Proton Capture Chains in Globular Cluster Stars. III. Abundances of Giants in the Second-Parameter Globular Cluster NGC 7006*

摘要

High-resolution spectra have been obtained of six red giants in the globular cluster NGC 7006 using the HIRES instrument on the Keck I Telescope. The [iron-peak/Fe] and [α-nuclei/Fe] elemental ratios are similar to those found in other halo globular clusters. Significant, but modest, star-to-star abundance differences are found for the elements O, Na, and Al, with the [O/Fe] abundance ratio being anticorrelated with both [Na/Fe] and [Al/Fe]. These anticorrelations indicate that O → N processed material, within which Ne has also been converted to Na and Mg to Al by proton addition reactions, has been brought to the surface of the cluster giants via a deep-mixing mechanism that cycles material through the O → N burning zone just outside the main hydrogen-burning shell. Despite the significant [Al/Fe] differences among the NGC 7006 giants, there is very little accompanying difference in [Mg/Fe]. This result, however, is not inconsistent with a deep-mixing origin for the [Al/Fe] enhancements; because Mg is initially much more abundant than Al, conversion of only a small fraction of Mg into Al can still produce a large percentage increase in the Al abundance. 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.