Close stars and accretion in Low Luminosity Active Galactic Nuclei

Details Close stars and accretion in Low Luminosity Active Galactic Nuclei Close stars and accretion in Low Luminosity Active Galactic Nuclei

2004-03-05

arxiv.org/abs/astro-ph/0403158v1

Mon.Not.Roy.Astron.Soc. 352 (2004) 1028

Astronomy and Astrophysics

Astrophysics

Submitted to MNRAS; 9 pages, 2 figures

Scientific paper

10.1111/j.1365-2966.2004.07993.x

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.

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