Very Large Telescope spectra of carbon stars in the Large Magellanic Cloud and their metallicity dependence

Very Large Telescope (VLT) L-band spectra of six carbon stars in the Large Magellanic Cloud are presented. The stars show absorption bands at 3.1 mum (HCN and C2H2), and 3.8 mum, which is probably due to C2H2. Two LMC stars show strong 3.5 mum HCN absorption. The equivalent widths of the 3.1 mum and 3.8 mum bands are systematically larger in LMC carbon stars than in carbon stars in the solar neighborhood. Moreover, the ratio of the equivalent widths of the 3.8 and 3.1 mm bands is much larger in the LMC, suggesting a higher ratio of n(C2H2)/n(HCN). The stronger absorption bands are in contrast to the assumption that if the elemental abundances are scaled from the carbon star's abundances in the solar neighbor, the abundances of these molecules are less at lower metallicity. We argue for a systematically larger C/O ratio in LMC carbon stars. In the Galactic carbon stars n(C)/n(O) approximate to 1.05-1.1 on average; our chemical model shows that the stronger molecular bands in the LMC carbon stars could be explained with n(C)/n(O) > 1.2. The higher C/O ratio can also explain the higher ratio of n(C2H2)/n(HCN) in LMC stars than in the solar neighborhood.