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M. M. Briley (U. Wisc. Oshkosh), D. Harbeck (U. Wisc. Madison), G. H. Smith (UCSC)
We report on an analysis of CN and CH band strengths of 42 bright (Mv < 0) giants in the globular clusters M13, M10, and NGC 7006. The clusters are all of similar metallicity ([Fe/H] = -1.5), however NGC 7006 is a classic second parameter cluster with a far redder HB morphology compared to M13 and M10. Our spectra, taken over several observing campaigns at the Lick 3-m Shane and McDonald 2.7-m Smith telescopes, have been flux calibrated and the resulting C and N abundances compare well for stars in common with previous studies in the literature.
In all three clusters significant (approximately 1 dex) depletions of carbon with increasing evolutionary state are evident. At the same time, large (0.3 dex or more) star to star variations in carbon are also present. An even larger (1 dex) scatter exists among the nitrogen abundances of the least luminous program stars and a tendency towards high nitrogen abundance stars with greater luminosity is observed.
We interpret these results as the operation of an extra-mixing process during the later stages of RGB ascent in which C-poor/N-rich material is mixed to the surface from a region just above the H-burning shell. The extent of mixing among the giants of NGC 7006 appears comparable to that of both M13 and M10, despite its redder HB morphology. Moreover, as has been demonstrated by Briley, Cohen & Stetson (2004 AJ, 127, 1579) for M13, the intrinsic scatter in C and N abundances of the giants in both M10 and NGC 7006 is also consistent with the presence of primordial star to star abundance differences that are being modified by this extra mixing. Partial support for this work was provided by the NSF under grant AST-0098489 and the UWO Faculty Development Program (to MMB).
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Bulletin of the American Astronomical Society, 36 #2
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