Search for carbon enhanced metal-poor stars

Abstract

It has been recognized that a large fraction, in excess of 20%, of stars with metallicities [Fe/H]<-2.0, exhibit large overabundances of carbon, [C/Fe]>+1.0. This fraction rises to 30% for [Fe/H]<-3.0, 40% for [Fe/H]<-3.5, and100% for [Fe/H]<-4.0. The vast majority of such stars were identified from objective-prism surveys, such as the HK survey of Beers and colleagues, and the Hamburg/ESO Survey (HES) of Christlieb and colleagues, based on weak lines of Ca II K.We propose to use the recently compiled set of line index and color information for over 1.5 million digitized spectra from the HES to assemble a list of star with unusually strong CH G bands, and obtain follow-up medium-resolution spectrawith the SOAR 4.1m telescope of a subsample of these candidates, in order to demonstrate the efficacy of searching for additional low-metallicity stars based on the expectation that many will also be strongly carbon enhanced. A list of HES stars with strong lines of carbon has been previously assembled by Christlieb et al. (2001). Spectra for most of these objects have been obtained over the past few years. Inspection of these data indicate that at least 50% of these targets are consistent with identification as Carbon-Enhanced Metal-Poor (CEMP) stars, while the others are roughly solar-abundance carbon-rich stars. However, this previous list was put together based on the SUM of molecular carbon lines, such as CN, C2, and CH, which overemphasizes cooler stars in the sample. CEMP tars with effective temperatures higher than about 5500K often only exhibit an unusual strength in just a single feature, the CH G band at 4300 A, and hence could have been missed in the previous assembly.