The Frequency of Carbon Stars among Extremely Metal-poor Stars

Abstract

We demonstrate that there are systematic scale errors in the [Fe/H]values determined by the Hamburg/ESO Survey (HES; and by inference bythe HK survey in the past) for certain extremely metal-poor, highlyC-enhanced giants. The consequences of these scale errors are that (1)the fraction of carbon stars at extremely low metallicities has beenoverestimated in several papers in the recent literature, (2) the numberof extremely metal-poor stars known is somewhat lower than has beenquoted in the recent literature, and (3) the yield for extremelymetal-poor stars by the HES is somewhat lower than is stated in therecent literature. A preliminary estimate for the frequency of carbonstars among the giants in the HES sample with -4 {\lt} [Fe/H] {\lt} -2.0dex is 7.4% +/- 2.9%; adding an estimate for the C-enhanced giants with[C/Fe] {\gt} 1.0 dex without detectable C_{2} bands raises thefraction to 14% +/- 4%. We rely on the results of an extensive set ofhomogeneous, detailed abundance analyses of stars expected to have[Fe/H] {\lt}= -3.0 dex selected from the HES to establish these claims.We have found that the Fe metallicity of the cooler(T_{eff}{\lt}~5200 K) C stars as derived from spectra taken withHIRES at Keck are a factor of ~10 higher than those obtained via thealgorithm used by the HES project to analyze the moderate-resolutionfollow-up spectra, and this algorithm is identical to that used untilvery recently by the HK survey. This error in the Fe abundance estimatefor C stars arises from a lowering of the emitted flux in the continuumbandpasses of the KP index (Ca II {$λ$}3933) and, particularly, theHP2 (H{$δ$}) index used to estimate [Fe/H] due to absorption fromstrong molecular bands.Based in part on observations obtained at the W. M. Keck Observatory,which is operated jointly by the California Institute of Technology, theUniversity of California, and the National Aeronautics and SpaceAdministration.