Radial Velocities of Late-Type Population II Stars

Abstract

AbstractEarly radial-velocity work on high-velocity stars has been summarized. Present samples of such stars have been reviewed. Selection methods and sample bias have been discussed. Spectroscopic as well as photometric methods have been examined. In order to find a reasonably bias-free sample of late-type stars of Population II, photometric survey techniques have to be applied. Our survey for cool Population II stars is presented. With uvby photometry we have covered a total of about 5000 stars in an attempt to derive an unbiased sample of late-type stars suitable for a study of galactic distribution and kinematical properties as a function of evolutionary phase. Especially for the metal-poor stars of our sample we have made highaccuracy CORAVEL scanner measurements of radial velocities. So far, around 1700 stars have been observed for radial velocities, most of them more than once. The CORAVEL measurements have been very successful also for stars with extremely low abundance of heavy’ elements. Many previously unknown high-velocity stars have been found. Some of them have very high radial velocities. A comparison between our radial-velocity data and our preliminary metallicity indices confirms the early evolutionary phase reached in our survey. Our results also seem to confirm the limited ranges occupied by Intermediate Population II in metallicity and in radial velocity.