1994A&A...290..148K

We have derived C, N and metal abundances for the metal-deficient late-type (C3,3CH) CH giant HD 187216 (α_2000.0=19h24m18.6s, δ_2000.0=+85deg21'56.5"). The oxygen abundance was fixed at logA(O)=7.0, assuming that it follows the trend of oxygen overabundance relative to iron found in halo stars in general. New model atmospheres of metal-poor carbon stars were calculated with continuum opacity sources and molecular lines of CO, CN, C₂, HCN, C₂H₂ and C₃. Numerical experiments with various assumed input parameters, such as effective temperature, T_eff, surface gravity, logg, microturbulent velocity, ξ_t, and dissociation energy of the CN molecule, D₀(CN), were performed when constructing the model atmospheres and calculating the synthetic spectra. The atmospheric model with T_eff=3500K, logg=0.4, ξ_t=3km/s, ¹²C/¹³C=8 and D₀(CN)=7.9eV was adopted for abundance analysis. The star was found to be extremely metal-deficient, [Fe/H]=-2.48. The carbon abundance is logA(C)=7.33, the nitrogen abundance is logA(N)=5.60 corresponding to [C/Fe]=+1.3, [N/Fe]=+0.2, and [N/C]=-1.1. The carbon isotopic abundance ratio is ¹²C/¹³C=7.0. The abundances of heavy elements produced in the s-process are larger than in early-type CH stars. The ratio of overabundance of heavier s-process elements to that of lighter ones, [hs/ls]=1.0, points to a very high neutron exposure in a single irradiation event. Search for binarity of HD 187216 has failed, and the star can be an intrinsic asymptotic giant branch (AGB) carbon star with some similarities to the C stars in the dwarf galaxies. If the CH characteristics are due to mass transfer it is most likely oxygen-rich material that has been donated. The star possesses both a low nitrogen abundance and a low ¹²C/¹³C ratio, in conflict with the standard stellar evolution theory.