CARBON AND NITROGEN ABUNDANCES IN EXTREMELY METAL-DEFICIENT RED GIANTS

• Published in: Publications of the Astronomical Society of the Pacific Vol. 94; no. 557; pp. 55 - 66
• Main Authors: KRAFT, ROBERT P., SUNTZEFF, NICHOLAS B., LANGER, G. E., CARBON, DUANE F., TREFZGER, CHARLES F., FRIEL, EILEEN, STONE, REMINGTON P. S.
• Format: Journal Article
• Language: English
• Published: The Astronomical Society of the Pacific 01-02-1982
• Subjects: Carbon; Carbon stars; Estimation methods; Giant stars; Globular star clusters; Metal abundance; Metal poor stars; Nitrogen; Stars; Subgiant stars
• ISSN: 0004-6280; 1538-3873
• DOI: 10.1086/130941

Low-resolution image-dissector scanner spectra have been obtained for 64 metal-poor ([Fe/H] ≾ — 1.4) halo giants selected from Bond's objective prism survey. Metallicities estimated from H and Ê line strengths, following Suntzeff, are shown to agree well with those derived by Bond on the basis of the Strömgren photometric index m₁ Estimates of carbon and nitrogen abundances were obtained from a comparison of observed and synthetic spectra in the regions of the NH(~ λ3360) and CH(~ λ4300) bands, respectively, following the work of Carbon et al. Abundance estimates [C/Fe] and [N/Fe] for halo field stars have been obtained using the same abundance methodology and using the same spectrographic equipment as had been employed in the study of 89 red-giant members of metal-poor globular clusters M3, M13, and M92. Thus systematic errors in abundances between cluster and field halo stars are suppressed and are shown to be very small. The halo sample was divided into Group I ([Fe/H] ≥ — 2.0) and Group II ([Fe/H] < — 2.0) stars, corresponding, respectively, to M3, M13 (Group I), and M92 (Group II). The distributions of [C/Fe] and [N/Fe] in Group I were found to mimic M3 but not M13: on the average M13 stars show more carbon depletion and nitrogen enhancement than do M3 or halo field stars. Group II stars on the average were found to show less carbon depletion and nitrogen enhancement than M92 stars: for example, some 40% of M92 giants show [N/Fe] > + 0.6 whereas no halo field giants have [N/Fe] above this value. Thus with respect to C and Í abundance distributions, globular cluster and field halo giants having the same [Fe/H] do not give the impression that they are samples of stars drawn from the same parent population. Reasons for these anomalies are discussed and hypotheses based on global vs. local considerations are compared. A picture suggesting that halo field stars of Group I (and possibly II) are the remains of dissolved clusters which exhibited the second parameter effect, as does M3, is found to have attractive possibilities, but also several difficulties.