Session 107 - The Galactic Center & Bulge.
Display session, Saturday, January 10
Exhibit Hall,

## [107.04] Stellar Collisions in the Galactic Center

V. C. Bailey, M. B. Davies (IoA, Cambridge, UK)

We investigate whether collisions are responsible for the paucity of massive red giants (2M_ødot-8M_ødot) as observed in the central 0.2pc of our galaxy by Genzel et al, (1996). In the central region, collisions involving the giants with other stars will occur on time-scales comparable to the lifetimes of the giants. However, our 3D hydrodynamical simulations of collisions between AGB stars and other cluster stars show that the initial mass loss of such encounters is only rarely a substantial fraction of the giants' envelopes. Some collisions dissipate enough of the impactor's kinetic energy to form common-envelope systems. These systems will then proceed to eject the envelope on a time-scale much shorter than the collision time-scale. However, our collision-rate analysis predicts too small a rate of encounters which form bound systems to totally explain the paucity of giant stars: for a velocity dispersion of 120km/s, only \sim0.6%-6% of collisions with impactor masses between 0.6M_ødot and 2M_ødot produce this outcome. As an alternative mechanism we consider encounters between the giants and binary stars. Although the collision rate for these 3-body encounters may be lower than for 2-body encounters, a larger fraction of such encounters lead to the formation of a common-envelope system. Our simulations of binary/giant encounters indicate that it is possible to destroy many giants in this manner. We present collision rates, mass-loss data and cross sections for these mechanisms, based upon our extensive grids of collisions pertinent to the galactic center.

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ref: Genzel R., Thatte N., Krabbe A., Kroker H., Tacconi-Garman L.E., 1996, ApJ, 472, 153