AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 53. Star Clusters Near and Far, Old and Young
Display, Tuesday, June 1, 1999, 10:00am-7:00pm, Southeast Exhibit Hall

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[53.02] Carbon Abundances on the M92 Giant Branch

S. Bellman, M.M. Briley (U Wisc. Oshkosh), G.H. Smith (Lick Obs., UCSC)

Star clusters provide a snapshot of a group of stars nearly homogeneous in age and composition spread over a wide range of evolutionary states. By comparing elemental abundances among the stars in a cluster, changes in composition due to internal stellar processes are illuminated.

In this project, carbon abundances were determined for a number of red giant and subgiant stars in the very metal-poor globular cluster M92 (NGC 6341). Determination was accomplished by comparing the spectral index S(4243) of program stars to index values computed for synthetic spectra of stellar models. Analysis of these index values confirms a clear pattern of decreasing carbon abundance as a function of increasing evolutionary state among cluster members. Carbon depletion in more evolved stars is evidence of internal mixing thought to be connected with a discontinuity in the luminosity function.

In M92, carbon depletion appears to onset at absolute magnitude approximately 0.4 mag. below the previously derived (by Fusi Pecci et al., 1990) bump for a generalized metal-poor cluster. This implies a time lag between the onset of carbon depletion and the occurrence of the bump, and that the mixing of carbon-poor material to the surface is taking place earlier than originally thought.

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