1996A&A...306..489K

In an attempt to increase the sample of metal-deficient late-type (i.e. cool) halo carbon stars analysed (only 3 such stars have previously been analysed spectroscopically), we obtained high-resolution visual spectra of 5 more candidates (and analysed in addition existing archive IR spectra for one of the stars) from the recent catalogue of Sleivyt e & Bartkevicius (1990), HD 25408 (C5,3J), HD 42272 (C5,4 CH), HD 59643 (C6,2CH), HD 189711 (C4,3 CH) and HD 197604 (C4,2 CH). From the spectra we have derived C/O ratios, N/C ratios, and metal abundances. If the oxygen abundance was fixed at logA(O)=7.4/8.3 (assuming that it follows the trend of oxygen overabundance relative to iron found in halo stars in general) we can furthermore derive [C/Fe] and [N/Fe]. 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. Two of the stars, HD 25408 and HD 42272, turned out not to be CH stars. The other three stars, although late-type, showed the C/O and [Fe/H] ratios common in early-type CH stars. From the total sample of the six confirmed cool halo-CH stars now analysed, we find evidence that not all metal-poor low mass halo carbon stars can have formed due to mass transfer in binary systems, as is usually assumed. At least 3 of the stars, and possible more, are likely to have formed as intrinsic carbon stars, with some similarities to the carbon star population in the dwarf galaxies.