Tidal disruption by a massive black hole and collisions in galactic nuclei

A possible model for the energy source of Seyfert galaxies and QSOs consists of a massive black hole sitting in a galactic core which accretes gas produced by tidal disruptions, collisions and mass loss from stars. Different stellar-d dynamical regimes may apply depending on the parameters of the galactic core. The growth of the black hole and the luminosity obtained in the process are discussed. An upper limit for the disruption rate of the order of the mass of the core divided by the relaxation time is derived. The radiation generated by accretion of stellar debris seems sufficient to explain the luminosity of Seyferts and QSOs but it may be necessary to invoke stellar collisions to explain the most luminous sources. (author)