The Rise of the AGB the Galactic Halo: M_g Isotopic Ratios and High Precision Elemental Abundances in M71 Giants

J. Meléndez, J. G. Cohen

Abstract
High-resolution (R ≈ 100 000), high signal-to-noise spectra of M71 giants have been obtained with HIRES at the Keck I Telescope in order to measure their Mg isotopic ratios, as well as elemental abundances of C, N, O, Na, Mg, Al, Si, Ca, Ti, Ni, Zr and La. We demonstrate that M71 has two populations, the first having weak CN, normal O, Na, Mg, and Al, and a low ratio of ²⁶Mg/Mg (~4%) consistent with models of galactic chemical evolution with no contribution from asymptotic giant branch (AGB) stars. The Galactic halo could have been formed from the dissolution of globular clusters prior to their intermediate mass stars reaching the AGB. The second population has enhanced Na and Al accompanied by lower O and by higher ²⁶Mg/Mg (~8%), consistent with models which do incorporate ejecta from AGB stars via normal stellar winds. All the M71 giants have identical [Fe/H], [Si/Fe], [Ca/Fe], [Ti/Fe] and [Ni/Fe] to within σ = 0.04 dex (10%). We therefore infer that the timescale for formation of the first generation of stars we see today in this globular cluster must be sufficiently short to avoid a contribution from AGB stars, i.e. less than ~0.3 Gyr. Furthermore, the Mg isotopic ratios in the second M71 population, combined with their elemental abundances for the light elements, demonstrate that the difference must be the result of adding in the ejecta of intermediate mass AGB stars. Finally we suggest that the low amplitude of the abundance variations of the light elements within M71 is due to a combination of its low mass and its relatively high Fe-metallicity.

Keywords
stars: abundances – stars: atmospheres – stars: evolution – globular clusters: individual (M71)

Notes
Based on observations obtained at the W.M. Keck Observatory, which is operated jointly by the California Institute of Technology, the University of California and the National Aeronautics and Space Administration

The Astrophysical Journal
Volume 699, Page 2017
June 2009

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