Ongoing Large Surveys for Metal-Poor Stars in the Galactic Halo

Abstract

We report on two major surveys for metal-poor stars in the galactic halo, the HK survey, and the Hamburg/ESO survey, which have been undertaken in order to provide targets for high-resolution spectroscopy with the Subaru HDS and other large telescopes. We compare basic properties of these two surveys and their current status, and add some historical remarks. The candidate selection procedures of both surveys are described in detail. We evaluate the candidate selection by comparing effective yields (EYs) of the survey techniques for the identification of metal-poor stars. It is found that EY for stars below [Fe/H]=-2.0 in the HES can be up to 80% for stars selected by automatic classification from machine-scanned unwidened plates, whereas in the HK survey, where stars are selected by visual inspection of widened survey plates, the EY is between 11% and 32%, depending on whether a pre-selection based on BV photometry has been applied. Finally, we describe techniques used for determining stellar parameters of the survey stars by means of moderate resolution follow-up spectroscopy, and additional UBV photometry. While follow-up observations of HES stars have just been started, the HK survey has already produced a list of ~4700 stars with estimates of [Fe/H] typically precise to +/-0.2 dex, some 1000 of which have [Fe/H]<-2.0, and roughly 100 of which have [Fe/H]<-3.0.