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J. P. Fulbright, R. P. Kraft (UCO/Lick Obs.)
The alpha-element (O, Mg, Si, Ca, and Ti) abundance of a star is an indicator of the star's enrichment history. Type II supernovae create large amounts of the alpha elements with respect to iron, while Type Ia supernovae do not. Until recently, the measured abundances of the alpha elements have yielded supersolar ratios ([alpha/Fe] ~ +0.3-0.5) for metal-poor stars ([Fe/H] < -1). This pattern has been used as evidence for the stars in the halo being formed in a fast collapse early in the history of the Galaxy.
However, recent results of King (1997), Carney et al (1997), Nissen & Schuster (1997), and Hanson et al (1998) show that metal-poor stars with lower alpha abundance ratios exist, and that the low [alpha/Fe] stars may have special kinematic properties. It is also possible that these stars represent an accreted population to the Galaxy.
In order to help determine the frequency and nature of these stars, we have begun a high-resolution, high-S/N survey of nearby subdwarf stars. At this time we have observed ~120 stars and have completed preliminary abundance analyzes on ~90 stars. It is hoped that by the end of the survey we will have alpha element abundances of ~200 stars. This poster paper will report on the progress of this project.
The author(s) of this abstract have provided an email address for comments about the abstract: jfulb@ucolick.org