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

Creators: Meléndez, J.; Cohen, J. G.

Abstract

High-resolution (R ≈ 100,000), high signal-to-noise spectra of M71 giants have been obtained with High Resolution Echelle Spectrometer 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 ^(26)Mg/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 ^(26)Mg/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.

Additional Information

© 2009 The American Astronomical Society. Received 2009 January 8; accepted 2009 May 14; published 2009 June 26. The entire Keck/HIRES user community owes a huge debt to Jerry Nelson, Gerry Smith, Steve Vogt, and many other people who have worked to make the Keck Telescope and HIRES a reality and to operate and maintain the Keck Observatory. We are grateful to the W. M. Keck Foundation for the vision to fund the construction of the W. M. Keck Observatory. The authors wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented herein would have been possible. We are grateful to NSF grant AST-0507219 and FCT (project PTDC/CTE-AST/65971/2006 and Ciencia 2007) for partial support. We thank B. Plez for providing the CH line list, and D. Yong and W. Aoki for sharing their spectrum of M71 A4. This publication makes use of data from the 2MASS, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration and the National Science Foundation.

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