Introduction Tidal Disruption of Stars by Supermassive Black Holes

There is strong evidence for the presence of massive black holes at the centers of many galaxies (see [16] for a review). Does this hold for all galaxies? Questions of particular interest in the context of galaxy evolution are what fraction of galaxies have passed through an active phase, and how many now have nonaccreting and hence unseen supermassive black holes (SMBHs) at their centers (e.g., [19, 20])? How do SMBHs grow to masses of 106-109MQ?

It has been pointed out that an unavoidable consequence, and one of the best diagnostics of the presence of supermassive black holes at the centers of nonactive galaxies would be occasional tidal disruptions of stars approaching the supermassive black hole [18, 20]. The flare, produced when the star is tidally disrupted and subsequently accreted, can be used to find and study these supermassive black holes and their immediate vicinity.

A star on a radial "loss-cone" orbit gets tidally disrupted once the tidal force ex-certed by the black hole exceeds the self-gravity of the star (e.g., [10]). This happens at the tidal radius, rt, given by cm' (2'1)

where MBH is the black hole mass, M* is the mass of the star in units of solar mass M0, and r* is the radius of the star in units of solar radius r0. About 50-90% of the gaseous debris becomes unbound and is lost from the system (e.g., [1]). The rest will eventually be accreted by the black hole. This produces a flare of radiation, lasting on the order of months to years [21].

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