A New Spin on Horizontal-Branch Stars: Anomalous Abundances and Rapid Rotation Rates

Citation

Behr, Bradford B. (2000) A New Spin on Horizontal-Branch Stars: Anomalous Abundances and Rapid Rotation Rates. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/rdxg-qg24. https://resolver.caltech.edu/CaltechETD:etd-09172008-111737

Abstract

High-resolution spectroscopy of blue horizontal-branch stars in metal-poor globular clusters reveals remarkable differences in photospheric composition and rotation rate between cooler and hotter stars. For the cooler stars (T_eff < 11000 K), the derived abundances are in good agreement with the canonical cluster metallicities, and we confirm the range of v sin i rotation velocities, some as high as 40 km s^-1, previously reported in the literature. In the hotter stars, however, metals are enhanced by factors of 100 or more, and helium is depleted by as much as two orders of magnitude. In addition, the hot stars are almost exclusively slow rotators, with v sin i < 8 km s^-1. The anomalous abundances are due to atomic diffusion mechanisms — gravitational settling of helium, and radiative levitation of metals - in the stable, non-convective atmospheres of these hot stars. The enhanced metal content in the stellar photospheres can alter the emergent spectral energy distributions, creating the photometric "gaps" observed in the clusters' color-magnitude diagrams. We discuss the connections between rotation and the diffusion mechanisms, and explore potential implications regarding the internal structure and evolution of these stars.

Thesis Files