Abundances in CH Subgiants: Evidence of Mass Transfer onto Main-Sequence Companions

Carbon, oxygen, and s-process abundances, as well as abundances of other metals, such as iron, calcium, or nickel, are presented for a sample of nine CH subgiants. The abundances are derived from high-resolution, high-S/N spectra. The CH subgiants in this sample are found to have mild metal-deficiencies (Fe/H ≈ 0.0 to -0.4), a range of s-process overabundances (s-process/Fe ≈ +0.2 to +1.0), and C/O ratios from 0.4 to 2.0. When compared to the barium giant stars, the CH subgiants tend to have larger C/O ratios, however, the barium giants have deep convective envelopes that are presumably absent from the CH subgiants, which lie on, or near, the main sequence. As the CH subgiants evolve up the red giant branch and develop deep convective envelopes, their carbon-rich atmospheres will be mixed with material less abundant in C and their C/O ratios will decrease. It is thus quite probable that the majority of the barium giant stars have evolved from CH subgiant progenitors. The heavy-element abundance distributions of the CH subgiants studied here, as well as published analyses of other barium and CH giant stars, indicate that the s-process abundance enhancements are created by neutrons from the 13C(α, n)16O neutron source.