The Frequency of Carbon-enhanced Metal-poor Stars in the Galaxy from the HERES Sample

By S. Lucatello, Timothy C. Beers, N. Christlieb, Paul Barklem, S. Rossi, B. Marsteller, T. Sivarani, Y. S. Lee. 2006

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We estimate the frequency of carbon-enhanced metal-poor (CEMP) stars among very metal-poor stars, based on an analysis of 349 stars with available high-resolution spectra observed as part of the Hamburg/ESO R-process Enhanced Star (HERES) survey. We obtain that a lower limit of 21% ± 2% of stars with [Fe/H] ≤ -2.0 exhibit [C/Fe] ≥ +1.0. These fractions are higher than those that have been reported by recent examinations of this question, based on substantially smaller samples of stars. We discuss the source of this difference and suggest that in order to take into account effects that result in a decrease of surface carbon abundance with advancing evolution, a definition of CEMP stars based on a [C/Fe] cutoff that varies as a function of luminosity is more appropriate. We discuss the likely occurrence of dilution and mixing for many CEMP stars, which, if properly accounted for, would increase this fraction still further.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/8239071fb90d0b8b10875736db84d218ab9382b1

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Binarity in carbon-enhanced metal-poor stars (2014, background): “…al. 2013b; Aoki et al. 2013; Lee et al. 2013), which then decreases to 9-21% for [Fe/H]<-2.0 (e.g., Norris, Ryan & Beers 1997a; Rossi, Beers & Sneden 1999; Christlieb 2003; Marsteller et al. 2005; Lucatello et al. 2006; Cohen et al. 2005; Frebel et al. 2006; Carollo et al. 2012; Lee et al. 2013)1.”
Neutron-Capture Elements in the Early Galaxy (2008, background): “Very C-rich stars ([C/Fe] ≥ +1.0) comprise a substantial fraction of very metal-poor stars, with estimates ranging from 14 ± 4% (Cohen et al. 2005) to 21 ± 2% (Lucatello et al. 2006) for stars with [Fe/H] ≤ −2.0.”
Stellar Abundances for the Galactic Archeology (SAGA) Database — Compilation of the Characteristics of Known Extremely Metal-Poor Stars (2008, methodology): “Previous work (e.g., Lucatello et al. 2006) has claimed that CEMP stars occupy ∼ 20% of EMP stars.”
THE s-PROCESS IN ASYMPTOTIC GIANT BRANCH STARS OF LOW METALLICITY AND THE COMPOSITION OF CARBON-ENHANCED METAL-POOR STARS (2011, background): “Roughly 10-20% (Cohen et al. 2005; Lucatello et al. 2006; Carollo et al. 2011) of old halo – 3 – stars are enriched in carbon ([C/Fe]1> 1) and are referred to as carbon-enhanced metal-poor (CEMP) stars.”
A holistic approach to carbon-enhanced metal-poor stars (2009, background): “Therefore we argue thatthe conclusions of Lucatello et al. (2006) and Aoki et al. (2007) are observationally biased so that the decrease of [C+N/H] as a function of luminosity is dominated by the metallicity trend.”
Carbon-enhanced Metal-poor Stars. I. Chemical Compositions of 26 Stars (2006, background): “Lucatello et al. (2006), based on high-resolution analyses of a much larger sample of stars, including many earlier type stars than the Cohen et al. (2005) sample, conclude that this fraction is at least 20% 2%.”
Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars - I. Stellar yield tables and the CEMPs (2008, background): “Beers & Christlieb 2005; Cohen et al. 2005; Lucatello et al. 2006), suggesting that an additional (or modified) source of C production was active in the early Universe.”
CARBON-ENHANCED METAL-POOR STARS IN SDSS/SEGUE. I. CARBON ABUNDANCE ESTIMATION AND FREQUENCY OF CEMP STARS (2013, background): “It appears that our derived value of the CEMP frequency (8%±1% for [Fe/H]≤ −2.0 and [C/Fe]≥ +1.0) is closer to those of Cohen et al. (2005) and Frebel et al. (2006) than to that of Lucatello et al. (2006), but again, one must recall the possible selection effect in making this comparison.”
CARBON-ENHANCED METAL-POOR STAR FREQUENCIES IN THE GALAXY: CORRECTIONS FOR THE EFFECT OF EVOLUTIONARY STATUS ON CARBON ABUNDANCES (2014, background): “Even though the sample sizes are considerably smaller than ours, we find that the cumulative CEMP star frequencies (for stars with [C/Fe] > +1.0 and [Fe/H] −2.0) from Lucatello et al. (2006) (21%±2”
Wind Roche-lobe overflow: Application to carbon-enhanced metal-poor stars (2013, methodology): “The observed CEMP to VMP ratio is approximately 20% (for example: 25% Marsteller et al. 2005; 9 ± 2% Frebel et al. 2006; 21 ± 2% Lucatello et al. 2006), with the fraction of CEMP stars rapidly increasing for decreasing iron content and for increasing distance from the Galactic plane (Carollo et al.…”