Aluminum-Oxygen Abundance Anticorrelations and Deep-Mixing Scenarios for Globular-Cluster Giants

摘要

We explore the conditions under which large aluminum enhancements might be produced by very deep mixing and proton-capture nucleosynthesis in bright globular cluster giants. We find that: (1) The initial abundances of neutron-rich magnesium seed nuclei must be surprisingly large for stars that belong to an old, metal-poor population. [Al/Fe] overabundances near 1.0 dex suggest that the initial 25Mg/24Mg ratio must be almost four times as large as it is in the Sun. If 25Mg and 26Mg isotopes make equal contributions to the aluminum overabundance, both ratios must be about twice as big as they are in the Sun. (2) Cluster giants with large aluminum enhancements (-1.0 dex) produced by very deep mixing should show smaller but observable Mg depletions (-0.2 dex) as a result of the destruction of 25Mg and 26Mg. (3) Large aluminum enhancements are very likely to be accompanied by significant hydrogen depletions and helium enhancements throughout the stellar envelope. Adding hydrogen from the envelope to the hydrogen burning shell may have an observable impact on giant-branch evolution; a helium-enriched envelope will surely have an important impact on the star's later location on the horizontal branch.