A Study of Millimeter Variability in FUor Objects

Wendeborn, J., Espaillat, C. C., Macías, E., Fehér, O., Kóspál, Á., Hartmann, L., Zhu, Zh., Dunham, M. M., Kounkel, M.

The Astrophysical Journal, Volume 897, Issue 1, id.54, 10 pp. (2020) | Link »


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.

Grain Growth in Newly Discovered Young Eruptive Stars

Kóspál, Á., Ábrahám, P., Carmona, A., Chen, L., Green, J. D., van Boekel, R., White, J. A.

The Astrophysical Journal Letters, Volume 895, Issue 2, id.L48, 6 pp. (2020) | Link »


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.

Measuring Turbulent Motion in Planet-forming Disks with ALMA: A Detection around DM Tau and Nondetections around MWC 480 and V4046 Sgr

Flaherty, K. Hughes, A. M., Simon, J. B., Qi, Ch., Bai, X.-N., Bulatek, A., Andrews, S. M., Wilner, D. J., Kóspál, Á.

The Astrophysical Journal, Volume 895, Issue 2, id.109, 18 pp. (2020) | Link »


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 0.25cs and 0.33cs, 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 (<0.08cs) and V4046 Sgr (<0.12cs). 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.

Outbursts in Global Protoplanetary Disk Simulations

Kadam, K., Vorobyov, E., Regály, Zs., Kóspál, Á., Ábrahám, P.

The Astrophysical Journal, Volume 895, Issue 1, id.41, 15 pp. (2020) | Link »


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.

The Surprisingly Low Carbon Mass in the Debris Disk around HD 32297

Cataldi, G., Wu, Y., Brandeker, A., Ohashi, N., Moór, A., Olofsson, G., Ábrahám, P., Asensio-Torres, R., Cavallius, M., Dent, W. R. F., Grady, C., Henning, Th., Higuchi, A. E., Hughes, A. M., Janson, M., Kamp, I., Kóspál, Á., Redfield, S., Roberge, A., Weinberger, A., Welsh, B.

The Astrophysical Journal, Volume 892, Issue 2, id.99, 15 pp. (2020) | Link »


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.

Magnetic field and accretion in EX Lup

Kóspál, Á., Donati, J. -F., Bouvier, J., Ábrahám, P.

Astronomy in Focus XXX, presented at IAU XXX General Assembly, Vienna, Austria. Proceedings of the IAU, 2020, pp. 125-125 | Link »


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.

V346 Nor: The Post-outburst Life of a Peculiar Young Eruptive Star

Á. Kóspál, Zs. M. Szabó, P. Ábrahám, S. Kraus, M. Takami, P. W. Lucas, C. Contreras Peña, A. Udalski

The Astrophysical Journal, Volume 889, 148 (2020) | Link »


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 Ks-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.

An UXor among FUors: extinction-related brightness variations of the young eruptive star V582 Aur

P. Ábrahám, Á. Kóspál, M. Kun, O. Fehér, G. Zsidi, J. A. Acosta-Pulido

Origins: From the Protosun to the First Steps of Life. Proceedings of the International Astronomical Union, 345, 390 (2020) | Link »


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.

Brightness variations of young Sun-like stars from ground-based and space telescopes

G. Zsidi, Á. Kóspál, P. Ábrahám, R. Szabó, B. Cseh, K. Sárneczky, Á. Sódor, R. Szakáts, K. Vida, J. Vinkó

Origins: From the Protosun to the First Steps of Life. Proceedings of the International Astronomical Union, 345, 380 (2020) | Link »


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.

Konkoly optical catalog of young stars for the Gaia Photometric Science Alerts

E. Varga-Verebélyi, M. Kun, E. Szegedi-Elek, P. Ábrahám, J. Varga, Cs. Kiss, Á. Kóspál, G. Marton, L. Szabados

Origins: From the Protosun to the First Steps of Life. Proceedings of the International Astronomical Union, 345, 378 (2020) | Link »


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.

Time-variability and disk geometry in Herbig Ae/Be disks

R. Szakáts, P. Ábrahám, Á. Kóspál

Origins: From the Protosun to the First Steps of Life. Proceedings of the International Astronomical Union, 345, 375 (2020) | Link »


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 RIJHKs 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.

Disks around FUor-type young eruptive stars with ALMA

F. Cruz-Sáenz de Miera, Á. Kóspál, P. Ábraham, H. B. Liu, M. Takami

Origins: From the Protosun to the First Steps of Life. Proceedings of the International Astronomical Union, 345, 369 (2020) | Link »


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.

Chemical modeling of FU Ori protoplanetary disks

T. Molyarova, V. Akimkin, D. Semenov, P. Ábrahám, Th. Henning, Á. Kóspál, E. Vorobyov, D. Wiebe

Origins: From the Protosun to the First Steps of Life. Proceedings of the International Astronomical Union, 345, 367 (2020) | Link »


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.

Global Protoplanetary Disk Simulations: Dead Zone Formation and FUor Outbursts

K. Kadam, E. Vorobyov, Zs. Regály, Á. Kóspál, P. Ábráham

Origins: From the Protosun to the First Steps of Life. Proceedings of the International Astronomical Union, 345, 324 (2020) | Link »


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.

Envelope-to-disk mass transport in the FUor-type young eruptive star V346 Normae

Á. Kóspál, P. Ábrahám, O. Fehér, F. Cruz-Sáenz de Miera, M. Takami

Origins: From the Protosun to the First Steps of Life. Proceedings of the International Astronomical Union, 345, 320 (2020) | Link »


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.

Spots, flares, accretion, and obscuration in the pre-main sequence binary DQ Tau

Á. Kóspál, P. Ábrahám, G. Zsidi, K. Vida, R. Szabó, A. Moór, A. Pál

Origins: From the Protosun to the First Steps of Life. Proceedings of the International Astronomical Union, 345, 314 (2020) | Link »


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.

Outbursts of young Sun-like stars may change how terrestrial planets form

P. Ábrahám, Á. Kóspál, L. Chen, A. Carmona

Origins: From the Protosun to the First Steps of Life. Proceedings of the International Astronomical Union, 345, 185 (2020) | Link »


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.

A mid-infrared interferometric survey of the planet-forming region around young Sun-like stars

J. Varga, P. Ábrahám, L. Chen, Th. Ratzka, K. É. Gabányi, Á. Kóspál

Origins: From the Protosun to the First Steps of Life. Proceedings of the International Astronomical Union, 345, 128 (2020) | Link »


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:

Episodic accretion in focus: revealing the environment of FU Orionis-type stars

O. Fehér, Á. Kóspál, P. Ábrahám, M. R. Hogerheijde, Ch. Brinch, D. Semenov

Origins: From the Protosun to the First Steps of Life. Proceedings of the International Astronomical Union, 345, 87 (2020) | Link »


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.

Variable Warm Dust around the Herbig Ae Star HD 169142: Birth of a Ring?

L. Chen, A. Moór, A. Kreplin, Á. Kóspál, P. Ábrahám, A. Matter, A. Carmona, K.-H. Hofmann, D. Schertl, G. Weigelt,

The Astrophysical Journal Letters, 887, L32 (2019) | Link »


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.

Spectral Evolution and Radial Dust Transport in the Prototype Young Eruptive System EX Lup

P. Ábrahám, L. Chen, Á. Kóspál, J. Bouwman, A. Carmona, M. Haas, A. Sicilia-Aguilar, C. Sobrino Figaredo, R. van Boekel, J. Varga

The Astrophysical Journal, Volume 887, 156 (2019) | Link »


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.

An ALMA Study of the FU Ori-type Object V900 Mon: Implications for the Progenitor

M. Takami, T.-Sh. Chen, H. B. Liu, N. Hirano, Á. Kóspál, P. Ábrahám, E. I. Vorobyov, F. Cruz-Sáenz de Miera, T. Csengeri, J. D. Green, M. Hogerheijde, T.-H. Hsieh, J. L. Karr, R. Dong, A. Trejo, L. Chen

The Astrophysical Journal, Volume 884, 146, (2019) | Link »


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.

Diagnosing 0.1-10 au Scale Morphology of the FU Ori Disk Using ALMA and VLTI/GRAVITY

H. B. Liu, A. Mérand, J. D. Green, S. Pérez, A. S. Hales, Y.-L. Yang, M. M. Dunham, Y. Hasegawa,Th. Henning, R. Galván-Madrid, Á. Kóspál, M. Takami, E. I. Vorobyov, Zh. Zhu

The Astrophysical Journal, 884, 97 (2019) | Link »


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.

Resolved ALMA Continuum Image of the Circumbinary Ring and Circumstellar Disks in the L1551 IRS 5 System

F. Cruz-Sáenz de Miera, Á. Kóspál, P. Ábrahám, H. B. Liu, M . Takami

The Astrophysical Journal Letters, 882, L4 (2019) | Link »


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.

Dynamical Gaseous Rings in Global Simulations of Protoplanetary Disk Formation

K. Kadam, E. Vorobyov, Zs. Régaly, Á. Kóspál, P. Ábrahám

The Astrophysical Journal, 882, 96 (2019) | Link »


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.

Identification of Young Stellar Object candidates in the Gaia DR2 x AllWISE catalogue with machine learning methods

G. Marton, P. Ábrahám, E. Szegedi-Elek, J. Varga, M. Kun, Á. Kóspál, E. Varga-Verebélyi, S. Hodgkin, L. Szabados, R. Beck, Cs. Kiss

Monthly Notices of the Royal Astronomical Society, 487, 2522 (2019) | Link »


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.

Dust evolution in the circumstellar disc of the unclassified B[e] star HD 50138

J. Varga, T. Gerják, P. Ábrahám, L. Chen, K. Gabányi, Á. Kóspál

Monthly Notices of the Royal Astronomical Society, 485, 3112 (2019) | Link »


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.

Protoplanetary Disk Science Enabled by Extremely Large Telescopes

H. Jang-Condell, S. Brittain, A. Weinberger, M. Liu, J. Faherty, J. Bae, S. Andrews, M. Ansdell, T. Birnstiel, A. Boss, L. Close, Th. Currie, S. J. Desch, S. Dodson-Robinson, Ch. Dong, G. Duchene, Ch. Espaillat, K. Follette, E. Gaidos, P. Gao, N. Haghighipour, H. Hartnett, Y. Hasegawa, M. Kama, J. S. Kim, Á. Kóspál, C. Lisse, W. Lyra, B. Macintosh, D. Mawet, P. McGehee, M. Meyer, E. Peretz, L. Perez, K. Pontoppidan, S. Sallum, C. Salyk, A. Szentgyorgyi, K. Wagner

Astro2020: Decadal Survey on Astronomy and Astrophysics, science white papers, no. 346; Bulletin of the American Astronomical Society, Vol. 51, Issue 3, id. 346 (2019) | Link »


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.

APEX Observations of the CO Envelope around the Young FUor-type Star V883 Ori

J. A. White, Á. Kóspál, Ch. Rab, P. Ábrahám, F. Cruz-Sáenz de Miera, T. Csengeri, O. Fehér, R. Güsten, Th. Henning, E. Vorobyov, M. Audard, A. Postel

The Astrophysical Journal, 877, 21 (2019) | Link »


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.

The Weakening Outburst of the Young Eruptive Star V582 Aur

G. Zsidi, P. Ábrahám, J. A. Acosta-Pulido, J. A.; Á. Kóspál, M. Kun, Zs. M. Szabó, A. Bódi, B. Cseh, N. Castro Segura, O. Hanyecz, B. Ignácz, Cs. Kalup, L. Kriskovics, L. Mészáros, A. Ordasi, A. Pál, K. Sárneczky, B. Seli, Á. Sódor, R. Szakáts

The Astrophysical Journal, Volume 873, 130 (2019) | Link »


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 , and I band multiepoch observations and new near-infrared J, H, and K 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.

Resolving the Radio Complexity of EXor and FUor-type Systems with the ngVLA

J. A. White, M. Audard, P. Ábrahám, L. Cieza, F. Cruz-Sáenz de Miera, M.M. Dunham, J. D. Green, M. Güdel, N. Grosso, A. Hales, L. Hartmann, K. Kadam, J. H. Kastner, Á. Kóspál, S. Perez, A. Postel, D. Ruiz-Rodriguez, Ch. Rab, E. I. Vorobyov, Zh. Zhu

Science with a Next Generation Very Large Array, ASP Conference Series, Vol. 517. ASP Monograph 7. Edited by Eric Murphy., p.177 (2018) | Link »


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.

Chemical Signatures of the FU Ori Outbursts

T. Molyarova, V. Akimkin, D. Semenov, P. Ábrahám, Th. Henning, Á. Kóspál, E. Vorobyov, D. Wiebe

The Astrophysical Journal, 866, 46 (2018) | Link »


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.

Near-infrared High-resolution Imaging Polarimetry of FU Ori-type Objects: Toward a Unified Scheme for Low-mass Protostellar Evolution

M. Takami, G. Fu, H. B. Liu, J. L. Karr, J. Hashimoto, T. Kudo, E. I. Vorobyov, Á. Kóspál, P. Scicluna, R. Dong, M. Tamura, T.-S. Pyo, M. Fukagawa, T. Tsuribe, M. M. Dunham, Th. Henning, J. de Leon

The Astrophysical Journal, 864, 20 (2018) | Link »


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.

VLTI/MIDI atlas of disks around low- and intermediate-mass young stellar objects

J. Varga, P. Ábrahám, L. Chen, Th. Ratzka, K. É. Gabányi, Á. Kóspál, A. Matter, R. van Boekel, Th. Henning, W. Jaffe, A. Juhász, B. Lopez, J. Menu, A. Moór, L. Mosoni, N. Sipos

Astronomy & Astrophysics, 617, A83 (2018) | Link »


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.

Spots, Flares, Accretion, and Obscuration in the Pre-main-sequence Binary DQ Tau

Á. Kóspál, P. Ábrahám, G. Zsidi, K. Vida, R. Szabó, A. Moór, A. Pál

The Astrophysical Journal, 862, 44 (2018) | Link »


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.