On the carbon abundance of subgiant stars in the globular cluster M 92

The authors find that >[C/Fe]< ≈ 0.0 in stars at the base of the subgiant branch and approaches +0.1 in stars near the main-sequence turnoff in the metal-poor globular cluster M92. The carbon abundance falls on the average by about 0.5 dex by the time stars reach M V 0 ≡ +1.5. These carbon depletions occur early enough and are approximately the right size to account for the large average nitrogen enhancement ([N/Fe] ≡ +0.65) observed in M92 subgiants near M V 0 ≡ +1.5. Although the present observations do not absolutely prove that C→N conversion and mixing alone control the C- and N-abundances seen in M92 giants, neither is there any longer a compelling reason to believe there were large primordial nitrogen abundance variations in M92. The mechanism responsible for the large (Δ[C/Fe] ≈ 1.0) carbon depletions observed as stars ascend the M92 giant branch to the red-giant tip remains unknown.