Close stars and accretion in Low Luminosity Active Galactic Nuclei

Abstract

Quasar accretion disks are believed to form stars by self-gravity. Low Luminosity Active Galactic Nuclei (LLAGN) are much dimmer galactic centers, and are often believed to be quasars that ran out of gaseous fuel. LLAGN accretion disks should thus co-exist with thousands to millions of stars or proto-stars left from the previous stronger accretion activity. In principle, these stars may produce several important effects: (i) contribute to the optical/UV spectra of some LLAGN; (ii) reprocessing of the stellar radiation in the dusty disks could dominate the LLAGN infra-red spectra; (iii) deplete the (accretion) gas disk much faster than it can accrete onto the supper-massive black hole (SMBH); (iv) stars, individually or in groups, may slow down and modulate the accretion flow significantly due to their inertia. In this way they may produce the LLAGN cut-off disks; (v) alternatively, frequent enough stellar collisions and resulting stellar disruptions could keep the inner disk empty. Here we explore these ideas. We find that, despite ``low'' luminosities of LLAGN, unrealistically high stellar densities are required to make a sizable radiative contribution to the (HST) optical/UV spectra of these galactic nuclei. Stellar contribution to the infrared spectrum is more likely. Further, if LLAGN are in a quasi steady-state for as long as 10^7 years or more, too high stellar densities would again be required to significantly affect the dynamics of accretion flow. However, if LLAGN are ``short''-lived phenomena, e.g. t < 10^5 years, quiescent states of quiescence-outburst cycles, then embedded stars may be much more important through the mass effects (iii) -- (v). With observations of LLAGN becoming progressively better, it will be more and more difficult to neglect the presence of close stars in and around nuclear accretion disks.