6 {M}_☉ {yr}}^-1. We infer that the observed envelope and outflow structure around V883 Ori could be caused by multiple outbursts, consistent with episodic accretion.

V582 Aur is a pre-main-sequence FU Orionis type eruptive star, which entered a brightness minimum in 2016 March due to changes in the line-of-sight extinction. Here, we present and analyze new optical B, V, R _C , and I _C band multiepoch observations and new near-infrared J, H, and K _S band photometric measurements from 2018 January-2019 February, as well as publicly available midinfrared Wide-field Infrared Survey Explorer (WISE) data. We found that the source shows a significant optical-near-infrared variability, and the current brightness minimum has not completely finished yet. If the present dimming originates from the same orbiting dust clump that caused a similar brightness variation in 2012, then our results suggest a viscous spreading of the dust particles along the orbit. Another scenario is that the current minimum is caused by a dust structure, that is entering and leaving the inner part of the system. The WISE measurements could be consistent with this scenario. Our long-term data, as well as an accretion disk modeling hint at a general fading of V582 Aur, suggesting that the source will reach the quiescent level in ~80 yr.

Episodic accretion may be a common occurrence in the evolution of young pre-main sequence stars and has important implications for our understanding of star and planet formation. Many fundamental aspects of what drives the accretion physics, however, are still unknown. The ngVLA will be a key tool in understanding the nature of these events. The high spatial resolution, broad spectral coverage, and unprecedented sensitivity will allow for the detailed analysis of outburst systems. The proposed frequency range of the ngVLA allows for observations of the gas, dust, and non-thermal emission from the star and disk.

The FU Ori-type young stellar objects are characterized by a sudden increase in luminosity by 1-2 orders of magnitude followed by a slow return to the pre-outburst state on timescales of ̃10-100 yr. The outburst strongly affects the entire disk, changing its thermal structure and radiation field. In this paper, using a detailed physical-chemical model, we study the impact of the FU Ori outburst on the disk chemical inventory. Our main goal is to identify gas-phase molecular tracers of the outburst activity that could be observed after the outburst with modern telescopes such as ALMA and NOEMA. We find that the majority of molecules experience a considerable increase in total disk gas-phase abundances due to the outburst, mainly due to the sublimation of their ices. Their return to the pre-outburst chemical state takes different amounts of time, from nearly instantaneous to very long. Among the former, we identify CO, NH₃, C₂H₆, C₃H₄, etc. Their abundance evolution tightly follows the luminosity curve. For CO, the abundance increase does not exceed an order of magnitude, while for other tracers, the abundances increase by 2-5 orders of magnitude. Other molecules, like H₂CO and NH₂OH, have longer retention timescales, remaining in the gas phase for ̃10-10³ yr after the end of the outburst. Thus, H₂CO could be used as an indicator of the previous outbursts in the post-outburst FU Ori systems. We investigate the corresponding time-dependent chemistry in detail and present the most favorable transitions and ALMA configurations for future observations.

We present near-IR imaging polarimetry of five classical FU Ori-type objects (FU Ori, V1057 Cyg, V1515 Cyg, V1735 Cyg, Z CMa) with an ~0.″1 resolution observed using HiCIAO+AO188 at the Subaru Telescope. We observed scattered light associated with circumstellar dust around four of them (i.e., all but V1515 Cyg). Their polarized intensity distribution shows a variety of morphologies with arms, tails or streams, spikes, and fragmented distributions, many of which were reported in our previous paper. The morphologies of these reflection nebulae significantly differ from many other normal young stellar objects (Class I-II objects). These structures are attributed to gravitationally unstable disks, trails of clump ejections, dust blown by a wind or a jet, and a stellar companion. We can consistently explain our results with the scenario that their accretion outbursts (FUor outbursts) are triggered by gravitationally fragmenting disks, and with the hypothesis that many low-mass young stellar objects experience such outbursts.

Context: Protoplanetary disks show large diversity regarding their morphology and dust composition. With mid-infrared interferometry the thermal emission of disks can be spatially resolved, and the distribution and properties of the dust within can be studied.

Aims: Our aim is to perform a statistical analysis on a large sample of 82 disks around low- and intermediate-mass young stars, based on mid-infrared interferometric observations. We intend to study the distribution of disk sizes, variability, and the silicate dust mineralogy.

Methods: Archival mid-infrared interferometric data from the MIDI instrument on the Very Large Telescope Interferometer are homogeneously reduced and calibrated. Geometric disk models are used to fit the observations to get spatial information about the disks. An automatic spectral decomposition pipeline is applied to analyze the shape of the silicate feature.

Results: We present the resulting data products in the form of an atlas, containing N band correlated and total spectra, visibilities, and differential phases. The majority of our data can be well fitted with a continuous disk model, except for a few objects, where a gapped model gives a better match. From the mid-infrared size-luminosity relation we find that disks around T Tauri stars are generally colder and more extended with respect to the stellar luminosity than disks around Herbig Ae stars. We find that in the innermost part of the disks (r ≲ 1 au) the silicate feature is generally weaker than in the outer parts, suggesting that in the inner parts the dust is substantially more processed. We analyze stellar multiplicity and find that in two systems (AB Aur and HD 72106) data suggest a new companion or asymmetric inner disk structure. We make predictions for the observability of our objects with the upcoming Multi-AperTure mid-Infrared SpectroScopic Experiment (MATISSE) instrument, supporting the practical preparations of future MATISSE observations of T Tauri stars.

DQ Tau is a young low-mass spectroscopic binary, consisting of two almost equal-mass stars on a 15.8 day period surrounded by a circumbinary disk. Here, we analyze DQ Tau’s light curves obtained by Kepler K2, the Spitzer Space Telescope, and ground-based facilities. We observed variability phenomena, including rotational modulation by stellar spots, brief brightening events due to stellar flares, long brightening events around periastron due to increased accretion, and short dips due to brief circumstellar obscuration. The rotational modulation appears as a sinusoidal variation with a period of 3.017 days. In our model, this is caused by extended stellar spots 400 K colder than the stellar effective temperature. During our 80 day long monitoring, we detected 40 stellar flares with energies up to 1.2 × 10³⁵ erg and duration of a few hours. The flare profiles closely resemble those in older late-type stars, and their occurrence does not correlate with either the rotational or the orbital period. We observe elevated accretion rates of up to 5 × 10^-8 M _☉ yr^-1 around each periastron. Our Spitzer data suggest that the increased accretion luminosity temporarily heats up the inner part of the circumbinary disk by about 100 K. We found an inner disk radius of 0.13 au, significantly smaller than expected from dynamical modeling of circumbinary disks. Interestingly, the inner edge of the disk corotates with the binary’s orbit. DQ Tau also shows short dips of <0.1 mag in its light curve, reminiscent of the well-known “dipper phenomenon” observed in many low-mass young stars.