Red Giant--Disk Encounters: Food for Quasars?

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Abstract
We explore the role that red giants might play in the central regions of Active Galactic Nuclei. Due to their large radii and the low binding energy of the stellar envelope, giants are vulnerable to envelope stripping from collisions with the accretion disk. Using hydrodynamic simulations we show that such collisions will typically deposit a substantial fraction of the envelope mass into the disk on each passage. Repeated encounters will then lead to the complete destruction of the star save for the dense core. We estimate the rate of fuel supply by this mechanism using simple models for the AGN disk and central stellar cluster. If the central stellar density is of order 10^7 solar masses per cubic pc, then stripping of giants could account for the activity of typical AGN provided that the accretion disk extends beyond 0.1 pc. For AGN with smaller disks, or clusters of lower central density, giant stripping could be an important source of gas enriched via stellar nucleosynthesis.Comment: 24 pages of LaTeX, including 10 of 11 figures. Uses aaspp.sty. Colour versions of Fig. 4 (not included) are available at http://www.ast.cam.ac.uk/~parm/star_disk.html . ApJ in pres
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