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The most updated list of my publications can be found on ADS .
Common Envelope Evolution
Common Envelope (CE) is considered an important stage in the evolution of many binary & multiple systems, in which an unstable mass transfer between the components, causes a rapid orbital decreasing and the expansion (and most widely believed a complete ejection) of its envelope. Due to its relatively short duration and low luminosity, there was no direct detection of systems experiencing this process, and the current theories encounter major problems when simulating this phase- at the end most of the envelope remain bounded, and the final separation is usually larger than expected from observations.
Our Recent Studies on the CE:
Eccentric CE Evolution
Most studies assume a complete circularization of the orbit by the CE onset, while observationally such eccentricities are detected in many post-CE binaries. Here we use smoothed particle hydro-dynamical simulations (SPH) to study the evolution of initially eccentric (0≤e≤0.95) CE-systems. We find that initially eccentric binaries only partially circularize. In addition, higher initial eccentricity leads to a higher post-CE eccentricity, with eccentricities of post-CE binaries as high as 0.2 in the most eccentric cases, and even higher if the initial peri-center of the orbit is located inside the star (e.g. following a kick into an eccentric orbit, rather than a smooth transition). CEE of more eccentric binaries leads to enhanced dynamical mass-loss of the CE compared with more circular binaries, and depends on the initial closest approach of the binary. We show that our results and the observed eccentricities of post-CE binaries suggest that the typical assumptions of circular orbits following CEE should potentially be revised. We expect post-CE eccentricities to affect the delay time distributions of various transients such as supernovae, gamma-ray bursts and gravitational-wave sources by up to tens of percents.
The evolution of the inner (light colors) and outer (dark) binary separations for triple systems with different initial inner separation (red- short period, blue- wide), compared with the corresponding binary system (yellow).
Triple CE - Circumstellar case
Common envelope (CE) evolution, which plays a major role in the evolution of compact binary systems, can similarly play a key role in the evolution of triples. Here, we use hydrodynamical simulations coupled with few-body dynamics to provide the first detailed models of the triple common envelope (TCE) evolution. We focus on the circumstellar case, where the envelope of an evolved giant engulfs a compact binary orbiting the giant, which then in-spirals into the core of the evolved star. Through our exploratory modelling, we find several possible outcomes of such TCE: the merger of the binary inside the third star's envelope; the disruption of the in-spiralling binary following its plunge, leading to a chaotic triple dynamics of the stellar core and the two components of the former disrupted binary. The chaotic evolution typically leads to the in-spiral and merger of at least one of the former binary components with the core, and sometimes to the ejection of the second, or alternatively its further now-binary CE evolution. The in-spiral in TCE leads to overall slower in-spiral, larger mass ejection, and the production of more aspherical remnant, compared with a corresponding binary case of similar masses, due to the energy/momentum extraction from the inner-binary. We expect TCE to play a key role in producing various types of stellar-mergers and unique compact binary systems, and potentially induce transient electromagnetic and gravitational wave sources.
Efficient CE Ejection Through Dust Driven Winds
In this paper, we propose that dust-driven winds can be produced following the CEE. These can evaporate the envelope following similar processes operating in the ejection of the envelopes of AGB (Asymptotic Giant Branch) stars. Pulsations in an AGB star drive the expansion of its envelope, allowing the material to cool down to low temperatures thus enabling dust condensation. Radiation pressure on the dust accelerates it, and through its coupling to the gas it drives winds that eventually completely erode the envelope. We show that the inspiral phase in CE binaries can effectively replace the role of stellar pulsation and drive the CE expansion to scales comparable with those of AGB stars, and gives rise to efficient mass-loss through dust-driven winds.
The characteristic strains of the gravitational-waves emitted by the system as simulated with SPH, sampled every 0.02 days (Blue) and our approximation (Red) showing that future GW detectors (DECIGO / BBO / LISA) could potentially detect the GWs emitted during the CE.
Detecting the CE with Future GWs detectors
Detection of gravitational-wave (GW) sources enables the characterization of binary compact objects (COs) and of their in-spiral. However, other dissipative processes can affect the in-spiral. Here, we show that the in-spiral of COs through a gaseous common envelope (CE) arising from an evolved stellar companion produces a novel type of GW sources, whose evolution is dominated by the dissipative gas dynamical friction effects from the CE, rather than the GW emission itself. The evolution and properties of the GW signals differ from those of isolated gas-poor mergers significantly. We find characteristic strains of ∼10-23-10-21 (10 kpc/{D}) for such sources - observable by next-generation space-based GW detectors (at rates of once per a few centuries for LISA, and about once a year for BBO). The evolution of the GW signal can serve as a probe of the interior regions of the evolved star, and the final stages of CE evolution, otherwise inaccessible through other observational means. Moreover, such CE mergers are frequently followed by observable explosive electromagnetic counterparts and/or the formation of exotic stars.
Supernova explosions from WD interactions
I use magnetohydrodynamical simulations with AREPO to study the explosions and remnants from collisions and mergers of binary white dwarf systems.
In particular, I study merger results of hybrid white dwarfs, which are remnants of evolved stars that most likely went through a CE phase, leaving a thick Helium (He) shell above their degenerate Carbon-Oxygen (CO) core. Mergers of hybrid He-CO WDs, can reproducing the detailed properties of not only normal Type-Ia SNe, but also those of the nonstandard classes.
The impact-parameter threshold of detonations in indirect CO WD collisions
In this paper, we explore both direct and indirect collisions and the conditions in which a detonation is induced and produces a luminous SNe. Using our simulations, we find a detonation criterion that can provide the critical impact parameter for an explosion to occur, depending on the density profile of the colliding WDs, their composition, and their collision velocities.
Initial configuration of the binary system. The area between the two purple lines is where the highest density in the "collision region" is, which is the one that should pass the threshold for Carbon detonation due to the collision.
Bondi-Hoyle Accretion
An important phenomenon in stellar interaction is the accretion of mass from one object to another. It changes the morphology of the surrounds, the stellar dynamics, and the outcome of the binary evolution.
Subsonic Accretion
In this paper, we carried 3D simulations in both MMR code AREPO and a static mesh version of ATHENA++ to study and verify the results of BH accretion in a subsonic motion. We simulated using the smallest accretor and compared our results with previously simulated cases and with a newly developed analytical model for the morphology of the surroundings.
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