CHEMICAL-COMPOSITION OF METAL-POOR CARBON STARS IN THE HALO

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 s pectra of 5 more candidates (and analysed in addition existing archive IR spectra for one of the stars) from the recent catalogue of Sleivyt t & Bartkevicius (1990), HD 25408 (C5,3J), HD 42272 (C5,4 CH), HD 5964 3 (C6,2CH), HD 189711 (C4,3 CH) and HD 197604(C4,2 CH). From the spect ra we have derived C/O ratios, N/C ratios, and metal abundances. If th e oxygen abundance was fixed at log A(O) = 7.4 divided by 8.3 (assumin g that it follows the trend of oxygen overabundance relative to iron f ound 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, C-2, HC N, C2H2 and C-3. Two of the stars, HD 25408 and HD 42272, turned out n ot to be CH stars. The other three stars, although late-type, showed t he 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 f ormed due to mass transfer in binary systems, as is usually assumed. A t least 3 of the stars, and possible more, are likely to have formed a s intrinsic carbon stars, with some similarities to the carbon star po pulation in the dwarf galaxies.