Carbon Stars in the Hamburg/ESO Survey: Abundances
Abstract
We have carried out a detailed abundance analysis using high-dispersion spectra from HIRES at Keck for a sample of 16 carbon stars found among candidate extremely metal-poor (EMP) stars from the Hamburg/ESO Survey (HES). We find that the Fe metallicities for the cooler C stars (T_(eff) ~ 5100 K) have been underestimated by a factor of ~10 by the standard HES tools. The results presented here provided crucial supporting data used recently by Cohen et al. to derive the frequency of C stars among EMP stars. C enhancement in these EMP C stars appears to be independent of Fe metallicity and approximately constant at ~1/5 the solar ϵ(C). The C enhancement shows some evidence of decreasing with decreasing T_(eff) (increasing luminosity), presumably due to mixing and dredge-up of C-depleted material. The mostly low ^(12)C/^(13)C ratios (~4) and the high N abundances in many of these stars suggest that material that has been through proton burning via the CN cycle comprises most of the stellar envelope. C enhancement in this sample is associated with strong enrichment of heavy nuclei beyond the Fe peak for 12 of the 16 stars. The remaining C stars from the HES, which tend to be the most Fe-poor, show no evidence for enhancement of the heavy elements. Very high enhancements of lead are detected in some of the C stars with highly enhanced Ba. The strong lead lines, the high Ba/Eu ratios, and the high ratios of abundances of the diagnostic elements in the first and second s-process peaks demonstrate that the s-process is responsible for the enhancement of the heavy elements for the majority of the C stars in our sample. The low ^(12)C/^(13)C ratios and large C and N enhancements of the EMP C stars are more extreme than those of intrinsic asymptotic giant branch C stars of near-solar Fe metallicity, but closer to the composition of CH stars. Our subsample of EMP C stars without s-process enhancement is reminiscent of the R-type C stars in the solar neighborhood; thus, we expect that they are formed by similar mechanisms. We suggest that both the s-process-enhanced and Ba-normal C stars result from phenomena associated with mass transfer in binary systems. This leads directly to the progression from C stars to CH stars and then to Ba stars as the Fe metallicity increases.
Additional Information
© 2006 The American Astronomical Society. Received 2005 December 13; accepted 2006 March 21. Based in part on observations obtained at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The entire Keck HIRES user community owes a huge debt to Jerry Nelson, Gerry Smith, Steve Vogt, and many other people who have worked to make the Keck telescope and HIRES a reality and to operate and maintain the Keck Observatory.We are grateful to the W. M. Keck Foundation for the vision to fund the construction of the W. M. Keck Observatory. We are grateful to W. Aoki for providing his Subaru HDS spectra of selected C stars to verify our ^(13)CH line list. We thank G. Wasserburg for helpful discussions and moral support. This publication makes use of data products from the TwoMicron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. J. G. C. is grateful for partial support from NSF grants AST 02-05951 and AST 05-07219. She is grateful for funds from the Ernest Fullam Award of the Dudley Observatory for help in initiating this work. N. C. and F. J. Z. acknowledge support from Deutsche Forschungsgemeinschaft through grant Re 353/44. N. C. is also supported by a Henri Chretien International Research Grant administered by the American Astronomical Society.Attached Files
Published - Cohen_2006_AJ_132_137.pdf
Accepted Version - 0603582.pdf
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Additional details
- Eprint ID
- 95655
- Resolver ID
- CaltechAUTHORS:20190521-160258047
- W. M. Keck Foundation
- NASA
- AST 02-05951
- NSF
- AST 05-07219
- NSF
- Dudley Observatory
- Re 353/44
- Deutsche Forschungsgemeinschaft (DFG)
- American Astronomical Society
- Created
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2019-05-21Created from EPrint's datestamp field
- Updated
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2021-11-16Created from EPrint's last_modified field