Red giant-disk encounters: Food for quasars?

We explore the role that red giants might play in the fueling and evolution of active galactic nuclei (AGNs). 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 ∼10⁷ M_⊙ pc^-3, then stripping of giants could account for the activity of typical AGNs provided that the accretion disk extends out to ∼0.1 pc. For AGNs with smaller disks, or clusters of lower central density, giant stripping could supply gas enriched via stellar nucleosynthesis to a disk replenished from some other source. We find that, for typical parameters, this mechanism is able to supply important quantities of gas to the disk at lower stellar densities than previously proposed stellar fueling models for AGNs.