The production of surface carbon depletions among globular cluster giants by interior mixing

Abstract

Published measurements of the carbon abundances of giants in several metal-poor globular clusters have indicated that [C/H] decreases progressively from the base to the tip of the giant branch by a total of 1.0 dex or more. Standard stellar models indicate that throughout much of giant branch evolution the convective envelope of a cluster star is not expected to be in contact with regions near the nuclear-burning shell wherein the |$\text C\to\text N$| process of hydrogen burning is taking place. The observations indicate, therefore, that transport of material must take place between the base of the convective envelope and the hydrogen-burning shell throughout much of the giant branch phase of evolution. A carbon abundance gradient of |$d[\text {C\H}]/dM_V \approx 0.25 \enspace \text {dex} \enspace \text {mag}^{-1}$| is observed on the giant branch of the cluster M92 by Carbon et al. We find that this gradient can be accounted for if circulation currents having speeds |$\sim 10^{-2}\enspace \text {cm}\enspace \text s^{-1}$| within the M92 giants are cycling material from the convective envelope down into the proximity of the hydrogen-burning shell, where it is processed and subsequently returned to the envelope. Giant branch mixing may be capable of producing significant modifications from the initial C and N abundance distribution of cluster stars. In order to account for the existence of CN-strong stars on the upper giant branches of globular clusters, it may be necessary to infer that the atmospheres of such stars have undergone substantial |$\text O \to \text N$| processing.