Carbon abundances and 12 C / 13 C from globular cluster giants

The behaviour of the ∆ν=2 CO bands around 2.3 μm was examined by comparing observed and synthetic spectra in stars in globular clusters of different metallicity. Changes in the C/C isotopic ratio and the carbon abundances were investigated in stars from 3500–4900 K in the galactic globular clusters M71, M5, M3 and M13, covering the metallicity range from –0.7 to –1.6. We found relatively low carbon abundances that are not affected by the value of oxygen abundance. For most giants the C/C ratios determined are consistent with the equilibrium value for the CN cycle. This suggests complete mixing on the ascent of the red giant branch, in contrast to the substantially higher values predicted across this range of parameters by the current generation of models. We found some evidence for a larger dispersion of C/C in giants of M71 of metallicity [μ] = [M/H] = -0.7 in comparison with the giants of M3, M5 and M13, which are more metal deficient. Finally, we show evidence for lower C/C in giants of globular clusters with lower metallicities, as predicted by theory.