Making Good Use of Bad Weather: Finding Extremely Metal-Poor Stars in the Clouds

Abstract

Current results are presented for a new survey effort to search for Extremely Metal-Poor (EMP) and Carbon Enhanced Metal-Poor (CEMP) stars from the Hamburg / ESO objective-prism survey (HES). These stars are important probes of Galactic Chemical Evolution, as they require specific scenarios and conditions for their formation. Recent results infer that their distribution among the various structural components of the Milky Way can also help describe the dynamical process(es) that led to the assembly of our Galaxy. We have developed a new method to search for metal-poor stars, based on identifying stars with apparently strong molecular CH G-band strengths for their colors. The hypothesis we exploit is that large over-abundances of carbon are common among metal-poor stars, as has been found by numerous studies over the past two decades. Estimates of the stellar atmospheric parameters, as well as carbon abundances, are now available for about 250 of the first candidates, based on follow-up medium-resolution spectra obtained with bad weather GMOS proposals on the Gemini 8m telescopes, and also with the Goodman HTS on the SOAR 4.1m telescope. We have over 5000 candidates, they are reasonably bright (10<B<16), and they cover most of the southern sky. Thus, they are perfect to switch to when weather or seeing prevents the execution of other programs. There are 13 newly discovered stars with [Fe/H]<−3.0 in our sample, including four with [Fe/H]<−3.5 and one with [Fe/H]∼−4.0. This work is important to provide reliable targets suitable for high-resolution spectroscopy on 8m class telescopes such as Subaru, to determine the detailed abundance patterns of these objects.