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Licenciate thesis: Tidal disruption of stars by non-spinning supermassive black holes
  Thesis defense

Friday 18 December 2015
from 13:00 to 15:00
at FP41
Speaker : Emanuel Gafton (Stockholm University; Dept. of Astornomy)
Abstract : The supermassive black holes that reside in the centres of most known galaxies are able to disrupt solitary stars that approach them on orbits with periapsis distances comparable to the tidal radius of the star. The observational signatures of such encounters may be used to probe the properties of the black hole, to test strong gravity and accretion theories, and to explain a number of puzzling observations around Sgr A*. Although a general picture of a typical tidal disruption can be easily obtained from analytical estimates (many of which are re-derived in this thesis), questions related to the stellar compression and bounce at pericentre, the energy and angular momentum redistribution between the different parts of the star, the long-term orbital evolution under the influence of relativistic effects such as apsidal and Lense–Thirring precession, and the fate of the tidal debris require the aid of detailed numerical simulations. In this thesis, we use a smoothed particle hydrodynamics code to simulate encounters between non-spinning supermassive black holes (modelled both with and without general relativistic effects) and solar-type stars (modelled as simple polytropes). In particular, we focus on partial tidal disruptions, which are strong enough to disrupt the outer layers of the star while still leaving a surviving, self-bound core. In a typical, marginally-unbound (parabolic) encounter, the surviving core receives an energy kick that places it on an unbound (hyperbolic) orbit. We found that relativistic effects can increase this kick significantly (by more than a factor of ∼ 2), enough to explain the origin of at least some of the observed hyper-velocity stars in our Galactic Centre, and enough even for the star to escape the Galactic gravitational potential well. For more information, please see http://compact-merger.astro.su.se/~em/licentiate.pdf

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