The disk accretion of a tidally disrupted star onto a massive black hole

We consider the consequences of the tidal disruption of a star by a massive black hole. We argue that the initial extremely eccentric orbit of the stellar debris will become a circular orbit near the tidal radius after experiencing strong shocks which thermalize the orbital energy on a relatively short time scale. The subsequent evolution of the accretion disk is studied using a time-dependent α-disk model. The luminosity evolves asymptotically toward the power law L ∝ t^-1.2, and the light-to-mass ratio of the disk-plus-black-hole exceeds unity for several thousand years after disruption, unless (1) the rate of disruptions is much lower than 10^-4 M_⊙ yr^-1, or (2) most of the stellar debris is ejected or accreted without significant emission before circularization occurs. We suggest that some fraction of galaxies (particularly dwarf ellipticals) should be extremely bright at far-ultraviolet wavelengths if they contain black holes of mass 10⁶-10⁸ M_⊙. On the other hand, our results may argue against the presence of massive black holes in nearby galaxies such as M32.