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Session 80 - Star Clusters in the Milky Way.
Display session, Friday, January 09
Exhibit Hall,

[80.06] A Catalog of Helium Abundance Determinations from Globular Cluster Photometry

E. Sandquist (Northwestern U.)

We provide a comprehensive list of re-determinations of the values for the helium abundance indicators R =N_HB/N_RGB, A (the exponent of the mass-luminosity relation for RR Lyraes), and \Delta (the magnitude difference between the HB and the main sequence at (B-V)_0 = 0.7) for globular clusters in the Milky Way and Magellanic Clouds as well as nearby dwarf spheroidals. This study corrects a prevalent error in the determination of R, and provides a more comprehensive error analysis for all three indicators. We attempt to correlate the indicators to identify anomalous abundances in spite of shortcomings in each indicator. From the R determinations, we identify 6 clusters which show high values, possibly as a result of environmental factors --- each cluster has a galactocentric radius of less than 7 kpc, and 5 of the 6 appear to be post-core-collapse. From A values, all indications point toward a constant helium abundance of Y\approx0.23 for virtually all Oosterhoff class I clusters. In general, uncertainties in reddening determinations must be reduced in order to improve the worth of the \Delta indicator, which is the most sensitive to helium.

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