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First Stars III
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Poster
Title: Refined Estimates of Carbon Abundances for Carbon-Enhanced Metal-Poor Stars Author(s): Silvia Rossi Abstract: Among the most metal-poor stars in the Galaxy, a large fraction of them, on the order of 20-25%, exhibit strong absorption lines due to the presence of molecular carbon. In many of these stars, the CH, CN, and C2 bands are quite prominent in medium-resolution spectra. It has been recently noted (Norris et al. 1997, Beers & Christlieb 2005; Rossi et al. 2005) that the frequency of carbon-enhanced stars ([C/Fe] > +1.0) in the Galaxy increases dramatically at the lowest metal abundances. However, it has also been realized that the extant calibration for determination of [C/Fe] and [Fe/H] by Rossi et al. (2005) based on the KP and GP indices suffers from difficulties near the edges of the parameter space, e.g., either at high Teff, low Teff, or when the GP index begins to saturate.Here we present results from a refined set of procedures for estimation of the [Fe/H] and [C/Fe] based on a much larger sample of calibration objects (on the order of 500 stars) then were available to Rossi et al. (2005), due to a dramatic increase in the number of stars with measurements obtained from high-resolution analyses in the past few years. We compare results obtained from a new calibration of the KP and GP indices with that obtained from a custom set of spectral synthesis based on MOOG. In cases where the GP index approaches saturation, it is clear that only spectral synthesis can achieve reliable results.
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