The chemical composition and evolutionary state of the early stars

Abstract

A sample of 11 stars classified R0-R5 has been analyzed to determine (i) iron-peak metal abundances, (ii) abundances of elements produced by the s-process, (iii) the light element abundances (C,N, and O), and (iv)= the carbon isotopic ratio /sup 12/C//sup 13/C. Abundances are reported relative to the Sun. With the expection of HD 100764, which has an iron abundance 0.6 dex lower than the Sun's, the iron abundance of 10 stars is near solar. The s-process elements have no demonstrable enhancement in the sample, unlike the Ba II stars of the same temperature and the cooler carbon stars. The enhancement of carbon relative to the G-K giants found in the analysis of eight stars is 0.7 dex, with one star having Croughly-equal O (HD 113801), while another star (HD 95405) is probably not a carbon star (C/O = 0.87). Relative to the ordinary G-K giants, nitrogen is enhanced by a small amount (0.2 dex). In contrast, oxygen has nearly the value found in the Sun and in normal giants, except in the case of HD 100764 where O is probably as underabundant as Fe. The /sup 12/C//sup 13/C ratio ranges for near 4 to 14 for nine stars, indicating anmore » enrichment of /sup 13/C with respect to the majority of faint giants. For HD 156074 the oxygen isotopic ratios /sup 16/O//sup 17/O and /sup 16/O//sup 18/O do not have detectable departures from the solar system values. The lack of oxygen depletion indicates that the CNO cycle operating near equilibrium is not responsible for the fact that C/O>1 in these stars. Based on the early R stars' position in the H-R diagram, their space number densities, and their enhanced carbon abundances, it is here suggested that they result from mixing at the time of, or closely following, the helium core flash.« less