The carbon-to-oxygen ratio in halo dwarfs

Abstract

C I and O I lines have been observed in 34 metal-poor dwarfs and subgiants, and both LTE and non-LTE analyses of these lines have been performed. Analysis of the lines is vitiated by a systematic temperature-dependent error, showing that present methods of analysis may not give the correct elemental abundances. The (C/O) ratios are, however, almost independent of effective temperature and are therefore reliable. These ratios are found to be essentially constant over the metallicity range (Fe/H) = -2.7 - -1.0 in the survey. The mean value is (C/O) = -0.57 +/- 0.12. Analysis of CH lines gives (C/Fe) = -0.14 +/- 0.20. It is considered that the O abundance in halo stars is more reliably indicated by the forbidden O I line, and it is suggested that (O/Fe) = +0.45 +/- 0.1 is representative for (Fe/H) less than -1. It is suggested that (C/Fe) = -0.1 and (O/Fe) about +0.5 are representative of the halo.