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D. Harbeck (MPIA), G. H. Smith (UCSC), E. K. Grebel (MPIA)
Globular clusters (GCs) are the oldest know star clusters known in the universe. Although star clusters seem to form from single gas clouds and therefore all stars in such clusters should start with the same initial conditions (age, chemical composition), there is evidence for abundance spreads within GCs. Especially the CNO-cycle element abundances can show strong star-to-star variations in globular clusters. The CN molecule band at 3883Å\ is a good tracer of of these abundance varaitions, and a scatter in CN was found for many GCs along the red giant branch down to the main sequence turn-off (MSTO).
We will present results from deep spectroscopy of main sequence stars in the GC 47 Tuc obtained at ESO's Very Large Telescope with FORS2/MXU. We measured the CN band absorption strength S3839 for stars as faint as V=20~mag. For these faint stars the CNO-cycle energy production rate is reduced by a factor of at least 10 compared to the MSTO stars. The observed scatter in the CN molecule absorption strength even for the low mass stars (0.7 solar masses) in our sample is interpreted as evidence for an external origin of the abundance variations (such as primordial variations or self-pollution); stellar evolution seems not to be of significant importance on the main sequence since the CNO process that could alter the content of C, N, and O has vanished.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.