Carbon-enhanced Metal-poor Stars. I. Chemical Compositions of 26 Stars

By Wak Aoki, Timothy C. Beers, N. Christlieb, J. Norris, S. Ryan, S. Tsangarides. 2006

376 citations recorded by the source corpus.

The chemical compositions of 26 metal-poor stars that exhibit strong CH and/or C2 molecular bands are determined. Twenty-two stars in our sample satisfy our definition for carbon-enhanced metal-poor (CEMP) stars based on the carbon abundance ratio ([C/Fe]) and the evolutionary status. In addition, we measure Na abundances for nine known carbon-enhanced stars. Combining our new sample with the results of previous work, we investigate the abundance and evolutionary status of a total of 64 CEMP stars. The following results are obtained: (1) All but one of the 37 stars with [Fe/H] ≥ -2.6 exhibit large excesses of barium ([Ba/Fe] > +0.5), while the other 27 stars with lower metallicity exhibit a large scatter in their barium abundance ratios (-1.2 < [Ba/Fe] < +3.3). (2) A correlation between the carbon and barium abundance ratios ([C/Fe] and [Ba/Fe]) is found in Ba-enhanced objects (comprising 54 stars), suggesting that the origin of the observed carbon excess in Ba-enhanced stars is nucleosynthesis in AGB stars, where the main s-process occurs. (3) The majority of the Ba-enhanced stars have -1.0 < [C/H] < 0.0, and a clear cutoff exists at [C/H] ~ 0, which we take as the limit of carbon enrichment by metal-poor AGB stars. The [C/H] values of Ba-normal stars are relatively low, with a wide distribution. (4) The difference in the distributions of evolutionary status between Ba-enhanced and Ba-normal CEMP stars suggested by our previous work is not statistically confirmed by the present, enlarged sample. (5) Excesses of Na are found in stars with extremely large enhancements of C, N, and Ba, suggesting efficient production of this element by AGB nucleosynthesis. The implications of these results on the origins of carbon in CEMP stars are discussed.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/68f08d4145384fc4c0074ffee2ceb5b3e3d125f9

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In Astronomy and Astrophysics

1. Timothy C. Beers
2. A. Karakas
3. Martin Asplund
4. A. Frebel
5. J. Norris
6. Roberto Gallino
7. C. Sneden
8. M. Walker

In Instrumentation

1. Timothy C. Beers
2. Martin Asplund
3. A. Frebel
4. M. Walker
5. A. Karakas
6. J. Norris
7. C. Sneden
8. R. Hirschi

Citations in the corpus

• The Dawes Review 2: Nucleosynthesis and Stellar Yields of Low- and Intermediate-Mass Single Stars (2014, neutral): “10 (e.g., Cohen et al. 2004; Sivarani et al. 2006; Jonsell et al. 2006; Aoki et al. 2007; Beers et al. 2007; Lucatello et al. 2011).”
• Near-Field Cosmology with Extremely Metal-Poor Stars (2015, methodology): “An incomplete list of such stars, with [Fe/H] ∼ < − 3.0, includes the following: 3.9.1 Divergent α-Element Abundances • The Mg-enhanced BS 16934-002, with [Fe/H] = −2.8, [Si/Fe] = +0.44, and [Ca/Fe] = +0.35, but [Mg/Fe] = +1.23 (Aoki et al. 2007b).”
• The s process: Nuclear physics, stellar models, and observations (2010, methodology): “Na in CS 31062-050, for which a single line has been detected (Aoki et al., 2007), is lower by approximately 1 dex than the model predictions for the M = 1.5M⊙ case (Fig.”
• HIGH-RESOLUTION SPECTROSCOPY OF EXTREMELY METAL-POOR STARS FROM SDSS/SEGUE. I. ATMOSPHERIC PARAMETERS AND CHEMICAL COMPOSITIONS (2012, methodology): “The fraction of CEMP-no stars among all CEMP stars is ∼ 20%, and increases with decreasing stellar metallicity (Aoki et al. 2007; Hollek et al. 2011).”
• The most metal-poor damped Lyα systems: insights into chemical evolution in the very metal-poor regime★ (2011, background): “…been suggested that their carbon-enhancement reflects the composition of the cloud of gas from which the CEMP star first condensed (Ryan et al. 2005; Aoki et al. 2007); and (5) the observed fraction of CEMP stars in the outer halo component of our Galaxy is roughly twice that of the inner halo…”
• THE MOST METAL-POOR STARS. II. CHEMICAL ABUNDANCES OF 190 METAL-POOR STARS INCLUDING 10 NEW STARS WITH [Fe/H] ⩽ −3.5,, (2012, methodology): “CEMP Objects We adopted the Aoki et al. (2007) definition for CEMP stars, which accounts for nucleosynthesis and mixing in evolved giants, namely (i) [C/Fe]≥ +0.70, for log(L/L⊙) ≤ 2.3 and (ii) [C/Fe]≥ +3.0− log(L/L⊙), for log(L/L⊙) > 2.3.”
• HIGH-RESOLUTION SPECTROSCOPY OF EXTREMELY METAL-POOR STARS IN THE LEAST EVOLVED GALAXIES: URSA MAJOR II and COMA BERENICES (2009, methodology): “Overall, C excesses in the most metalpoor stars point toward the important role of C in the early universe and as a potential cooling agent of the primordial ISM (Beers & Christlieb 2005; Aoki et al. 2007a; Frebel et al. 2007a).”
• THE s-PROCESS IN ASYMPTOTIC GIANT BRANCH STARS OF LOW METALLICITY AND THE COMPOSITION OF CARBON-ENHANCED METAL-POOR STARS (2011, background): “In fact, Aoki et al. (2007) classified both CS 22960-053 and CS30322-023 as CEMP-s stars following Ryan et al. (2005).”
• The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models (2011, methodology): “The most metal−poor stars among CEMP-s and CEMP-s/r are CS 22960–053 (Aoki et al. 2007), CS 30322–023 (Aoki et al. 2007; Masseron et al. 2006), HE 1005–1439 (Aoki et al. 2007; Schuler et al. 2008), HE 1410–0004 (Cohen et al. 2006), SDSS 0126+06 (Aoki et al. 2008), and SDSS J1349– 0229 (Behara et…”
• THE MOST METAL-POOR STARS. IV. THE TWO POPULATIONS WITH [Fe/H] ≲ −3.0 (2012, background): “First, the suggestion of a continuous distributionof [C/Fe] in Figure 1 is in some contrast to what one sees in Figure 3 ([C/Fe] vs. [Fe/H]) and Figure 5 ([C/Fe] vs log(L/L⊙) of Aoki et al. (2007a), where clear separations are evident.”
• Neutron-Capture Elements in the Early Galaxy (2008, background): “On the other hand, similar to CS 22880-074 (Aoki et al. 2007a), 13 other CEMP-s stars do not show any r-process-enhancement, including HE 0024-2523 (Lucatello et al. 2003), HE 2158-0348 (Cohen et al. 2006), and HE 0202-2203 (Barklem et al. 2005).”
• A grid of MARCS model atmospheres for late-type stars. I. Methods and general properties (2008, background): “…is a group of low-metallicity stars with considerably enriched carbon abundances, so-called carbon-enriched metal-poor (CEMP) stars, (see, e.g., Aoki et al. 2007), which may show carbon enrichments ranging in the interval between a factor of 10 and 1000 relative to a solar C/Fe, as well as…”
• Wind Roche-lobe overflow: Application to carbon-enhanced metal-poor stars (2013, background): “The largest group of CEMP stars are the s-process rich CEMP-s stars, which display barium enhancements of [Ba/Fe] > +0.5 and account for at least 80% of all CEMP stars (Aoki et al. 2007).”
• CARBON-ENHANCED METAL-POOR STARS IN SDSS/SEGUE. I. CARBON ABUNDANCE ESTIMATION AND FREQUENCY OF CEMP STARS (2013, methodology): “As discussed by Lucatello et al. (2006), Spite et al. (2006), and Aoki et al. (2007), the apparent lack of an increase in the frequency of CEMP stars among giants could well be associated with extra mixing of CNO-processed material from their interiors, diluting the C-rich material in their…”
• DETAILED CHEMICAL ABUNDANCES IN THE r-PROCESS-RICH ULTRA-FAINT DWARF GALAXY RETICULUM 2 (2016, methodology): “About 20 r-II stars are known, and all are located in the halo, with the exception of one star in UMi (Shetrone et al. 2001; Sadakane et al. 2004; Aoki et al. 2007b) and one in the Milky Way bulge (Johnson et al. 2013).”
• Binarity in carbon-enhanced metal-poor stars (2014, background): “…authors prefer a limit that is dependent on the luminosity of the star to take into account internal mixing of carbon on the red giant branch (e.g. Aoki et al. 2007). c© 0000 RAS low in this paper (note that the different classes as defined here are not necessarily mutually exclusive): • CEMP-r:…”