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1998AJ....115.1500K - Astron. J., 115, 1500-1515 (1998/April-0)
Proton capture chains in globular cluster stars. III. Abundances of giants in the second-parameter globular cluster NGC 7006.
KRAFT R.P., SNEDEN C., SMITH G.H., SHETRONE M.D. and FULBRIGHT J.
Abstract (from CDS):
Plots of [Na/Fe] versus [O/Fe] and [Al/Fe] versus [Mg/Fe] indicate that the amount of proton-capture-processed material that has been mixed into the envelopes of the NGC 7006 giants is less extreme than within some giants of the globular clusters M13 and M15. In particular, unlike the NGC 7006 giants, most giants in M13 and some in M15 exhibit significantly reduced [O/Fe] and [Mg/Fe] ratios and very high [Al/Fe]. This difference in the [Mg/Fe] behavior may be related to differences in horizontal-branch morphology between M13 and NGC 7006. Within some M13 giants, it is possible that deep mixing has accessed regions in which not only has Mg been converted to Al but significant amounts of helium have also been produced from hydrogen burning. Deep mixing in such stars may not only significantly reduce the surface [Mg/Fe] ratio but may also increase the [He/H] ratio by amounts that will affect the zero-age horizontal-branch (HB) position that a giant will eventually occupy. This could explain the extension of the M13 HB to very blue colors. By contrast, giants in NGC 7006 experience modest amounts of interior mixing that may be deep enough to detectably alter the surface [Al/Fe] ratio, but not the [Mg/Fe] and [He/H] ratios. These stars will have a relatively low [He/H] ratio and so will adopt relatively red positions when they evolve onto the horizontal branch; the HB of NGC 7006 does indeed comprise a large percentage of stars redder than the RR Lyrae gap. In this scenario, deep mixing within cluster red giants may be one of the ``second parameters'' that affects horizontal-branch morphology.
Abstract Copyright: ∼
Journal keyword(s): Galaxy: Globular Clusters: Individual: NGC Number: NGC 7006 - Stars: Abundances - Stars: Evolution
Simbad objects: 17
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