AAS 201st Meeting, January, 2003
Session 108. Old Stellar Populations
Oral, Wednesday, January 8, 2003, 2:00-3:30pm, 618-619

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[108.04D] Chemical Inhomogeneities in the Old Stellar Populations of the Local Group

D.-R. Harbeck (MPIA)

I present a survey for spatial population gradients in nine Local Group (LG) dwarf spheroidal (dSph) galaxies, as traced by different morphologies of the horizontal branch (HB) and the red giant branch (RGB). I find that dSphs often show radial gradients in the sense that the red HB stars are more centrally concentrated than the blue HB stars. The same is true for the RGB, indicating that population gradients in the dSphs are correlated with metallicity gradients. The role of other "internal second parameters" like age seems to be less important. The population gradients indicate a complex star formation history of these galaxies and suggest that self-enrichment during their star forming epoch was most efficient in the centers of the galaxies, where the potential well is deepest. In order to investigate the role of deep mixing in RGB stars as a "second parameter" of the HB morphology I use CN molecule absorption bands as a tracer of mixing processes. For second parameter globular clusters (GCs) with [Fe/H]~-1.6 dex, including NGC 7006, no correlation between the HB morphology and the fraction of CN-enhanced and CN-normal stars can be found, suggesting that CN variations in GCs do not directly reflect significant differences in the evolutionary path of RGB stars. In order to investigate the true origin of CN abundance variations in GCs, I present CN measurements for stars 2.5 below the main-sequence turn-off in 47 Tuc. I find significant bimodal differences in CN absorption strength for the faint stars in the sample. These low-mass stars can not sustain CNO hydrogen burning, which together with mixing processes could alter the relative C and N surface abundance. I conclude that internal stellar evolution cannot be the only origin of CN variations in 47 Tuc. External pollution, e.g., by intermediate mass AGB stars, is favoured.

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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.