Session 12 - Stellar Evolution - Theory.
Display session, Wednesday, January 07
Exhibit Hall,

## [12.09] Mass Loss on the Horizontal Branch

H. Yong, P. Demarque (Yale U.), S. Yi (NASA/GSFC)

Mass loss is an important mechanism in various stages of stellar evolution. Despite this, mass loss remains one of the more poorly understood aspects of stellar evolution. We present preliminary results of a systematic study of mass loss effects on the evolution of horizontal branch stars. Since mass loss is poorly understood, the simplest approach was taken. Several constant mass loss rates of 10^-9, 3 \times 10^-9, and 10^-10 M_ødot yr^-1 were assumed in the evolution of horizontal branch stars and compared to those with no mass loss. Horizontal branch models were constructed with envelope masses ranging from 0.005 M_ødot to 0.46 M_ødot. While it is unclear whether mass loss occurs at all on the horizontal branch, we examine its effects, first, to attempt to explain the origin of some extreme blue horizontal branch stars and second, to constrain mass loss rates on the horizontal branch. We also study the effect it has in stars of different metallicities. Preliminary results suggest that mass loss tends to evolve stars blueward in the HR diagram in comparison to their normal course of evolution. This effect is somewhat enhanced with larger metallicities.