arXiv subjects

Vardan Elbakyan

Publications and source records attributed to Vardan Elbakyan.

At least 19 recordsLinked to original sources

arXiv2026

Hot water emission during an outburst in a classical T Tauri star

In this paper, we present observations of an eruptive young star in the Rosette Nebula, identified by the Gaia Science Alerts system using Gaia time series data. We aim to investigate the evolution of the brightness and mass accretion rate of V557 Mon throughout its outburst and subsequent decline. In addition, we trace the evolution of the inner accretion disk during the outburst by monitoring molecular emission features. We compiled multi-band photometric time series from Gaia, ZTF, and several 1 m-class ground-based telescopes and obtained optical and near-infrared spectra at multiple epochs covering the outburst and fading phases. Stellar parameters were derived from quiescent colour/spectra and spectral energy distribution (SED) fitting. We also measured the mass accretion rate and fit models to molecular emission bands. Since late 2024, V557 Mon has undergone a year-long outburst consistent with EXor variability. Based on quiescent photometry, V557 Mon has a spectral type of M1 with an extinction of AV = 1.8+_0.3 mag, consistent with a 0.4-0.5 M_sol star at an age of 2 Myr. Our multi-epoch spectra and u-band photometry indicate a peak accretion rate of 6.3x10^(-7)M_sol/yr during the outburst, roughly 70 times higher than in quiescence. We report the detection of hot water vapour emission bands, together with TiO, VO, and CO emission features. Using ExoMol models, we measured the inner-disk temperature changed from 3000 K to 2000 K during the fading phase of the outburst. We report a recent EXor outburst in a low-mass Class II YSO. Our observations reveal the transient formation of a hot molecular inner disk, traced by variable water vapour emission during the EXor event. A positive correlation is found between the molecular excitation temperature and the overall stellar brightness.

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arXiv2026

From Fragments to Flares: Migration, Tidal Disruption, and Observable Bursts in Massive Protostellar Disks

We investigate how resolving the inner few astronomical units of a massive protostellar disk affects the migration, disruption, and accretion signatures of an inward-moving fragment. In particular, we aim to determine whether the predicted burst strength and duration depend on the adopted sink cell size. We present a new three-dimensional radiation-hydrodynamic simulation of a $\sim$5$M_{\odot}$ protostar surrounded by a self-gravitating disk, comparing the original 30 AU sink model to a refined model with a 1 AU sink that resolves the inner disk. The resulting gas structures are post-processed with radiative transfer calculations to derive synthetic photometry and multi-band images. Both simulations produce a major accretion burst as a migrating fragment is tidally disrupted, but their detailed behavior differs markedly. The refined model shows faster migration, a complete tidal disruption of the fragment, and a shorter, sharper outburst (more consistent with observations) with nearly the same peak accretion rate as the 30 AU model, which yields a broader, smoother event. The refined run produces much stronger near- and mid-infrared emission, reflecting the formation of a compact, hot inner disk. Resolving the inner few AU qualitatively changes the dynamics and observable appearance of fragment-driven bursts. Diffuse fragment disruption can reproduce decade-long events, but the much shorter ($<$3 yr) bursts observed in some massive protostars likely require the tidal disruption of more compact objects such as second Larson cores. Our trajectory analysis indicates that second Larson cores can migrate sufficiently close to the star to be tidally destroyed, offering a plausible mechanism for the fastest FU-Ori-like bursts observed in massive protostars.

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arXiv2026

Gaia24ccy: An outburst followed the footsteps of its predecessor

Accretion-driven outbursts in young stellar objects remain poorly understood, largely limited by a statistically small sample of closely followed-up events. This underscores the importance of a thorough exploration of each outbursting object. We studied a peculiar outbursting system, Gaia24ccy, which exhibited two $Δg \sim$ 3.8 mag outbursts in 2019 and 2024. The system consists of two unresolved, nearly identical, and rapidly rotating young stars: Gaia24ccy A (1.1419 days) and Gaia24ccy B (1.7898 days). Periodogram analyses just before the onset of the outbursts suggest Gaia24ccy B to be the outbursting component. Unlike any previously known EXor sources, the two outburst profiles show a very similar evolution: both rose at the same rate for the first 15 days, followed by many 'sub-bursts' on the timescale of 10-20 days. The 2019 outburst lasted 145-255 days, while the 2024 outburst persisted for 367 days. We infer the unstable region to lie at $r_{\rm trigger} \simeq 0.019-0.047$ au ($\sim5-12.3 R_\star$). The accreted mass per event $M_{\rm acc}\sim10^{-5} M_\odot$ can be provided by a compact inner-disk reservoir. The photometric rise/decay timescales and the mid-infrared color evolution favor a thermal-viscous trigger in a hot inner disk, while the appearance of rich emission-line spectra indicates concurrent magnetospheric compression - together best described by a hybrid picture. Finally, we explain the reddening of the mid-infrared color observed during the outburst as a consequence of the competing emission from the viscous disk and the photosphere.

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arXiv2026

Observational Signatures of Planetary Tidal Disruption Events Around Solar-Mass Stars

The tidal disruption of planets by their host stars represents a growing area of interest in transient astronomy, offering insights into the final stages of planetary system evolution. We model the hydrodynamic evolution and predict the multi-wavelength observational signatures of planetary TDEs around a solar-mass host, focusing on Jupiter-like and Neptune-like progenitors and examining how different eccentricities of the planet's pre-disruption orbit shape the morphology and emission of the tidal debris. We perform 2D hydrodynamic simulations using the FARGO3D code to model the formation and viscous evolution of the resulting debris disk. We employ a viscous alpha-disk prescription and include a time-dependent energy equation to compute the disk's effective temperature and subsequently derive the bolometric and multi-band photometric light curves. Our simulations show that planetary TDEs produce a diverse range of luminous transients. A Jupiter-like planet disrupted from a circular orbit at the Roche limit generates a transient peaking at $L_{bol} \approx 10^{38}$ erg s$^{-1}$ after a 12-day rise. In contrast, the same planet on an eccentric orbit (e=0.5) produces a transient of comparable peak luminosity but on a much shorter timescale, peaking in only 1 day and followed by a highly volatile light curve. We find that the effect of eccentricity is not universal, as it accelerates the event for Jupiter but delays it for Neptune. A robust "bluer-when-brighter" colour evolution is a common feature as the disk cools over its multi-year lifetime. The strong dependence of light curve morphology on the initial orbit and progenitor mass makes these events powerful diagnostics. This framework is crucial for identifying planetary TDEs in time-domain surveys.

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arXiv2026

On the accuracy of mass and size measurements of young protoplanetary disks

Knowing the masses and sizes of protoplanetary disks is of fundamental importance for the contemporary theories of planet formation. However, their measurements are associated with large uncertainties. In this proof of concept study, we focus on the very early stages of disk evolution, concurrent with the formation of the protostellar seed, because it is then that the initial conditions for subsequent planet formation are likely established. Using three-dimensional hydrodynamic simulations of a protoplanetary disk followed by radiation transfer postprocessing, we constructed synthetic disk images at millimeter wavelengths. We then calculated the synthetic disk radii and masses using an algorithm that is often applied to observations of protoplanetary disks with ALMA, and compared the resulting values with the actual disk mass and size derived directly from hydrodynamic modeling. We paid specific attention to the effects of dust growth on the discrepancy between synthetic and intrinsic disk masses and radii. We find that the dust mass is likely underestimated in Band 6 by factors of 1.4-4.2 when Ossenkopf & Henning opacities and typical dust temperatures are used, but the discrepancy reduces in Band~3, where the dust mass can be even overestimated. Dust growth affects both disk mass and size estimates via the dust-size-dependent opacity, and extremely low values of dust temperature (~ several Kelvin) are required to recover the intrinsic dust mass when dust has grown to mm-sized grains and its opacity has increased. Dust mass estimates are weakly sensitive to the distance to the source, while disk radii may be seriously affected. We conclude that the accuracy of measuring the dust mass and disk radius during the formation of a protoplanetary disk also depends on the progress in dust growth. (Abridged)

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arXiv2025

VVV-WIT-13: an eruptive young star with cool molecular features

Here we investigate an infrared eruptive source, identified from the decade-long VISTA Variables in the Via Lactea survey (VVV). We named this target after a group of variable sources discovered by VVV, as VVV-WIT-13, with WIT standing for "What Is This?", due to its unique photometric variation behaviour and the mysterious origin of the outburst. This target exhibited an outburst with a 5.7 mag amplitude in the Ks-band, remained on its brightness plateau for 3.5 years, and then rapidly faded to its pre-eruptive brightness afterwards. We aim to reveal the variable nature and outburst origin of VVV-WIT-13 by presenting our follow-up photometric and spectroscopic observations along with theoretical models. We gathered photometric time series in both near- and mid-infrared wavelengths. We obtained near-infrared spectra during the outburst and decaying stages on XSHOOTER/VLT and FIRE/Magellan, and then fitted the detected molecular absorption features using models from ExoMol. We applied 2D numerical simulations to re-create the observables of the eruptive phenomenon. We observe deep AlO absorption bands in the infrared spectra of VVV-WIT-13, during the outburst stage, along with other more common absorption bands (e.g. CO). Our best-fit model suggests a 600 K temperature of the AlO absorption band. In the decaying stage, the AlO bands disappeared, whilst broad blue-shifted H2 lines arose, a common indicator of stellar wind and outflow. The observational evidence suggests that the CO and TiO features originate from an outflow or a wind environment. We find that VVV-WIT-13 is an eruptive young star with instability occurring in the accretion disk. One favoured theoretical explanation of this event is a disrupted gas clump at a distance of 3 au from the source. If confirmed, this would be the first such event observed in real time.

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arXiv2025

Episodic accretion in high-mass star formation: An analysis of thermal instability for axially symmetric disks

High-mass young stellar objects exhibit episodic accretion bursts similar to their low-mass counterparts. Understanding these outbursts is crucial for elucidating massive star formation and disk evolution around high-mass protostars. We investigate thermal instability's role in triggering accretion outbursts using a two-dimensional hydrodynamical model that fully resolves the vertical structure of the inner disk. This approach provides a more realistic depiction of axially symmetric disk dynamics and assesses observable burst signatures. We simulate the inner 10 astronomical units of a circumstellar disk around a high-mass protostar, incorporating viscous heating and radiative transport in radial and vertical directions. Unlike previous one-dimensional studies, our two-dimensional model resolves time-dependent vertical disk structure, capturing complex radial-vertical dynamics. Our simulations show thermal instability causes significant structural changes. Steep temperature gradients and vigorous convection develop at outburst onset, with gas flows differing between midplane and upper layers. Energy release produces 15-30 year outbursts with peak accretion rates of $2-3\times10^{-4}~\rm M_{\odot}~\rm{yr}^{-1}$. While observable, these bursts are insufficiently bright with rise times differing from rapid observed events. Our models lack the "reflares" seen in one-dimensional calculations. Resolving full vertical disk structure is essential for accurate thermal instability modeling. While thermal instability significantly influences episodic accretion, it appears insufficient alone to explain observed HMYSO outburst diversity. Additional mechanisms are required for comprehensive understanding.

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arXiv2024

Dust growth and pebble formation in the initial stages of protoplanetary disk evolution

Aims. The initial stages of planet formation may start concurrently with the formation of a gas-dust protoplanetary disk. This makes the study of the earliest stages of protoplanetary disk formation crucially important. Here we focus on dust growth and pebble formation in a protoplanetary disk that is still accreting from a parental cloud core. Methods. We have developed an original three-dimensional numerical hydrodynamics code, which computes the collapse of rotating clouds and disk formation on nested meshes using a novel hybrid Coarray Fortran-OpenMP approach for distributed and shared memory parallelization. Dust dynamics and growth are also included in the simulations. Results. We found that the dust growth from $\sim 1~μ$m to 1-10~mm already occurs in the initial few thousand years of disk evolution but the Stokes number hardly exceeds 0.1 because of higher disk densities and temperatures compared to the minimum mass Solar nebular. The ratio of the dust-to-gas vertical scale heights remains rather modest, 0.2--0.5, which may be explained by the perturbing action of spiral arms that develop in the disk soon after its formation. The dust-to-gas mass ratio in the disk midplane is highly nonhomogeneous throughout the disk extent and is in general enhanced by a factor of several compared to the fiducial 1:100 value. Low St hinders strong dust accumulation in the spiral arms compared to the rest of the disk and the nonsteady nature of the spirals is also an obstacle. The spatial distribution of pebbles in the disk midplane exhibits a highly nonhomogeneous and patchy character. The total mass of pebbles in the disk increases with time and reaches a few tens of Earth masses after a few tens of thousand years of disk evolution. Abridged.

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arXiv2023

On the origin of accretion bursts in FUORs

Accretion luminosity of young star FU Ori increased from undetectable levels to hundreds of Solar luminosities in 1937 and remains nearly as high at the present time. In a recent paper we showed how Extreme Evaporation (EE) of a young gas giant planet that migrated to a 10 day orbit around the star may power FU Ori. However, our model assumed a power-law mass-radius relation for the evaporating planet. Here we employ a stellar evolution code to model mass losing planets. We find that adiabatic planets expand rapidly, which results in runaway FUOR bursts. Super-adiabatic planets contract while losing mass; their outbursts are dimming with time. Long steadily declining bursts such as FU Ori require relatively fine tuned internal planetary structure, which may be rare. More commonly we find that super-adiabatic planets contract too rapidly and their EE falters, leading to FUOR burst stutter. This stutter allows a single planet to produce many short repeating bursts, which may be relevant to bursts observed in V346 Nor, V899, V1647. We compute broad band spectra of our best fitting scenario for FU Ori. Since the outburst is triggered behind the planet location, the mid-IR emission rises many months before the optical, similar to bursts in Gaia-17bpi and Gaia-18dvy. We show that in outbursts powered by the classic thermal instability, mid-IR lags the optical, whereas the dead zone activation models predict mid-IR light precede the optical burst by many years to decades. We comment on the stellar flyby scenario for FU Ori.

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arXiv2023

Distinguishing Magnetized Disc Winds from Turbulent Viscosity through Substructure Morphology in Planet-forming Discs

The traditional paradigm of viscosity-dominated evolution of protoplanetary discs has been recently challenged by magnetized disc winds. However, distinguishing wind-driven and turbulence-driven accretion through observations has been difficult. In this study, we present a novel approach to identifying their separate contribution to angular momentum transport by studying the gap and ring morphology of planet-forming discs in the ALMA continuum. We model the gap-opening process of planets in discs with both viscous evolution and wind-driven accretion by 2D multi-fluid hydrodynamical simulations. Our results show that gap-opening planets in wind-driven accreting discs generate characteristic substructures that differ from those in purely viscous discs. Specifically, we demonstrate that discs, where wind-driven accretion dominates the production of substructures, exhibit significant asymmetries. Based on the diverse outputs of mock images in the ALMA continuum, we roughly divide the planet-induced features into four regimes (moderate-viscosity dominated, moderate-wind dominated, strong-wind dominated, inviscid). The classification of these regimes sets up a potential method to constrain the strength of magnetized disc wind and viscosity based on the observed gap and ring morphology. We discuss the asymmetry feature in our mock images and its potential manifestation in ALMA observations.

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arXiv2023

Extreme evaporation of planets in hot thermally unstable protoplanetary discs: the case of FU Ori

Disc accretion rate onto low mass protostar FU Ori suddenly increased hundreds of times 85 years ago and remains elevated to this day. We show that the sum of historic and recent observations challenges existing FU Ori models. We build a theory of a new process, Extreme Evaporation (EE) of young gas giant planets in discs with midplane temperatures exceeding 30, 000 K. Such temperatures are reached in the inner 0.1 AU during thermal instability bursts. In our 1D time-dependent code the disc and an embedded planet interact through gravity, heat, and mass exchange. We use disc viscosity constrained by simulations and observations of dwarf novae instabilities, and we constrain planet properties with a stellar evolution code. We show that dusty gas giants born in the outer self-gravitating disc reach the innermost disc in a $\sim$ 10,000 years with radius of $\sim 10 R_J$. We show that their EE rates are $\sim 10^{-5}$ Msun/yr; if this exceeds the background disc accretion activity then the system enters a planet-sourced mode. Like a stellar secondary in mass-transferring binaries, the planet becomes the dominant source of matter for the star, albeit for $\sim$ O(100) years. We find that a $\sim$ 6 Jupiter mass planet evaporating in a disc fed at a time-averaged rate of $\sim 10^{-6}$ Msun/yr appears to explain all that we currently know about FU Ori accretion outburst. More massive planets and/or planets in older less massive discs do not experience EE process. Future FUOR modelling may constrain planet internal structure and evolution of the earliest discs.

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arXiv2023

Computing the gravitational potential on nested meshes using the convolution method

Aims. Our aim is to derive a fast and accurate method for computing the gravitational potential of astrophysical objects with high contrasts in density, for which nested or adaptive meshes are required. Methods. We present an extension of the convolution method for computing the gravitational potential to the nested Cartesian grids. The method makes use of the convolution theorem to compute the gravitational potential using its integral form. Results. A comparison of our method with the iterative outside-in conjugate gradient and generalized minimal residual methods for solving the Poisson equation using nonspherically symmetric density configurations has shown a comparable performance in terms of the errors relative to the analytic solutions. However, the convolution method is characterized by several advantages and outperforms the considered iterative methods by factors 10--200 in terms of the runtime, especially when graphics processor units are utilized. The convolution method also shows an overall second-order convergence, except for the errors at the grid interfaces where the convergence is linear. Conclusions. High computational speed and ease in implementation can make the convolution method a preferred choice when using a large number of nested grids. The convolution method, however, becomes more computationally costly if the dipole moments of tightly spaced gravitating objects are to be considered at coarser grids.

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arXiv2022

Gap opening by planets in discs with magnetised winds

Planets open deep gaps in protoplanetary discs when their mass exceeds a gap opening mass, $M_{\rm gap}$. We use one- and two-dimensional simulations to study planet gap opening in discs with angular momentum transport powered by MHD disc winds. We parameterise the efficiency of the MHD disc wind angular momentum transport through a dimensionless parameter $α_{\rm dw}$, which is an analogue to the turbulent viscosity $α_{\rm v}$. We find that magnetised winds are much less efficient in counteracting planet tidal torques than turbulence is. For discs with astrophysically realistic values of $α_{\rm dw}$, $M_{\rm gap}$ is always determined by the residual disc turbulence, and is a factor of a few to ten smaller than usually obtained for viscous discs. We introduce a gap opening criterion applicable for any values of $α_{\rm v}$ and $α_{\rm dw}$ that may be useful for planet formation population synthesis. We show that in discs powered by magnetised winds, growing planets detach from the disc at planet masses below $\sim 0.1M_{\rm Jup}$ inside 10 AU. This promotes formation of super-Earth planets rather than gas giants in this region, in particular precluding formation of hot jupiters in situ. On larger scales, ALMA gap opening planet candidates may be less massive than currently believed. Future high-resolution observations with instruments such as the extended ALMA, ngVLA, and SKA are likely to show abundant narrow annular features at $R < 10$ AU due to ubiquitous super-Earth planets.

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arXiv2022

ALMA constraints on assembly of Core Accretion planets

Resolved dust continuum and CO line ALMA imaging, and in some cases detection of H$α$ emission, hint that young massive planets are abundant at wide separations in protoplanetary discs. Here, we show how these observations can probe the runaway phase of planetary growth in the Core Accretion theory. Planets in this phase have the right range of masses to account for the predominantly moderate contrast gaps and rings seen in ALMA observations. However, we find that these planets gain mass and migrate inward very rapidly. As a result, the phase when they could produce gaps with properties similar to those observed is very short, i.e., $t_{\rm gap} \lesssim 0.1$~Myr, independently of the disc viscosity parameter. This would require many tens to hundreds of gas giant planets to be born per ALMA system, violating the available mass budget of solids in realistic discs. This also predicts preponderance of discs with very wide gaps or complete inner disc holes, which is not observed. We show that suppression of both planet accretion and migration by a factor of at least ten is a possible solution to these serious problems. Future population synthesis models of planet formation should aim to address both exoplanetary data of older discless planetary systems and ALMA discs with embedded planets in one framework.

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arXiv2021

Millimeter-sized Dust Grains Appear Surviving the Water-sublimating Temperature in the Inner 10 au of the FU Ori Disk

Previous observations have shown that the $\lesssim$10 au, $\gtrsim$400 K hot inner disk of the archetypal accretion outburst young stellar object, FU Ori, is dominated by viscous heating. To constrain dust properties in this region, we have performed radio observations toward this disk using the Karl G. Jansky Very Large Array (JVLA) in 2020 June-July, September, and November. We also performed complementary optical photometric monitoring observations. We found that the dust thermal emission from the hot inner disk mid-plane of FU Ori has been approximately stationary and the maximum dust grain size is $\gtrsim$1.6 mm in this region. If the hot inner disk of FU Ori which is inward of the 150-170 K water snowline is turbulent (e.g., corresponding to a Sunyaev & Shakura viscous $α_{t}\gtrsim$0.1), or if the actual maximum grain size is still larger than the lower limit we presently constrain, then as suggested by the recent analytical calculations and the laboratory measurements, water-ice free dust grains may be stickier than water-ice coated dust grains in protoplanetary disks. Additionally, we find that the free-free emission and the Johnson B and V bands magnitudes of these binary stars are brightening in 2016-2020. The optical and radio variability might be related to the dynamically evolving protostellar or disk accretion activities. Our results highlight that hot inner disks of outbursting objects are important laboratories for testing models of dust grain growth. Given the active nature of such systems, to robustly diagnose the maximum dust grain sizes, it is important to carry out coordinated multi-wavelength radio observations.

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arXiv2018

Gravitational fragmentation and formation of giant protoplanets on tens-of-au orbits

Migration of dense gaseous clumps that form in young protostellar disks via gravitational fragmentation is investigated to determine the likelihood of giant planet formation. High-resolution numerical hydrodynamics simulations in the thin-disk limit are employed to compute the formation and long-term evolution of a gravitationally unstable protostellar disk around a solar-mass star. We show that gaseous clumps that form in the outer regions of the disk (>100 AU) through disk fragmentation are often perturbed by other clumps or disk structures, such as spiral arms, and migrate toward the central star on timescales from a few 10^3 to few 10^4 yr. The migration timescale is slowest when stellar motion in response to the disk gravity is considered. When approaching the star, the clumps first gain mass (up to several tens of M_Jup), but then quickly lose most of their diffuse envelopes through tidal torques. Part of the clump envelope can be accreted on the central star causing an FU-Ori-type accretion and luminosity outburst. The tidal mass loss helps the clumps to significantly slow down or even halt their inward migration at a distance of a few tens of AU from the protostar. The resulting clumps are heavily truncated both in mass and size compared to their wider-orbit counterparts, keeping only a dense and hot nucleus. During the inward migration, the temperature in the clump interiors may exceed the molecular hydrogen dissociation limit (2000 K) and the central region of the clump can collapse into a gas giant protoplanet. We argue that FU-Orionis-type luminosity outbursts may be the end product of disk fragmentation and clump inward migration, ushering the formation of giant protoplanets on tens-of-au orbits in systems such as HR~8799.

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arXiv2018

Knotty protostellar jets as a signature of episodic protostellar accretion?

We aim at studying the causal link between the knotty jet structure in CARMA 7, a young Class 0 protostar in the Serpens South cluster, and episodic accretion in young protostellar disks. We used numerical hydrodynamics simulations to derive the protostellar accretion history in gravitationally unstable disks around solar-mass protostars. We compared the time spacing between luminosity bursts Δτ_mod, caused by dense clumps spiralling on the protostar, with the differences of dynamical timescales between the knots Δτ_obs in CARMA 7. We found that the time spacing between the bursts have a bi-modal distribution caused by isolated and clustered luminosity bursts. The former are characterized by long quiescent periods between the bursts with Δτ_mod = a few * (10^3-10^4) yr, whereas the latter occur in small groups with time spacing between the bursts Δτ_mod= a few * (10-10^2) yr. For the clustered bursts, the distribution of Δτ_mod in our models can be fit reasonably well to the distribution of Δτ_obs in the protostellar jet of CARMA 7, if a certain correction for the (yet unknown) inclination angle with respect to the line of sight is applied. The K-S test on the model and observational data sets suggests the best-fit values for the inclination angles of 55-80 deg., which become narrower (75-80 deg.) if only strong luminosity bursts are considered. The dynamical timescales of the knots in the jet of CARMA 7 are too short for a meaningful comparison with the long time spacings between isolated bursts in our models. The exact sequences of time spacings between the luminosity bursts in our models and knots in the jet of CARMA 7 were found difficult to match. (abridged)

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arXiv2017

Formation of freely floating sub-stellar objects via close encounters

We numerically studied close encounters between a young stellar system hosting a massive, gravitationally fragmenting disk and an intruder diskless star with the purpose to determine the evolution of fragments that have formed in the disk prior to the encounter. Numerical hydrodynamics simulations in the non-inertial frame of reference of the host star were employed to simulate the prograde and retrograde co-planar encounters. The initial configuration of the target system (star plus disk) was obtained via a separate numerical simulation featuring the gravitational collapse of a solar-mass pre-stellar core. We found that close encounters can lead to the ejection of fragments that have formed in the disk of the target prior to collision. In particular, prograde encounters are more efficient in ejecting the fragments than the retrograde encounters. The masses of ejected fragments are in the brown-dwarf mass regime. They also carry away an appreciable amount of gas in their gravitational radius of influence, implying that these objects may possess extended disks or envelopes, as also suggested by Thies et al. (2015). Close encounters can also lead to the ejection of entire spiral arms, followed by fragmentation and formation of freely-floating objects straddling the planetary mass limit. However, numerical simulations with a higher resolution are needed to confirm this finding.

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