Spectroscopic analysis of two CH subgiant stars: HD 50264 and HD 87080

Abstract

We present the abundance pattern of two CH subgiant stars HD 50264 and HD 87080 based on high-resolution optical spectra. We also determined the spectroscopic stellar atmospheric parameters, temperature and microturbulent velocity as well as stellar surface gravity from a solution of excitation and ionization equilibria of Fe i and Fe ii lines under the assumption of local thermodynamic equilibrium. The abundance analysis reveals HD 50264 with a metallicity of $\rm [Fe/H]=-0.34$ and HD 87080 with a metallicity of $\rm [Fe/H]=-0.51$. We compare the abundance pattern with abundances of disk stars and also with other stars of the same class. We found that iron group, α -elements, manganese, as well as sodium and aluminum of HD 50264 and HD 87080 follow the abundance pattern of the disk stars. The heavy-element abundance pattern of both stars shows enhancements by a factor of 4–6 with respect to the sun. By heavy-element we mean the elements that have been synthesized by neutron capture, such as barium, yttrium and zirconium. We also discuss the abundances of the s-process elements and compare our results with other binary systems that display enrichment due to neutron-capture reactions, through several diagrams involving the index [hs/ls] and [s/Fe], the metallicity and [C/Fe] ratio. We compare the observed abundance of the heavy elements with theoretical nucleosynthesis calculations. This shows that the s-process are best fit by models in which the seed nuclei are exposed to single neutron irradiation and with a neutron exposure of $\tau=0.9$ and $\tau=1.0$ respectively for HD 50264 and HD 87080. We also use the [Rb/Zr] ratio in order to investigate how this ratio behaves versus metallicity for other binary systems, including HD 50264 and HD 87080 that are s-process enriched.