Amit Kashi

A circumbinary disc in the final stages of common envelope and the core‐degenerate scenario for Type Ia supernovae

We study the final stages of the common envelope (CE) evolution and find that a substantial fraction of the ejected mass does not reach the escape velocity. To reach this conclusion we use a self-similar solution under simplifying assumptions. Most of the gravitational energy of a companion white dwarf (WD) is released in the envelope of a massive asymptotic giant branch (AGB) or the red giant branch (RGB) star in a very short time. This rapid energy release forms a blast wave in the envelope. We follow the blast wave propagation from the centre of the AGB outwards, and show that ∼1–10 per cent of the ejected envelope remains bound to the remnant binary system. We suggest that due to angular momentum conservation and further interaction with the binary system, the fall-back material forms a circumbinary disc around the post-AGB Core and the companion WD. The interaction of the circumbinary disc with the binary system will reduce the orbital separation much more than expected of the dynamical phase (where the envelope is ejected) of the CE alone. The smaller orbital separation favours a merger at the end of the CE phase or a short time after, while the core is still hot. This is another channel for the formation of a massive WD with super-Chandrasekhar mass that might explode as a Type Ia supernova. We term this the core-degenerate (CD) scenario.

Explaining the Type Ia supernova PTF 11kx with a violent prompt merger scenario

We argue that the multiple shells of circumstellar material (CSM) and the supernovae (SN) ejecta interaction with the CSM starting 59 days after the explosion of the Type Ia SN (SN Ia) PTF 11kx, are best described by a violent prompt merger. In this prompt merger scenario the common envelope (CE) phase is terminated by a merger of a WD companion with the hot core of a massive asymptotic giant (AGB) star. In most cases the WD is disrupted and accreted onto the more massive core. However, in the rare cases where the merger takes place when the WD is denser than the core, the core will be disrupted and accreted onto the cooler WD. In such cases the explosion might occur with no appreciable delay, i.e., months to years after the termination of the CE phase. This, we propose, might be the evolutionary route that could lead to the explosion of PTF 11kx. This scenario can account for the very massive CSM within � 1000 AU of the exploding PTF 11kx star, for the presence of hydrogen, and for the presence of shells in the CSM.

Explaining the Supernova Impostor SN 2009ip as Mergerburst

We propose that the energetic major outburst of the supernova (SN) impostor SN 2009ip in 2012 September (outburst 2012b) was a mergerburst event, where two massive stars merged. The previous outbursts of 2009 and 2011 might have occurred near periastron passages of the binary system prior to the merger, in a similar manner to the luminosity peaks in the 19th century Great Eruption of the massive binary system Eta Carinae. The major 2012b outburst and the 2012a pre-outburst resemble the light curve of the mergerburst event V838 Mon. A merger of an evolved star with a mass of M 1 ~ 60-100 M ? and a secondary main-sequence star of M 2 ~ 0.2-0.5 M 1 can account for the energy of SN 2009ip and for the high velocities of the ejected gas. The ejected nebula is expected to have a non-spherical structure, e.g., bipolar or even a more complicated morphology.

Formation of Bipolar Planetary Nebulae by Intermediate-luminosity Optical Transients

We present surprising similarities between some bipolar planetary nebulae (PNe) and eruptive objects with peak luminosity between novae and supernovae. The latter group is termed ILOT for intermediate-luminosity optical transients (other terms are intermediate-luminosity red transients and red novae). In particular, we compare the PN, NGC 6302 and the pre-PNe OH231.8+4.2, M1-92, and IRAS 22036+5306 with the ILOT NGC 300 OT2008-1. These similarities lead us to propose that the lobes of some (but not all) PNe and pre-PNe were formed in an ILOT event (or several close sub-events). We suggest that in both types of objects the several months long outbursts are powered by mass accretion onto a main-sequence (MS) companion from an asymptotic giant branch (AGB, or extreme-AGB) star. Jets launched by an accretion disk around the MS companion shape the bipolar lobes. Some of the predictions that result from our comparison is that the ejecta of some ILOTs will have morphologies similar to those of bipolar PNe, and that the central stars of the PNe that were shaped by ILOTs should have an MS binary companion with an eccentric orbit of several years long period.

Possible implications of mass accretion in Eta Carinae

Abstract We apply the previously suggested accretion model for the behavior of the super-massive binary system η Car close to periastron passages. In that model it is assumed that for ∼ 10 weeks near periastron passages one star is accreting mass from the slow dense wind blown by the other star. We find that the secondary, the less massive star, accretes ∼ 2 × 10 - 6 M ⊙ . This mass possesses enough angular momentum to form a disk, or a belt, around the secondary. The viscous time is too long for the establishment of equilibrium, and the belt must be dissipated as its mass is being blown in the reestablished secondary wind. This process requires about half a year, which we identify with the recovery phase of η Car. We show that radiation pressure, termed radiative braking, cannot prevent accretion. In addition to using the commonly assumed binary model for η Car, we also examine alternative models where the stellar masses are larger, and/or the less massive secondary blows the slow dense wind, while the primary blows the tenuous fast wind and accretes mass for ∼ 10 week near periastron passages. We end by some predictions for the next event (January–March 2009).

The orientation of the η Carinae binary system

We examine a variety of observations that shed light on the orientation of the semimajor axis of the η Carinae massive binary system. Under several assumptions, we study the following observations: the Doppler shifts of some He i P Cygni lines that are attributed to the secondarys wind, of one Fe ΙΙ line that is attributed to the primarys wind and of the Paschen emission lines which are attributed to the shocked primarys wind, are computed in our model and are compared with observations. We compute the hydrogen column density towards the binary system in our model, and find a good agreement with that deduced from X-ray observations. We calculate the ionization of the surrounding gas blobs by the radiation of the hotter secondary star, and compare with observations of a highly excited [Ar ΙΙΙ] narrow line. We find that all of these support an orientation where for most of the time the secondary - the hotter, less-massive star - is behind the primary star. The secondary comes closer to the observer only for a short time near periastron passage in its highly eccentric (e ≃ 0.9) orbit. Further supporting arguments are also listed, followed by a discussion on some open and complicated issues.

Mergerburst transients of brown dwarfs with exoplanets

We explore the properties of an optical transient event formed by the destruction of a planet by a brown dwarf (BD) – a BD–planet mergerburst. When a massive planet approaches a BD towards a merging process it will be tidally destroyed and will form an accretion disc around the BD. The viscosity in the disc sets the characteristic time for the event – several days. We suggest that BD–planet mergerburst events have light curves resembling those of other intermediate luminosity optical transient events, such as V838 Mon, but at shorter time-scales and lower luminosities. With the high percentage coverage of the sky, we expect that such events will be detected in the near future.

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.

An indication for the binarity of P Cygni from its 17th century eruption

I show that the 17th century eruption of the massive luminous blue variable (LBV) star P Cygni can be explained by mass transfer to a B-type binary companion in an eccentric orbit, under the assumption that the luminosity peaks occurred close to periastron passages. The mass was accreted by the companion and liberated gravitational energy, part of which went to an increase in luminosity. I find that mass transfer of ~0.1 M ⊙ to a B-type binary companion of ~3-6 M ⊙ can account for the energy of the eruption, and for the decreasing time interval between the observed peaks in the visual light curve of the eruption. Such a companion is predicted to have an orbital period of ~7 years, and its Doppler shift should be possible to detect with high-resolution spectroscopic observations. Explaining the eruption of P Cygni by mass transfer further supports the conjecture that all major LBV eruptions are triggered by interaction of an unstable LBV with a stellar companion.

Using X-ray observations to explore the binary interaction in Eta Carinae

We study the usage of the X-ray light curve, column density towards the hard X-ray source, and emission measure (density square times volume), of the massive binary system η Carinae to determine the orientation of its semimajor axis. The source of the hard X-ray emission is the shocked secondary wind. We argue that, by itself, the observed X-ray flux cannot teach us much about the orientation of the semimajor axis. Minor adjustment of some unknown parameters of the binary system allows to fit the X-ray light curve with almost any inclination angle and orientation. The column density and X-ray emission measure, on the other hand, impose strong constrains on the orientation. We improve our previous calculations and show that the column density is more compatible with an orientation where for most of the time the secondary – the hotter, less massive star – is behind the primary star. The secondary comes closer to the observer only for a short time near periastron passage. The 10-week X-ray deep minimum, which results from a large decrease in the emission measure, implies that the regular secondary wind is substantially suppressed during that period. This suppression is most likely resulted by accretion of mass from the dense wind of the primary luminous blue variable star. The accretion from the equatorial plane might lead to the formation of a polar outflow. We suggest that the polar outflow contributes to the soft X-ray emission during the X-ray minimum; the other source is the shocked secondary wind in the tail. The conclusion that accretion occurs at each periastron passage, every five and a half years, implies that accretion had occurred at a much higher rate during the Great Eruption of η Car in the 19th century. This has far reaching implications for major eruptions of luminous blue variable stars.