RR Lyrae Stars of the Inner Halo and Thick Disk

Michigan State University, East Lansing MI

Investigators

Abstract

AST 0205813 Smith RR Lyrae stars are among the oldest stars in the Galaxy, and have long been recognized as important probes of the processes by which the halo and thick disk formed. Nevertheless, our knowledge of the RR Lyrae population within several kiloparsecs of the Sun remains very incomplete. Dr. Horace Smith at Michigan State University will use observations obtained by the Robotic Optical Transient Search Experiment (ROTSE) to identify and characterize RR Lyrae stars north of declination -30 degrees. RR Lyrae stars of Bailey types ab and c will be detected to a limiting magnitude of <V> = 15, corresponding to a distance of about 8 kpc. This survey is expected to include about 5000 RR Lyrae stars. Most of these variable stars will be newly discovered in this survey. The ROTSE survey will enable Dr. Smith to investigate how the density and properties of RR Lyrae stars change as a function of distance from the galactic center and distance from the galactic plane. He will test the hypothesis that field RR Lyrae reveal the presence of a dual halo: an inner flattened system, with RR Lyrae stars that have the characteristics of those in Oosterhoff type II globular clusters, and an outer, less flat, system with RR Lyrae stars that have the characteristics of those in Oosterhoff type I globular clusters. The RR Lyrae sample will also be used to make a new estimate of the local RR Lyrae density and of the scale height of the RR Lyrae of the thick disk. The ROTSE survey will make it possible to trace the distribution and properties of RR Lyrae stars throughout the Galaxy, from the bulge, through the solar vicinity, to the distant halo. It will also provide a database to be used in selecting stars for follow-up spectroscopic studies. Dr. Smith will begin this process with the determination of spectroscopic metal-abundances and radial velocities for the hitherto often neglected RR Lyrae of Bailey type c. These will be used to determine a statistical parallax solution and to study the motions of RRc variables. The ROTSE survey will also systematically identify multi-mode RR Lyrae stars in the solar vicinity, including double-mode stars and Blazhko effect variables. ***