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Snacktime for Hungry Black Holes: Theoretical Studies of the Tidal Disruption of Stars

  • Author(s): Strubbe, Linda Elisabeth
  • Advisor(s): Quataert, Eliot
  • et al.
Abstract

A star that wanders too close to the massive black hole (BH) in the center of a galaxy is headed for trouble: within a distance rT ~ r*(MBH/M*)1/3 (where r* and M* are the star's radius and mass, and MBH is the BH's mass), the BH's tidal gravity overcomes the binding gravity of the star, and the star is shredded into a stream of stellar debris. Studying this process of tidal disruption has the potential to give us insights into how central BHs and their surrounding stellar population grow and evolve. Motivated by new and upcoming rapid-cadence optical transient surveys, which should detect and allow study of tidal disruption events (TDEs) in unprecedented detail, I make theoretical predictions of the observable properties of these events to aid in their detection, identification, and interpretation. I find that stellar debris falling towards the BH is likely driven off again by radiation pressure at early times when the feeding rate is super-Eddington: this outflow has a large photosphere and relatively cool temperature, producing a luminous (~ 1043 - few × 1044 erg s-1) transient event at optical wavelengths. I predict that new transient surveys such as the Palomar Transient Factory are likely to find tens to hundreds of these events. I further predict the spectroscopic signature of super-Eddington outflows&mdash broad, blueshifted absorption lines in the ultraviolet&mdash which should help confirm and teach us more about TDE candidates. Finding that the observable appearance of TDEs depends not only on BH mass but on pericenter radius of the star's last fateful orbit, I derive a theoretical expression for the disruption rate as a function of pericenter and apply it to the galaxy NGC 4467 using real observational data, laying the groundwork for more extensive studies in the future. Finally, I also present my work on the debris disk surrounding the star AU Mic, in which I propose an explanation for the physical processes of dust dynamics that give rise to the observed disk profile.

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