ABUNDANCE PATTERNS IN RED-GIANT CH STARS

Abstract

ABSTRACT A detailed abundance analysis of seven red-giant CH stars has been conducted. Using high resolution echelle and coude spectrographic data and modern analysis techniques involving model stellar atmospheres and computer codes which model LTE radiative transfer, the abundances of carbon, nitrogen and oxygen, the carbon isotope ratios and the abundances of the s-process elements have been determined. The red-giant CH stars are a class of metal-poor giants with equivalent spectral types of G and K which show enhanced abundances of s-process elements. Studies of their kinematics indicate that the red-giant CH stars are members of the halo of our galaxy (Hartwick & Cowley 1985). Therefore, the CH stars provide valuable information about the early chemical evolution of our galaxy. In addition, it is known that the red-giant CH stars are members of binary systems like the barium stars (McClure 1984), so they are also valuable in the study of mass transfer. The carbon isotope ratios are found to break the red-giant CH stars into two groups--those with 12C 13C >/= 20 and those with 12C 13C ~4 (a value associated with equilibrium burning of the CN cycle) and carbon and nitrogen abundances are found to be greatly enhanced, [C+N/Fe] ~+1.0. In addition, it is found that the red-giant CH stars show large enhancements of the s-process elements with the enhancements of the heavy s-process peak elements (Ba through Sm) being larger than the light s-process peak elements (Sr, Y and Zr) by a factor of approximately six. Taken with the orbital data available for the red-giant CH stars, the carbon, nitrogen and oxygen abundances and the carbon isotope ratios are consistent with a picture in which carbon and s-process elements were dumped onto the incipient red-giant CH star from an AGB star. The accreted material is mixed deep enough into the star's interior to be subjected to CN burning. In addition, the greatly enhanced s-process abundances stars strongly suggest that the 13C (alpha ,n)16O reaction is the source of neutrons for the s-process. In addition, the abundance patterns in the globular-cluster CH star, M22 III-106, have been analyzed. It is found to have an abnormally large carbon abundance, but the C/O ratio is much less than unity and the star shows no evidence for enhanced abundances of the s-process elements. Therefore, M22 III-106 is most likely not a CH star in the sense that term is applied to galactic CH stars. The most probable explanation for the carbon overabundance in M22 III-106 is that CN processing has not been as complete in this star as in other giants of similar composition in M22.