Subgiant CH stars. II. Chemical compositions and the evolutionary connection with barium stars

R. Earle Luck, Howard E. Bond

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122 Scopus citations

Abstract

Through model-atmosphere analyses of high-resolution photographic echelle spectrograms, we have determined chemical abundances in 15 subgiant CH stars, five barium stars, and four objects that we classify as metal-deficient barium stars. The stars in each of these groups show enhanced surface abundances of carbon and the very heavy elements produced by s-process neutron-capture reactions. The atmospheric parameters of subgiant CH stars are found to be typical of F- and G-type main-sequence stars and subgiants. Barium stars are more luminous G- and K-type giants. Both classes of objects constitute ∼ 1 % of their respective populations, and exhibit a high incidence ( ∼ 100%) of membership in wide spectroscopic binaries. Subgiant CH stars occur on the main sequence, but only in the range 6000 K ≤ Teff ≤ 6800 K. Among hotter stars, diffusive processes may mask the peculiarities of subgiant CH stars, but there does appear to be a true scarcity of such objects among low-mass dwarfs. The degree of enhancement of s-process elements, and the s-process neutron exposure, show similar ranges among subgiant CH and barium stars. The metal-deficient barium stars appear to be the Population II analogs of the classical barium red giants. The classical red-giant CH stars differ from the metal-deficient and classical barium stars in showing strong C2 bands (implying that C/O > 1) and much higher s-process neutron exposures. We argue that subgiant CH stars are created when an AGB star in a wide binary contaminates its main-sequence companion with carbon and s-processed material. Later, when the subgiant CH star evolves to the red-giant stage, it is seen as a barium star. The close similarities of the abundances and frequencies of occurrence of subgiant CH stars and barium stars imply that the deepening of the convective envelop when the star becomes a red giant does not dilute the peculiar abundances. Therefore, subgiant CH stars must have thick mantles of processed material accreted from their now-unseen companions. The required mass of accreted material is sufficient to move subgiant CH stars up the main sequence, accounting for the rarity of low-mass subgiant CH stars.

Original languageEnglish (US)
Pages (from-to)515-540
Number of pages26
JournalAstrophysical Journal, Supplement Series
Volume77
Issue number4
DOIs
StatePublished - Dec 1991

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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