arXiv subjects

Amit Kashi

Publications and source records attributed to Amit Kashi.

At least 37 records · Page 2Linked to original sources

arXiv2019

ASASSN-13db 2014-2017 Eruption as an Intermediate Luminosity Optical Transient

The low mass star ASASSN-13db experienced an EXor outburst in 2013, which identified it as a Young Stellar Object (YSO). Then, from 2014 to 2017 it had another outburst, longer and more luminous than the earlier. We analyze the observations of the second outburst, and compare it to eruptions of Intermediate Luminosity Optical Transients (ILOTs). We show that the decline of the light curve is almost identical to that of the V838 Mon, a prototype of a type of ILOT known as Luminous Red Nova (LRN). This similarity becomes conspicuous when oscillations that are associated with rotation are filtered out from the light curve of ASASSN-13db. We suggest that the eruption was the result of accretion of a proto-planet of a few Earth masses. The proto-planet was shredded by tidal forces before it was accreted onto the YSO, releasing gravitational energy that powered the outburst for $\approx 800$ days, and ended in a $\approx 55$ days decline phase. When the accretion material started depleting the accretion rate lowered and the eruption light curve declined for almost two months. Then it exhausted completely, creating a sharp break in the light curve. Another possibility is that the mass was a result of an instability in the proto-planetary disk that lead to a large episode of accretion from an inner viscous disk. We find that the variation of the temperature of the outburst is consistent with the surface temperature expected from a depleted viscous accretion disk. The 2014-2017 outburst of ASASSN-13db may be the least energetic ILOT to have been discovered to date, with an energy budget of only $\approx 10^{42}$ erg.

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arXiv2019

Simulating the Response of the Secondary Star of Eta Carinae to Mass Accretion at Periastron Passage

We use high resolution 3D hydrodynamical simulations to quantify the amount of mass accreted onto the secondary star of the binary system Eta Carinae, exploring two sets of stellar masses that had been proposed for the system, the conventional mass model ($M_1=120 M_{\odot}$ and $M_2=30 M_{\odot}$) and the high mass model ($M_1=170 M_{\odot}$ and $M_2=80 M_{\odot}$). The system consists of two very massive stars in a highly eccentric orbit. Every cycle close to periastron passage the system experiences a spectroscopic event during which many lines change their appearance, accompanied by a decline in x-ray emission associated with the destruction wind collision structure and accretion of the primary wind onto the secondary. We take four different numerical approaches to simulate the response of the secondary wind to accretion. Each affects the mass loss rate of the secondary differently, and in turn determines the amount of accreted mass. The high mass model gives for most approaches much more accreted gas and a longer accretion phase. We find that the effective temperature of the secondary can be significantly reduced due to accretion. We also test different eccentricity values and a higher primary mass loss rate and find their effect on the duration of the spectroscopic event. We conclude that the high mass model is better compatible with the amount of accreted mass, $\approx 3 \times 10^{-6} M_{\odot}$, required for explaining the reduction in secondary ionization photons during the spectroscopic event and compatible with its observed duration.

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arXiv2018

Common envelope jets supernova (CEJSN) impostors resulting from a neutron star companion

We propose a new type of repeating transient outburst initiated by a neutron star (NS) entering the envelope of an evolved massive star, accreting envelope material and subsequently launching jets which interact with their surroundings. This interaction is the result of either a rapid expansion of the primary star due to an instability in its core near the end of its nuclear evolution, or due to a dynamical process which rapidly brings the NS into the primary star. The ejecta can reach velocities of 10,000 km/s despite not being a supernova, and might explain such velocities in the 2011 outburst of the luminous blue variable progenitor of SN 2009ip. The typical transient duration and kinetic energy are weeks to months, and up to approximately 1 foe, respectively. The interaction of a NS with a giant envelope might be a phase in the evolution of the progenitors of most NS-NS binary systems that later undergo a merger event. If the NS spirals in all the way to the core of the primary star and brings about its complete disruption we term this a `common envelope jets supernova' (CEJSN), which is a possible explanation for the peculiar supernova iPTF14hls. For a limited interaction of the NS with the envelope we get a less luminous transient, which we term a CEJSN impostor.

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arXiv2018

Counteracting Tidal Circularization with the Grazing Envelope Evolution

We show that substantially enhanced mass loss at periastron passages, as is expected in the grazing envelope evolution (GEE), can compensate for the circularization effect of the tidal interaction in binary systems composed of an asymptotic giant branch (AGB) star and a main sequence secondary star. By numerically integrating the equations of motion we show that under our assumptions the binary system can maintain its high eccentricity as the AGB star evolves toward the post-AGB phase. Our results can explain the high eccentricity of some post-AGB intermediate binaries (post-AGBIBs), i.e., those with an orbital periods in the range of several months to few years. In the framework of the GEE, the extra energy to sustain a high mass loss rate comes from the accretion of mass from the giant envelope or its slow wind onto a more compact secondary star. The secondary star energizes the outflow from the AGB outer envelope by launching jets from the accretion disk.

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arXiv2018

Simulations and Modeling of Intermediate Luminosity Optical Transients and Supernova Impostors

More luminous than classical novae, but less luminous than Supernovae, lies the exotic stellar eruptions known as Intermediate luminosity optical transients (ILOTs). They are divided into a number of sub-groups depending on the erupting progenitor and the properties of the eruption. A large part of the ILOTs is positioned on the slanted Optical Transient Stripe (OTS) in the Energy-Time Diagram (ETD) that shows their total energy vs. duration of their eruption. We describe the different kinds of ILOTs that populate the OTS and other parts of the ETD. The high energy part of the OTS hosts the supernova impostors -- giant eruption (GE) of very massive stars. We show results of 3D hydrodynamical simulations of GEs that expose the mechanism behind these GEs, and present new models for recent ILOTs. We discuss the connection between different kinds of ILOTs, and suggest that they have a common energy source -- gravitational energy released by mass transfer. We emphasize similarities between Planetary Nebulae (PN) and ILOTs, and suggest that some PNe were formed in an ILOT event. Therefore, simulations used for GEs can be adapted for PNe, and used for learning about the influence of the ILOT events on the the central star of the planetary nebula.

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arXiv2018

Periodicity in the Light Curve of P Cygni - Indication for a Binary Companion?

We use observations of the Luminous Blue Variable (LBV) P Cygni spanning seven decades, along with signal processing methods, to identify a periodicity in the stellar luminosity. We find a distinct period of $4.7 \pm 0.3$ years together with shorter periods. The periodicity is a possible indication of a binary companion passing in an eclipse-like event from the dense LBV wind, and if so it is the first observational indication that P Cygni is a binary system. This may support models that contribute giant LBV eruptions to interaction with a binary companion. We discuss other interpretations for the periodicity as well.

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arXiv2018

The orientation of Eta Carinae and the powering mechanism of intermediate luminosity optical transients (ILOTs)

Contrary to recent claims, we argue that the orientation of the massive binary system Eta Carinae is such that the secondary star is closer to us at periastron passage, and it is on the far side during most of the time of the eccentric orbit. The binary orientation we dispute is based on problematic interpretations of recent observations. Among these observations are the radial velocity of the absorption component of He I P-Cyg lines, of the He II $\lambda4686$ emission line, and of the Br$γ$ line emitted by clumps close to the binary system. We also base our orientation on observations of asymmetric molecular clumps that were recently observed by ALMA around the binary system, and were claimed to compose a torus with a missing segment. The orientation has implications for the modeling of the binary interaction during the nineteenth century Great Eruption (GE) of Eta Carinae that occurred close to periastron passage. The orientation where the secondary is closer to us at periastron leads us to suggest that the mass-missing side of the molecular clumps is a result of accretion onto the secondary star during the periastron passage when the clumps were ejected, probably during the GE. The secondary star accreted a few solar masses during the GE and the energy from the accretion process consists the majority of the GE energy. This in turn strengthens the more general model according to which many intermediate-luminosity optical transients (ILOTs) are powered by accretion onto a secondary star.

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arXiv2017

An intermediate-luminosity-optical-transient (ILOT) model for the young stellar object ASASSN-15qi

We construct a scenario where the outburst of the young-stellar-object ASASSN-15qi is an intermediate luminosity optical transient (ILOT). In this scenario a sub-Jupiter young planet was tidally destructed on to a young main-sequence star. The system is young, therefore the radius of the planet is larger than its final value, and consequently its density is smaller. The lower density allows the tidal destruction of the young Saturn-like planet on to the main-sequence star of mass $\approx 2.4~\rm{M_\odot}$, resulting in a formation of a disc and a gravitationally-powered ILOT. Unlike the case of the more energetic ILOT V838~Mon, the mass of the destructed planet is too low to inflate a giant envelope, and hence the merger remnant stays hot. If our suggested model holds, this ILOT possesses two interesting properties: (1) its luminosity and total energy are below those of novae, and (2) it is not as red as other ILOTs. The unusual outburst of ASASSN-15qi, if indeed is an ILOT, further increases the diversity of the already heterogeneous group of ILOTs. We mark the region on the energy-time diagram occupied by such young ILOTs.

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arXiv2017

Type II intermediate-luminosity optical transients (ILOTs)

We propose that in a small fraction of intermediate luminosity optical transients (ILOTs) powered by a strongly interacting binary system, the ejected mass in the equatorial plane can block the central source from our line of sight. We can therefore observe only radiation that is reprocessed by polar outflow, much as in type~II active galactic nuclei (AGN). An ejection of $M_{\rm ej,e}=10^{-4} ~\rm{M_\odot} ~ (1 ~\rm{M_\odot})$ at 30 degrees from the equatorial plane and at a velocity of $v_{\rm e} = 100 ~\rm{km~s^{-1}}$ will block the central source in the NIR for about 5 years (500 years). During that period of time the object might disappear in the visible band, and be detected only in the IR band due to polar dust. We raise the possibility that the recently observed disappearance of a red giant in the visible, designated N6946-BH1, is a type~II ILOT rather than a failed supernova. For this case we estimate that the ejected mass in the polar direction was $M_{\rm ej,p}\approx 10^{-3} ~\rm{M_\odot}$. Our scenario predicts that this event should reinstate its visible emission in several decades.

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arXiv2016

Simulating the Onset of Grazing Envelope Evolution of Binary Stars

We present the first three-dimensional gas-dynamical simulations of the grazing envelope evolution (GEE) of stars, with the goal of exploring the basic flow properties and the role of jets at the onset of the GEE. In the simulated runs, a secondary main-sequence star grazes the envelope of the primary asymptotic giant branch (AGB) star. The orbit is circular at the radius of the AGB primary star on its equator. We inject two opposite jets perpendicular to the equatorial plane from the location of the secondary star, and follow the evolution for several orbital periods. We explore the flow pattern by which the jets eject the outskirts of the AGB envelope. After one orbit the jets start to interact with gas ejected in previous orbits and inflate hot low-density bubbles.

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arXiv2016

Accretion at the periastron passage of Eta Carinae

We present high resolution numerical simulations of the colliding wind system $η$ Carinae, showing accretion onto the secondary star close to periastron passage. Our hydrodynamical simulations include self gravity and radiative cooling. The smooth stellar winds collide and develop instabilities, mainly the non-linear thin shell instability, and form filaments and clumps. We find that a few days before periastron passage the dense filaments and clumps flow towards the secondary as a result of its gravitational attraction, and reach the zone where we inject the secondary wind. We run our simulations for the conventional stellar masses, $M_1=120 ~\rm{M_\odot}$ and $M_2=30 ~\rm{M_\odot}$, and for a high mass model, $M_1=170 ~\rm{M_\odot}$ and $M_2=80 ~\rm{M_\odot}$, that was proposed to better fit the history of giant eruptions. As expected, the simulations results show that the accretion processes is more pronounced for a more massive secondary star.

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arXiv2016

Explaining Two Recent Intermediate Luminosity Optical Transients (ILOTs) by a Binary Interaction and Jets

We propose that two recent intermediate luminosity optical transients (ILOTs), M31LRN 2015 and SN 2015bh (SNHunt 275; PTF 13efv) can be accounted for with a stellar binary model involving mass transfer that leads to the launching of jets. We inspect observations of the ILOT M31LRN 2015 and conclude that it cannot be explained by the onset of a common envelope evolution (CEE). Instead we conjecture that a $M \simeq 1 - 3 ~\rm{M_\odot}$ main sequence star accreted $\simeq 0.04 ~\rm{M_\odot}$ from the giant star, possibly during a periastron passage. The main sequence star accreted mass through an accretion disc, that launches jets. The radiation from the disk and the collision of the jets with the ambient gas can account for the luminosity of the event. Along similar lines, we suggest that the 2013 eruption of SN 2015bh (SNHunt~275) can also be explained by the High-Accretion-Powered ILOT (HAPI) model. In this case a massive secondary star $M_2 \gtrsim 10 ~\rm{M_\odot}$ accreted $\approx 0.05 ~\rm{M_\odot}$ from a much more massive and more evolved star during a periastron passage. If the much more energetic 2015 outburst of SN 2015bh (SNHunt 275) was not a supernova explosion, it might have been a full almost head-on merger event, or else can be accounted for by a the HAPI-jets model in a very highly eccentric orbit.

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arXiv2016

Orbital Parameters for the $250 M_\odot$ Eta Carinae Binary System

We show that recent observations of He I and N II lines of $η$ Carinae may provide support for an orbital orientation where the secondary star is closest to us at periastron passages. This conclusion is valid both for the commonly assumed masses of the two stars, and for the higher stellar masses model where the very massive evolved primary star mass is $M_1=170 M_\odot$ and its hot secondary star mass is $M_2 = 80 M_\odot$. The later model better explains the change in the orbital period assuming that the ninetieth century Great Eruption was powered by accretion onto the secondary star. Adopting the commonly used high eccentricity $e \simeq 0.9$ and inclination $i=41^\circ$, we obtain a good fit to newly released Doppler shift observations of He I emission and absorption lines assuming they are emitted and absorbed in the acceleration zone of the secondary stellar wind. Our model in which the secondary star is in the foreground at periastron is opposite to the view presented recently in the literature.

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arXiv2016

Reverberation Mapping of the Broad Line Region: application to a hydrodynamical line-driven disk wind solution

The latest analysis efforts in reverberation mapping are beginning to allow reconstruction of echo images (or velocity-delay maps) that encode information about the structure and kinematics of the broad line region (BLR) in active galactic nuclei (AGNs). Such maps can constrain sophisticated physical models for the BLR. The physical picture of the BLR is often theorized to be a photoionized wind launched from the AGN accretion disk. Previously we showed that the line-driven disk wind solution found in an earlier simulation by Proga and Kallman is virialized over a large distance from the disk. This finding implies that, according to this model, black hole masses can be reliably estimated through reverberation mapping techniques. However, predictions of echo images expected from line-driven disk winds are not available. Here, after presenting the necessary radiative transfer methodology, we carry out the first calculations of such predictions. We find that the echo images are quite similar to other virialized BLR models such as randomly orbiting clouds and thin Keplerian disks. We conduct a parameter survey exploring how echo images, line profiles, and transfer functions depend on both the inclination angle and the line opacity. We find that the line profiles are almost always single peaked, while transfer functions tend to have tails extending to large time delays. The outflow, despite being primarily equatorially directed, causes an appreciable blue-shifted excess on both the echo image and line profile when seen from lower inclinations ($i \lesssim 45^\circ$). This effect may be observable in low ionization lines such as $\rm{H}β$.

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arXiv2016

Operation of the jet feedback mechanism (JFM) in intermediate luminosity optical transients (ILOTs)

We follow the premise that most intermediate luminosity optical transients (ILOTs) are powered by rapid mass accretion onto a main sequence star, and study the effects of jets launched by an accretion disk. The disk is formed due to large specific angular momentum of the accreted mass. The two opposite jets might expel some of the mass from the reservoir of gas that feeds the disk, and therefore reduces and shortens the mass accretion process. We argue that by this process ILOTs limit their luminosity and might even shut themselves off in this negative jet feedback mechanism (JFM). The group of ILOTs is a new member of a large family of astrophysical objects whose activity is regulated by the operation of the JFM.

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arXiv2015

Recovery from Giant Eruptions in Very Massive Stars

We use a hydro-and-radiative-transfer code to explore the behavior of a very massive star (VMS) after a giant eruption -- i.e., following a supernova impostor event. Beginning with reasonable models for evolved VMSs with masses of $80~M_\odot$ and $120~M_\odot$, we simulate the change of state caused by a giant eruption via two methods that explicitly conserve total energy: 1. Synthetically removing outer layers of mass of a few $M_\odot$ while reducing the energy of the inner layers. 2. Synthetically transferring energy from the core to the outer layers, an operation that automatically causes mass ejection. Our focus is on the aftermath, not the poorly-understood eruption itself. Then, using a radiation-hydrodynamic code in 1D with realistic opacities and convection, the interior disequilibrium state is followed for about 200 years. Typically the star develops a $\sim 400 ~\rm{km}~\rm{s}^{-1}$ wind with a mass loss rate that begins around $0.1 ~M_\odot~\rm{yr^{-1}}$ and gradually decreases. This outflow is driven by $κ$-mechanism radial pulsations. The 1D models have regular pulsations but 3D models will probably be more chaotic. In some cases a plateau in the mass-loss rate may persist about 200 years, while other cases are more like $η$ Car which lost $>10~M_\odot$ and then had an abnormal mass loss rate for more than a century after its eruption. In our model, the post-eruption outflow carried more mass than the initial eruption. These simulations constitute a useful preliminary reconnaissance for 3D models which will be far more difficult.

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arXiv2013

On the Virialization of Disk Winds: Implications for the Black Hole Mass Estimates in AGN

Estimating the mass of a supermassive black hole (SMBH) in an active galactic nucleus (AGN) usually relies on the assumption that the broad line region (BLR) is virialized. However, this assumption seems invalid in BLR models that consists of an accretion disk and its wind. The disk is likely Keplerian and therefore virialized. However, the wind material must, beyond a certain point, be dominated by an outward force that is stronger than gravity. Here, we analyze hydrodynamic simulations of four different disk winds: an isothermal wind, a thermal wind from an X-ray heated disk, and two line-driven winds, one with and the other without X-ray heating and cooling. For each model, we check whether gravity governs the flow properties, by computing and analyzing the volume-integrated quantities that appear in the virial theorem: internal, kinetic, and gravitational energies, We find that in the first two models, the winds are non-virialized whereas the two line-driven disk winds are virialized up to a relatively large distance. The line-driven winds are virialized because they accelerate slowly so that the rotational velocity is dominant and the wind base is very dense. For the two virialized winds, the so-called projected virial factor scales with inclination angle as $1/ \sin^2{i}$. Finally, we demonstrate that an outflow from a Keplerian disk becomes unvirialized more slowly when it conserves the gas specific angular momentum -- as in the models considered here, than when it conserves the angular velocity -- as in the so-called magneto-centrifugal winds.

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arXiv2013

Powering the Second 2012 Outburst of SN 2009ip by Repeating Binary Interaction

We propose that the major 2012 outburst of the supernova impostor SN 2009ip was powered by an extended and repeated interaction between the Luminous Blue Variable (LBV) and a more compact companion. Motivated by the recent analysis of Margutti et al. (2013) of ejected clumps and shells we consider two scenarios. In both scenarios the major 2012b outburst with total (radiated + kinetic) energy of ~5 x 10^{49} erg was powered by accretion of 2-5 solar masses onto the companion during a periastron passage (the first passage) of the binary system approximately 20 days before the observed maximum of the light curve. In the first scenario, the surviving companion scenario, the companion was not destructed and still exists in the system after the outburst. It ejected partial shells (or collimated outflows or clumps) for two consecutive periastron passages after the major one. The orbital period was reduced from ~38 days to ~25 days as a result of the mass transfer process that took place during the first periastron passage. In the second scenario, the merger scenario, some partial shells/clumps were ejected also in a second periastron passage that took place ~20 days after the first one. After this second periastron passage the companion dived too deep into the LBV envelope to launch more outflows, and merged with the LBV.

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