Close stars and accretion in low-luminosity active galactic nuclei

Abstract

Quasar accretion discs are believed to form stars by self-gravity. Low-luminosity active galactic nuclei (LLAGNs) are much dimmer galactic centres, and are often believed to be quasars that ran out of gaseous fuel. LLAGN accretion discs should thus coexist with thousands to millions of stars or protostars left from the previous stronger accretion activity. In principle, these stars may produce several important effects: (i) contribute to the optical/ultraviolet spectra of some LLAGNs; (ii) the dusty discs could reprocess stellar radiation in the infrared frequencies and then dominate the LLAGN spectra in that region; (iii) deplete the (accretion) gas disc much faster than it can accrete on to the supermassive black hole; (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 discs; (v) alternatively, frequent enough stellar collisions and resulting stellar disruptions could keep the inner disc empty. Here we explore these ideas. We find that, despite 'low' luminosities of LLAGNs, unrealistically high stellar densities are required to make a sizable radiative contribution to the Hubble Space Telescope optical/ultraviolet spectra of these galactic nuclei. Stellar contribution to the infrared spectrum is more likely. Further, if LLAGNs are in a quasi-steady state for as long as 107 yr or more, too high stellar densities would again be required to significantly affect the dynamics of accretion flow. However, if LLAGNs are 'short-lived' phenomena, e.g. t ≲ 105 yr, the low-activity states of quiescence-outburst cycles, then embedded stars may be much more important through the mass effects (iii)-(v). With observations of LLAGNs becoming progressively better, it will be more and more difficult to neglect the presence of close stars in and around nuclear accretion discs.