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D.R. Alves (GSFC), K.H. Cook, E. Wishnow (LLNL)
Previously, we reported the first precise (10%) N-band photometry for red giants in the Galactic globular clusters (GCs) M3 and M13. These data are sensitive to the infrared emission from dusty circumstellar material. We found a difference in the infrared excesses between the red giants observed in M3 and M13, and suggested that mass loss, and not cluster age, was the likely 2nd parameter that affects the stellar populations in these GCs. We report here on our subsequent analysis and interpretation of this problem. 2 of 17 stars observed have large infrared excesses, and this is consistent with the ratio of the number of asymptotic giant branch (AGB) stars to red giant branch (RGB) stars near the maximum luminosity of red giants in GCs. The AGB stars lose mass more prolifically because mass-loss rates are inversely proportional to mass (as predicted theoretically and found empirically), and because AGB stars have lower masses than RGB stars in GCs (about 0.55 and 0.75 solar masses, respectively). Higher mass-loss rates yield larger infrared excesses from condensing dust. The AGB and RGB stars observed in M3 provide a key calibration between mass and infrared excess. The infrared excesses of the RGB stars in M3 and M13 thus imply masses of 0.75 and 0.70, and ages of 11.4 and 12.7 Gyr, respectively. This age difference may be the 2nd parameter. Although not yet rigorously quantitative, this analysis demonstrates a new way to estimate GC ages based on mass loss.
Data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. Support of this work has also been provided by the National Research Council.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.