The Frequency of Carbon Stars among Extremely Metal-poor Stars

We demonstrate that there are systematic scale errors in the [Fe/H] valuesdetermined by the Hamburg/ESO Survey (and by inference by the HK Survey in thepast) for certain extremely metal poor highly C-enhanced giants. Theconsequences of these scale errors are that a) the fraction of carbon stars atextremely low metallicities has been overestimated in several papers in therecent literature b) the number of extremely metal poor stars known is somewhatlower than has been quoted in the recent literature c) the yield for extremelymetal poor stars by the HES Survey is somewhat lower than is stated in therecent literature. A preliminary estimate for the frequency of Carbon starsamong the giants in the HES sample with -4 < [Fe/H] < -2.0 dex is 7.4 +-2.9%;adding an estimate for the C-enhanced giants with [C/Fe] > 1.0 dex withoutdetectable C2 bands raises the fraction to 14 +-4$%. We rely on the results of an extensive set of homogeneous detailed abundanceanalyses of stars expected to have [Fe/H] < -3.0 dex selected from the HES toestablish these claims. We have found that the Fe-metallicity of the cooler(Teff < 5200K) C-stars as derived from spectra taken with HIRES at Keck are afactor of ~10 higher than those obtained via the algorithm used by the HESproject to analyze the moderate resolution follow-up spectra, which isidentical to that used until very recently by the HK Survey. This error inFe-abundance estimate for C-stars arises from a lowering of the emitted flux inthe continuum bandpasses of the KP (3933 A line of CaII) and particularly theHP2 (Hdelta) indices used to estimate [Fe/H] due to absorption from strongmolecular bands.Comment: Accepted to the ApJL after a very lengthly duel with the 3 simultaneous referee