Born in Fire: Program

The pre-main-sequence evolution is shaped by the mass accretion process, with eruptive behaviours in nature. The episodic accretion events have great impacts on the evolution of the young system, as most of the stellar mass is thought to be accreted during this stage. It also affects the formation of young planets by heating the protoplanetary disc. In this talk, I will present the latest search results of eruptive behaviours of Class I YSOs from the decade-long photometric surveys (VVV & NEOWISE). In particular, we confirmed 15 new FUor-type outbursts with data from the VVV survey and XSHOOTER spectrograph. We also discovered an eruptive Class II object with delays between the optical and infrared rising stages, suggesting the instability originated at 0.1 AU. I will also present some statistical results on protostellar eruptive behaviours.

Since 2015, over its nine continuous years of operation, the Gaia Photometric Science Alerts programme announced unexpected brightness changes for more than a thousand known, candidate, or suspected young stellar objects (YSOs). Here I report on our ongoing project at Konkoly Observatory, performing a statistical investigation of the eruptive phenomenon on the basis of the unbiased all-sky Gaia survey. We identify candidate eruption events and determine their main parameters (duration, amplitude, accretion rate, total emitted energy, etc.), and study their distribution. We estimate the incidence of different kinds of eruptive events within the YSO population in the vicinity of the Solar System, and provide input for studies of the impact of protostellar outbursts on the circumstellar disks, the birthplace of planets.

Young stellar objects (YSOs) are accreting material from their disc via magnetic field lines. About half of them show daily-weekly photometric variations with an amplitude of a few times 0.1 mag. Some YSOs show brightness variations on longer time-scales: months, years, centuries. Eruptive YSOs show brightness variations with an amplitude of a few magnitudes. These outbursts are caused by a sudden increase of the mass accretion rate by a few orders of magnitude. Eruptive YSOs are commonly divided into two main classes: EX Lupi-type stars (EXors) and FU Orionis-type stars (FUors). The former show brightenings of 2-4 mag, last for less than a year and are recurrent, the latter brighten by up to 5 magnitudes and last for several decades. So far the number of confirmed FUors is limited to no more than a dozen while the number of known EXors is limited to less than 25, including candidates. The Gaia Photometric Science Alerts System, with its large sky coverage and approximately monthly cadence, provides an efficient tool to identify new eruptive YSOs. Based on the Gaia Science alerts, our group recently published the discovery of three FUors (Gaia18dvy, Gaia21elv, and Gaia20bdk), three EXors (Gaia20eae, Gaia19fct, and Gaia23bab), and other types (e.g. Gaia21bty and Gaia18cjb). Our follow-up observations include photometry using 1-m class telescopes, and spectroscopy using the TNG, the NOT, the NTT, the GTC, the LBT, and the VLT. I will present results on follow-up observations of eruptive YSOs found from Gaia alerts.

Accretion outbursts in young stars come in many different forms, most notably long term FUor an short term EXor outbursts. Such outbursts have a profound effect on the dusk structure and composition, which in turn greatly impacts planet formation. It is now thought that most young stars will go though an outburst phase, meaning our understanding of these mechanisms must improve. In this talk, I will present the interferometric view of the inner disk of 4 FUor objects and explore how the outburst has effected the thermal and spatial structure of the disk. This includes evidence of boundary layer accretion and viscous heating in the inner disks of these unusual objects.

FU Ori objects are a class of young stellar objects (YSO) that undergo extreme photometric outbursts with amplitudes of 4-5 magnitudes in the V band. During an outburst, the disk-to-star accretion rate rises by a factor of 100-10,000, crushing the magnetosphere of the star so that the material accretes directly onto the stellar surface. It is believed that FU Ori outbursts represent a stage of pre-main-sequence evolution critical to stellar mass assembly. However, due to the rarity of the outbursts (< 20 have actually been observed) and their decades-long timescales, many of the details of the accretion physics remain unknown. Persistent follow-up of two recent FU Ori objects, HBC 722 and V960 Mon, has enabled us to study the evolution of their disks in the 10 years following their outbursts in detail. We combined many epochs of multi-band photometry, broadband spectrophotometry, and high resolution spectroscopy to constrain the physical parameters of the disks at different times post-outburst. We have demonstrated that the magnetosphere of the star in V960 Mon is re-establishing itself as the accretion rate in the disk falls. In HBC 722, we see the outer boundary of the disk instability expanding in the first 5 years following the peak of its initial outburst. I will discuss these observations in the context of potential models for the FU Ori outbursts. I will also highlight the importance of collecting a broad range of observations at many epochs following an FU Ori outburst to enable similar detailed studies in the future.

The universality of episodic accretion and its potential impact on stellar and planetary formation is still under debate. Improvement in the statistics of the members of the eruptive variable class is needed to better understand the episodic accretion phenomenon. With this goal in mind, we have started a project to collect all of the information available on the spectrophotometric characteristics of confirmed members of this variability class. In this talk, I will discuss the different criteria that were used to include YSOs in this list, as well as provide some simple statistics based of the available data. The list currently has 162 YSOs confirmed to belong to the eruptive variable class. In addition, we include 248 candidate eruptive variable YSOs, which either lack spectroscopic information or the available spectroscopic data is not sufficient for an unambiguous classification. The catalogue of confirmed and candidate eruptive YSOs will be maintained and updated in the future to serve as an important reference for the star formation community.

The traditional picture of star formation suggests that protostars, together with their circumstellar disks, form due to the axisymmetric collapse of dense protostellar cores. However, stars form in turbulent giant molecular clouds, where the initial conditions for star and disk formation cannot be represented as isolated spheres. Numerical simulations of molecular clouds that follow the collapse of many protostellar cores show that the star formation is highly asymmetrical, with material usually falling onto protostellar systems via elongated channels, or streamers. Simulations suggest that such an infall of material can induce instabilities in the disk which can further trigger accretion bursts onto the protostars. Interestingly, streamers are now being detected around systems with known accretion bursts like FU Ori (Hales et al., in press) and M512 (Cacciapuoti et al. 2024). In this talk I will present how we used the first-of-its-kind code TIPSY (Trajectory of Infalling Particles in Streamers around Young stars, Gupta et al., 2024) to ascertain the infalling nature of these observed structures and then to quantify their impact on the protostellar systems.

FU-Orion-type objects are characterized by short (tens of years) episodic outbursts, during which their luminosity increases by orders of magnitude. A possible cause of such flares may be a gravitational perturbation of a circumstellar disks during close encounters in young star-forming regions. Numerical simulations show that to generate a burst with characteristics close to that of FU Orionis, a close flyby with a periastron no larger than several tens of AU is required. However, the separation between the two known components of FU Orionis amounts to several hundreds of AU. Considering that the burst occurs near the periastron, simple mathematical estimates show that the components of FU Orionis must have an extremely high relative velocity to account for such a large separation, which is an order of magnitude higher than the observed velocity dispersion in young star clusters. Using numerical hydrodynamics modeling we showed that flybys with a much larger periastron on the order of 500 AU can lead to luminosity bursts, but these bursts are not triggered by a mere gravitational perturbation of the circumstellar disk by the intruder star. Instead, a gravitational disturbance in the form of a pressure wave from a passing-by star can trigger a cascade process, during which the thermal instability first develops in the inner disk, followed by the magnetorotational instability. As a result, a sharp increase in the mass accretion rate onto the star occurs, which is also manifested in an increase in luminosity by more than 2 orders of magnitude. This cascade mechanism of FU Orionis outburst relaxes strict requirements on the relative velocity of the FU Orionis components and raises the likelihood of such events.

Protoplanetary systems are traditionally believed to acquire their mass and angular momentum during the protostellar phase, strongly constraining disk evolution, planet formation, and the inclination and spin in the system. Recent findings, however, suggest that material from the protostellar environment is added through accretion streamers and late infall. This process not only increases the mass but also introduces significant angular momentum, potentially altering the star-disk systems orientation and allow late disks to maintain their size in a wind-driven scenario, without the need for viscous expansion. Based on ab initio simulations of a star forming region and theoretical derivations, I will argue that the orientation of star-disk systems can change considerably during accretion, in particular for higher mass stars. While the change in orientation varies with stellar mass, it is consistent across different stars relative to their mass accrual. The post-collapse accretion phase tends to be more anisotropic than the initial collapse phase, aligning with the idea that infall occurs via streamers along a preferred direction, unlike the more isotropic early collapse. Environmental factors leads to a large variation in mass accretion history, with some stars gaining most of their mass during initial collapse and others receiving over half of their final mass from late infall. Stars experiencing significant late infall have higher angular momentum budgets, correlating with larger disk sizes. Late accretors, due to their significant infall, can appear younger than they are, being classified as Class 0 objects despite being 1 Myr old. This new understanding, supported by our magnetohydrodynamical models and recent observations of late infall, requires significant revisions in late-time disk and planet formation theories.~

Recent observations of high-mass young stellar objects (HMYSOs) with masses M∗≥10 M⊙ uncovered outbursts with accretion rates exceeding ～10−3 M⊙ yr−1. We utilise 1D time-dependent models of protoplanetary discs around HMYSOs to study burst properties. We find that discs around HMYSOs are much hotter than those around their low-mass cousins. As a result, a much more extended region of the disc is prone to the thermal hydrogen ionisation and magnetorotational (MRI) activation instabilities. The outbursts triggered by these instabilities, however, always have too low accretion rates and are one to several orders of magnitude too long compared to those observed from HMYSOs to date. On the other hand, bursts generated by tidal disruptions of gaseous giant planets formed by the gravitational instability of the protoplanetary discs yield properties commensurate with observations, provided that the clumps are in the post-collapse configuration with planet radius Rp≥10 Jupiter radii.

Our two dimensional radiation hydrodynamic models show the evolution of the vertical thermal and dynamic structure of the inner protoplanetary disk impacted by thermal instability-induced episodic accretion events. We describe the evolution of the thermal instability as a consequence of MRI activation in the dead zone on the basis of S-curves, investigate the disk's dust content in the burst regions and the emergence of pressure maxima, and analyse the occurring small-scale modulation of the accretion rate in addition to the large-scale outbursts. Our models highlight different important aspects of the inner disk evolution during and in between episodic accretion events and can incentivise further theoretical and observational investigations.

Accretion and Outflows are the fundamental processes in the formation of low mass stars that remain poorly understood. Initially, it was understood that the Pre-Main sequence (PMS) stars accrete at steady rate from their circumstellar envelope. However, the works of Kenyon et al. 1990 and Evans et al. 2009 revealed that the predicted luminosity of low mass Class I young stellar objects (YSOs) based on steady accretion rate is an order lower than the observed luminosities. The most promising solution to this apparent paradox is episodic accretion as a widespread phenomenon among YSOs. Indeed, accretion bursts have been known for decades to occur in ‘peculiar’ objects like FU Oris. Systematic optical and infrared surveys are being conducted to test and quantify accretion variations. In the radio part of the spectrum, the jet emanating from YSOs is known to have faint free-free emission. These ‘thermal radio jets’ have also been found to be variable and/or to have episodic ejections. The radio data permits to directly obtain the mass-loss rate from the wind/jet emission, but it does not provide direct information on accretion variations. Therefore to test this in a systematic way, we started in 2012 a radio monitoring of a few nearby star formation regions using JVLA in coordination with the KMOS-VLT to observe the variations of NIR lines and the radio continuum respectively. The goals of our current work is to probe the connection between accretion and ejection in an evolutionary unbiased sample of YSOs, and to test for episodic accretion/ejection and in this platform we wish to show our results.~

Eruptive young stars experience fast increases in brightness in the visual and near-IR due to enhanced accretion from their circumstellar disks. Their status on whether they constitute a sub-class of pre-MS stars or they are a typical step in early-type stellar evolution is under debate. FUori-type eruptive young stars experience a rapid brightening (e.g., ~200x brighter than in quiescence) that can last from a few months to years, caused sudden spikes in mass accretion rates as high as 1e-4 Msun/yr, followed by a very slow dimming. This long-term variability makes them ideal laboratories in studying the evolution of protoplanetary disks within a few years after an eruptive event. V900 Mon is one such star, whose irregular reflection nebula was discovered by an amateur astronomer. It is thought to have erupted in the 1990s and has yet to return to quiescence. Sub-mm observations probed a wide-angled bipolar outflow and a compact disk. We have now harnessed the power of mid-infrared interferometry with MATISSE/VLTI and integral field spectroscopy with MUSE/VLT to study the protoplanetary disk and outflow of V900 Mon. In this talk, we will discuss our recent results (Lykou et al. 2024) and potential implications in disk-wind interaction in FUors based on the jet-like signature that was discovered with MUSE, and the MATISSE data that could indicate sub-micron dust at the bottom of said outflow.~

The JCMT Transient Survey recently discovered that the Class 0 protostar HOPS 358 decreased in 350 GHz continuum brightness by ~25% over the course of four years before brightening again for the next four. Intriguingly, the JCMT lightcurve can be fit by a long period sinusoid with a period roughly eight years and an amplitude of 10% the mean flux. A shorter period overtone is also apparent with a period of 1.75 years and a smaller 3\% amplitude. I will present a study of nine epochs of ALMA observations of HOPS 358 taken over the course of the decline and subsequent rise in brightness seen with the JCMT to test whether the variation seen on ~15", covering both disk and envelope, is also observed on smaller, <1" scales that primarily probe HOPS 358's protostellar disk. With images covering a range of spatial scales, I will discuss the interesting structure we find in the protostellar disk, and further consider how this structure may inform and/or be related to planet formation at early times.

Young stellar objects (YSOs) undergo accretion-driven episodic outbursts. Studying the impact of these outbursts on the environment is fundamental for our understanding of planet formation. The FU Orionis objects (briefly FUors) represent a small but rather pivotal class of YSOs, whose outbursts are primar- ily characterised by a rapid, multi-magnitude increase in brightness at optical and near-infrared wavelengths, attributed to the sudden increase of the accretion flow from the disk onto the protostar, a process lasting for several decades, likely centuries. These highly energetic outbursts may have a long-lasting influence on the chemistry and molecular inventory of disks around eruptive young stars and the elevated temperatures can, temporarily, increase abundances of complex organic molecules (COMs). Improvements in observational techniques and instrumentation have opened a new path for astrochemical research. Despite the growing number of observational studies and model predictions of molecular emission, there is a lack of dedicated line surveys for FUors over wide frequency ranges that cover multiple transitions of many molecules, which is required for deriving their column densities. We carried out the first dedicated (almost) continuous large band with millimeter line survey of a low-mass young eruptive star, the classical FUor V1057 Cyg, known to have the highest peak accretion rate ever observed in a FUor. This source has shown extraordinary multiple flares in the 1720 MHz OH maser transition, and NH3 observation have shown that it is still associated with its natal dense core, thus it is a good candidate to search for other molecular species. We performed a wide spectral line survey of this source, complemented by on-the-fly maps of selected molecules, with the IRAM 30-m and APEX 12-m telescopes from 73 to 263 GHz, of selected frequencies ranged centered around 227, 291, and 344 GHz. In this talk, I will present results of this survey, which provides the first in depth view at the molecular inventory of a young eruptive star’s environment. Due to the wide frequency coverage we detected emission from many molecular species (and isotopologues of several), covering multiple transitions for many. We identify simple molecules of C-, N-, O-, S-bearing species, deuterated species, ionic species, and complex organic molecules. Several molecular species trace large-scale structures in the environment of V1057 Cyg with hints of small-scale fragmentation. The position-velocity diagram shows concentrated peaks even in the single-dish data, indicating past outburst activity. We used population diagrams and a simple radiative transfer analysis to derive column densities and excitation temperatures, and position- velocity diagrams to get insight into the past outburst activity of the source. This study highlights the importance of using astrochemistry to study outbursting sources in different evolutionary stages, and reveals that V1057 Cyg is an excellent target for high angular resolution interferometric observations.

Planets are not only 'born in fire', but they are born in a highly 'explosive' environment. Protostars are highly variable objects, as they commonly undergo flaring events, i.e. 'explosions', that temporarily increase light emission across the electromagnetic spectrum. Flares are thought to dramatically impact the physics and chemistry of surrounding disk, and therefore driving variations in the environment where planets form. However, there is limited observational evidence on this phenomenon. We present results from a multi-wavelength campaign designed to map the physics and chemistry of flaring events in real time in the Orion Nebula Cluster. Chandra was used to identify stars actively undergoing flaring events, which then triggered rapid (days or less) follow up observations by ground based telescopes. We monitored coronal mass ejections using the VLBA, magnetic activity with the Hobby-Ebberly telescope, and gas-phase chemistry and ionization with ALMA. This campaign reveals previously unexplored information on 'explosive' stars, thus providing a more complete picture of the highly dynamic regions where planet formation occurs.

Disks around young stars are the birthplaces of planets, with their chemical compositions playing a crucial role in the materials available for planet formation. However, these disks are not static environments. The increased temperatures during accretion (out)bursts significantly perturb the chemistry of the disk. In particular, molecular species in the midplane region of a quiescent disk are subjected to temperatures so low that they are frozen icy mantles encasing dust grains. Once the disk temperature rises, these molecules quickly sublimate, hereby modifying the spatial distribution and chemical composition of the gas available for dust growth and planet formation. Here I will present our studies of the effects of (out)bursts in the chemistry of young protostellar disks. I will show the serendipitous ALMA discovery of hot and compact emission of multiple complex organic molecules in four protostars with evidence of FUor-type accretion outbursts. I will put these results into context by comparing our four targets, plus others outbursting young stars from the literature, with young stars considered to be in quiescence. Lastly, I will mention some ideas when designing future astrochemical studies of (out)bursting young stars.

Many bright disks show signs of ongoing planet formation such as deep cavities and rings. These may be carved by massive planets but how the planets form remains one of the biggest questions to date. On top of that, their chemical composition depends on their formation location with respect to the snowlines, the midplane radius where a major volatile freezes out. One possible solution to solve the planet formation puzzle is that planets form around the water snowline. We will resolve and locate the 2D water snow surface for the first time in a disk around a young outbursting star using HDO, an isotopologue of water observed with ALMA. In addition, we will use observations of another chemically rich disk to study the effect of the water snowline on the chemical composition of the planet forming material through freshly sublimated ices.

The Very Low Luminosity Object (VeLLO) in the DC3272+18 cloud has undergone an outburst in the past ~10^4 yr. This is evident from the current position of its CO snowline, which is ~3 times further out than the present luminosity of the central protostar would allow for [1]. It is currently in the quiescent phase of the episodic accretion process, and has an internal luminosity of only 0.04 Lsun [1,2], but due to its past outburst (Lburst = 1-4 Lsun, [1]) a plethora of molecules has been detected in the gas phase with the Atacama Pathfinder Experiment [3]. In my talk I will discuss how we can utilize the line fluxes, column densities and abundance ratios of the detected molecules to characterize the different physical components of the VeLLO, a in general understudied group, and how the past outburst has influenced its chemical inventory. Detection highlights include molecules that have a) sublimated due to the increased temperature in the envelope during the heating event such as methanol (CH3OH), formaldehyde (H2CO), and sulfur oxide (SO), and b) freshly formed gas phase species that have formed from processed and sublimated ices such as nitric oxide (NO). I will argue how the detection of NO, which has been detected for the first time in a source of this type, can be directly linked to its formation at the position that the water snowline got shifted to during the burst, which suggests that NO could potentially be used to trace the water snowline in outbursting sources. [1] Hsieh, T.-H., Murillo, N. M., Belloche, A. et al., 2018, ApJ, 854, 15 [2] Kim G., Lee, C. W., Maheswar, G. et al. 2019, ApJS, 240, 18 [3] Kulterer, B. M., Wampfler, S.-F., Ligterink, N. F. W. et al., 2024, „Post-Outburst Chemistry in a Very Low Luminosity Object: Does Nitric Oxide Trace the Water Snowline”, submitted to A&A

Protostellar discs are the link between stars and planets: they form with the star, and they are the environments in which planet formation takes place. Thanks to ALMA radio telescope, we are collecting plenty of data about these systems, showing a high degree of complexity in their structure, morphology and kinematics. Many discs exhibit substructures that are consistent with - and often interpreted as - the theoretically expected signature of planet-disc interaction. Under the hypothesis of the planetary interpretation, a robust conclusion is that a substantial part of the planet formation process must overlap with the time when protostellar discs are young, likely to be self-gravitating and, possibly, gravitationally unstable. Hence, a natural question to ask is: what is the role of the disc self-gravity in the context of plant formation? In this talk, I investigate both gas kinematics and dust dynamics of young protostellar discs. As for the gas kinematics, gravitational instability leaves clear observational signatures, known as “GI wiggles”. Through their characterisation, it is possible to investigate protoplanetary disc mass and cooling. These information shed light on fundamental questions like the amount of mass available for planet formation, and the transport of angular momentum. As for the dust dynamics, it is well known that the spiral perturbations induced by gravitational instability can trap dust very efficiently, possibly making the dust unstable itself. This mechanism could, in principle, solve the conundrum of planetesimal formation, forming planetary cores during the early stages of protostellar disc life. In this context, I present a new analytical framework to characterise the interplay between gravitational instability and the aerodynamical coupling between gas and dust, fundamental to understand planet formation. To conclude, I test this model through numerical simulations, showing that dust collapse inside gas spiral arms is a viable way to form planetary cores in young protostellar systems.

The formation of giant planets has traditionally been divided into two pathways: core accretion and gravitational instability. However, in recent years, gravitational instability has become less favored, primarily due to the scarcity of observations of fragmented protoplanetary disks around young stars and low occurrence rate of massive planets on very wide orbits. In this study, we discuss several observations of the young outbursting object V960 Mon. The images reveal a vast structure of intricately shaped spiral arms and large-scale interaction with its surroundings. ALMA 1.3 mm data acquired just two years after the onset of the outburst of V960 Mon detects several clumps of continuum emission aligned along a spiral arm that coincides with the scattered light structure. We interpret the localized emission as fragments formed from a spiral arm under gravitational collapse. Estimating the mass of solids within these clumps to be of several Earth masses, we suggest this observation to be the first evidence of gravitational instability occurring on planetary scales. In this talk I will discuss the significance of this finding for planet formation and its potential connection with the outbursting state of V960 Mon.

Although gravitational instability has been long-considered as a promising pathway to planet formation, observational identification of gravitational instability is still very limited. In this talk, I present a clear and convincing case of gravitational instability in the planet-forming disk around AB Aur, a nearby Class II YSO. High quality ALMA observations of CO isotopologue emission reveal global spiral arms in the gas density and kinematics at scales of 100-1000 au. The data show detailed agreement with numerical and analytical models. In particular, we detect the telltale kinematic signature of self-gravitating spiral arms —the "GI wiggle"— in 13CO and C18O simultaneously. Through quantitative comparisons to models, we are able to constrain the AB Aur disk mass. With multiple protoplanet candidates identified amongst the spiral arms in previous observations, the AB Aur system represents a direct observational connection between gravitational instability and planet formation.

To fully understand how planets are formed, we need to explain the presence of water and carbon-deficient planets like Earth and Mars, as well as icy giants like Neptune and Uranus. This requires understanding how water-ice-coated and water-ice-free dust grains grow. The observational constraints on the latter have been sparse due to the small scales (sub-au) involved. Fortunately, in the FU Orionis type accretion outburst YSOs, the water snowlines can be enlarged to comparably larger than 10 au radii thanks to the viscous heating, which opens a new window for studying dry dust coagulation. Our case study on FU Ori suggests that water-ice-free dust may be stickier than previously thought, with a lower limit of fragmentation velocity estimated at 10 m/s, which is higher than previously assumed by one order of magnitude. The sticky dry dust may be prone to form planetesimal and rocky planets in situ, which could help explain the origins of the water and carbon-deficient planets and the asteroid belt in the inner Solar System.

The water snowline, i.e., the boundary inside which the temperature allows for water ice to sublimate in the gas phase, is particularly important for dust growth, planetesimal formation, and planetary composition. However, its location in the very inner part of discs makes it difficult to resolve spatially with current observatories. Discs around stars undergoing FUor-type accretion outbursts represent a unique opportunity to study the interplay of dust and water ice, as such events push the water snowline outwards, in regions that can be resolved. In this talk, I will detail how an FUor-type outburst can affect the growth and properties of dust particles, the building blocks of planets, using results from dust coagulation simulations. I then apply these findings to an outbursting source, demonstrating how we can use these objects as laboratories to probe the structure and cohesion of icy aggregates. Based on new analysis of archival ALMA data of V883 Ori, coupled with predictions from dust evolution models, I will discuss how icy aggregates respond to the sublimation of their water ice mantles, and what that means in the grand scheme of planet formation.

Some active young stars are known to host intriguing accretion outbursts, which is characterized by a sudden 1-6 mag optical brightening. While it is generally accepted that episodic accretion is associated with the infalling material in the inner disk, the origin of this physical mechanism is still highly debated. We present ALMA Band 3 and 4 (3.2 mm / 2.0 mm) high-resolution (< 0.1 arcsec or < 16 au) observations of EX Lup, the archetype of a well-known subclass of young eruptive stars (Yamaguchi, Liu, and Takami, in prep). For the first time, the dust continuum emission has revealed the presence of a lopsided inner ring located southeast of the star and annular outer ring-gap structures. The lopsided inner ring casts a shadow resolved with the VLT/SPHERE near-infrared imaging polarimetry. The asymmetry of the inner ring could result from a vortex instability induced by a nearby massive planet, while the gap in the outer ring could be a footprint of planetary migration toward the host star. These observations suggest that an unseen massive planet near the star regulates mass accretion by accumulating material in the inner ring, possibly triggering the episodic accretion outburst.

Stellar binaries can exhibit episodic FUor-like outbursts, which lead to abrupt luminosity increases of 10 to 1000 times in a few years or decades. So far, most computational models have considered highly symmetric radiation and temperature fields to determine the dynamical behaviour of discs surrounding (or next to) outbursting stars. This assumption is not well-suited for discs in multiple stellar systems. In this study, we present the first results obtained using the hybrid code Phantom-mcfost, which efficiently combines hydrodynamical SPH simulations and Monte Carlo radiative transfer computations in 3D. We aim to study the impact of asymmetric and evolving radiation fields on protoplanetary discs in binary stellar system. Here, we consider two main configurations: an equal-mass binary system (composed of two twin Solar-like stars) and an unequal-mass binary, where one of the stars experiences a FUor-like outburst. We focus on the disc around the primary quiescent star. Our results show highly asymmetrical (radial, vertical & azimuthal) disc profiles – with a significant heating from the secondary outbursting star affecting the outermost disc regions. This mechanism has important implications for disc dynamics and dust composition, as the ice surface within the disc varies with the binary phase. This periodic modulation is particularly relevant during outburst events.

We investigate the occurrence of accretion bursts, dust accumulation, and the prospects for planetesimal formation in a gravitationally unstable (GI) magnetized protoplanetary disk with triggered magnetorotational instability (MRI), in Herbig Ae (HAe) stars. We use numerical magnetohydrodynamics simulations in the thin-disk limit (FEOSAD code) to model the formation and long-term evolution of a gravitationally unstable magnetized protoplanetary disk, including dust dynamics and growth. Massive dust rings that are susceptible to streaming instability (SI) form within the inner disk region owing to the radially varying strength of GI. Gradual warming of the dust rings, thanks to high opacity, increases the gas temperature above a threshold for the MRI to develop via thermal ionization of alkaline metals. The characteristics of ensuing bursts are analyzed and compared to their cousins in low-mass stars. Additionally, we analyze the effects of dust sublimation within these dusty rings forming near the central HAe protostar. Our work shows that GI-induced dusty rings within the inner disk around HAe stars, while initially susceptible to streaming instability, tend to shrink or be destroyed over time due to periodic accretion bursts and dust sublimation. Our results also suggest that the potential for disks around HAe stars to be dust-depleted significantly reduces the chances of planetesimal formation in pre-main sequence Herbig Ae stars.

During an astonishing event of a young star brightening over a month’s timescale, we discovered a reconfiguration of the accretion funnel by tracing the azimuthal and latitudinal migration of an accretion-induced hotspot over the stellar surface. EX Lupi, a historically renowned outbursting YSO (Wang et al. 2023), underwent an accretion-driven brightening by ∆g = 2 mag during the March of 2022. With an unprecedented cadence of spectroscopic and photometric observations with SALT, CTIO, TMMT, and LCRO, along with publicly available data from ASAS-SN, TESS, AAVSO, and literature, we could trace the accretion-induced hotspot’s location over the stellar surface for over 1.5 decades. We used the phases of periodically varying light curves and radial velocities (RVs) to trace the location of the hotspot. At the onset of the outburst, the hotspot migrated azimuthally over the stellar surface by 112◦± 5◦, along the rotation direction. Such an accretion rate-dependent hotspot migration was predicted by Kulkarni and Romanova (2013) by 3D MHD simulations. We explain this migration as a result of an increase in relative angular speed between the inner disk and the central star, due to the movement of the inner edge of the disk closer to the star during the outburst. Our 3D MHD simulations were able to reproduce the observations, and they provide insights into the magnetospheric accretion dynamics in EX Lupi. Post-outburst, the hotspot unexpectedly managed to stay migrated, we hypothesize a warmed and thicker inner disk along with occasional episodes of small brightening could have supported it at the current location. The increased RV amplitudes of various metallic lines indicated that the hotspot also moved down in longitude by ∼ 10◦. Manuscript: Singh et. al. (2024).

Most of what we know about the accretion process is focused on the accretion parameters we compute for pre-main sequence stars by assuming the magnetospheric accretion model. However, recent results to constrains accretion rates on Class I and 0 and the discovery of streamers, suggest a more complicated picture. In this talk I will review the state of the art about the accretion evolution and how eruptive accretion impact the stellar and planet formation.

Young Stellar Objects (YSOs) are observed to undergo powerful accretion events known as FUors. Until recently, the protoplanetary disk that surrounds the central star in these objects was thought to evolve due to the action of turbulent viscosity, caused by magnetorotaional instability (MRI). However, recent evidence suggests that these disks may evolve primarily due to magnetohydrodynamic (MHD) disk winds, which carry both mass and momentum away from the disk. These winds are intimately related to the phenomenon of episodic accretion, as the outflow during FUor outbursts is known to increase by up to an order of magnitude. We investigate the episodic accretion in YSOs with the help of hydrodynamic simulations, wherein the disk evolves with concurrent action of viscosity, self-gravity and magnetic disk winds. The simulations start with the core collapse phase of the parent molecular cloud and the long-term evolution of the disk is considered over a timescale of Myr. The disk exhibits episodic accretion; the duration and frequency of the outbursts, their rise times, and brightness amplitudes resemble the expectations from the observations of FUors. In this presentation, I focus on the dust content of the outflows during FUor events. The simulations suggest that the winds are over an order of magnitude more dusty, as compared to in quiescence, with most of the ejected dust originating from the innermost few au of the disk. Our results are in general agreement with observational findings and can explain the mechanism behind dusty winds during outbursting events.

Young stellar objects are thought to commonly undergo sudden accretion events that result in a sudden rise in bolometric luminosity. This outburst likely corresponds with the onset of planet formation, and could impact the formation of planets. The reason behind this dramatic enhancement of accretion is an active area of research, and the mass of the system is a critical clue. Using Northern Extended Millimeter Array, we survey five FU Ori-like objects with the primary goal of determining the system's mass using an optically thin line of CO. We estimate the mass of a central region for each object using both continuum and C17O emission. The C17O emission likely includes both disk and inner envelope material, with masses ranging from 0.33-3.1 M$_{\odot}$. In addition to the mass determinations, we identify bright lines captured over NOEMA's wide bandwidth. In all sources we see multiple lines of SO and H2CO in addition to the common CO isotopologues. One source, V1057 Cyg shows 48 individual transitions among 12 different molecular species. The disks and central regions around outbursting young stellar objects appear to be massive objects, often containing envelope emission putting these sources on the edge of Class I/II sources.

Strong accretion outbursts on to protostars are associated with emission dominated by a viscously heated disc, which is characterized by high luminosities. We report the discovery and characterization of a strong mid-IR (3.4, 4.6 μm) outburst in the embedded protostar SSTgbs J21470601+4739394 (hereafter SSTgbsJ214706). SSTgbsJ214706 has steadily brightened in the mid-infrared by ∼2 mag over the past decade, as observed by NEOWISE. Follow-up investigations with the Gemini near-IR spectrograph reveal that SSTgbsJ214706 is a binary system with a spatially extended outflow. The outburst is occurring on the more embedded south-east (SE) component, which dominates the mid- and far-infrared emission from the source. The outbursting component exhibits a spectrum consistent with an FU Ori-type outburst, including the presence of enhanced absorption observed in the molecular bands of CO. The luminosity of the SE component is estimated to be 0.23 L⊙ before the outburst and 0.95 L⊙ during the outburst, which is one to two orders of magnitude fainter than bonafide FU Ori outbursts. We interpret this eruption as an FU Ori-type outburst, although the possibility of brightening following an extinction episode cannot be ruled out. We discuss the implications and potential explanations for such a low-luminosity eruption.

Near-infrared (~2 μm) imaging observations at 8-m telescopes have recently revealed complicated spatial distributions of circimstellar dust toward FU Ori-type objects (FUors). A promising explanation is that these are associated with gravitationally unstable disks and clump ejections. The METIS coronagraphs on the E-ELT 39-m telescope would allow us to further investigate their nature. The imaging capability of METIS at longer wavelengths (3-10 μm) would enable us to observe deeper into the dusty environments at angular resolution improved by up to a factor of ~2 (~0".02). Following our previous conference presentation in 2022*, we present extended simulations for such observations, creating mock images of the FUor disks based on existing 1.65-μm images and using the HEEPS simulator. (* ...("Planet and binary formation in gravitationally unstable protoplanetary discs in the high-resolution era", Leicester, UK)