A New Calibration of [Fe/H] and [C/Fe] Estimates for Medium-Resolution Spectra of Carbon-Enhanced Metal-Poor Stars

Statistics – Methodology

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Scientific paper

In recent years it has become clear that a significant fraction (on the order of at least 20%) of Galactic halo stars with [Fe/H] < -2.0 exhibit strong enhancements of carbon, with [C/Fe] > +1.0. The availability of many thousands of medium-resolution spectra from previous (and ongoing) surveys such as the HK survey of Beers and colleagues, and the Hamburg/ESO Survey of Christlieb and collaborators, provide the opportunity to identify and quantify the distribution of [C/Fe] over a wide range of [Fe/H] and stellar evolutionary states. In order to quickly obtain this information, previous attempts have been made to develop a calibration of line index estimates of [Fe/H] and [C/Fe] based on the observed strengths of the CaII K line and the CH G-band feature. The methodology developed by Rossi et al. (2005) relied on a sample of some 120 stars with available high-resolution spectroscopic estimates of [Fe/H] and [C/Fe], as well as 2MASS J-K colors, to carry out such a calibration. Unfortunately, the sample of calibration objects did not include numerous stars with effective temperatures (and carbon abundances) over the full range that is required for some applications. Recently the numbers of potential calibration objects has increased dramatically, to over 500 stars, due to the completion of several large high-resolution spectroscopic studies (e.g., Barklem et al. 2005, Cohen et al. 2005, Spite et al. 2005, and Aoki et al. 2006). In addition, new carbon-enhanced model atmospheres from which synthetic spectra and colors can be estimated have become available. We explore the use of these new data and tools and develop a revised calibration that is expected to be superior in many respects to previous attempts.

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