2001ApJ...562L.137M

The tidal disruption of a star by a supermassive black hole is expected to lead to a short bright flare followed by an extended period of low-level emission. Existing models of the late-time accretion of the stellar debris via a thin disk imply that the Galactic center source Sgr A* should be brighter than currently observed by several orders of magnitude. A similar problem exists for M31 and M32. This problem could be avoided if thin disk accretion transitions to low radiative efficiency accretion (e.g., advection-dominated or convection-dominated accretion flows) at low accretion rates (via ``evaporation'' of the thin disk). Alternatively, we show that the outer portions of a thin disk may become neutral and unable to sustain MHD turbulence on a timescale of less than the time between tidal disruption events; this may dramatically shorten the duration of the late accretion phase.: