2004/02/19 by Jonathan R. Hargis, Eric L. Sandquist, Michael Bolte · 1 citation
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #Globular cluster #Hertzsprung–Russell diagram #Horizontal branch #Luminosity #Luminosity function #Mass segregation #Metallicity #Photometry (optics) #Stellar evolution #Stellar, planetary, and galactic studies #Subgiant #astro-ph
paper · pdf · doi:10.1086/386329
published as Astrophys.J. 608 (2004) 243-260 · 63 pages, 29 figures, accepted for ApJ. Quality of images are degraded; please e-mail lead author for high-quality PS/PDF preprint
arxiv created 2004/02/19 · openalex publication_date 2004/06/07 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/06
In this paper we present the V and I luminosity functions and color-magnitude diagrams derived from wide-field (23' × 23') BVI photometry of the intermediate-metallicity ([Fe/H] ~ -1.3) Galactic globular cluster M12. Using observed values (and ranges of values) for the cluster metallicity, reddening, distance modulus, and age, we compare these data with recent α-enhanced stellar evolution models for low-mass metal-poor stars. We describe several methods of making comparisons between theoretical and observed luminosity functions to isolate the evolutionary timescale information that the luminosity functions contain. We find no significant evidence of excesses of stars on the red giant branch, although the morphology of the subgiant branch in the observed luminosity function does not match theoretical predictions in a satisfactory way. Current uncertainties in T eff -color transformations (and possibly also in other physics inputs to the models) make more detailed conclusions about the subgiant branch morphology impossible. Given the recent constraints on cluster ages from the WMAP experiment, we find that good-fitting models that do not include He diffusion (both color-magnitude diagrams and luminosity functions) are too old (by ~1-2 Gyr) to adequately represent the cluster luminosity function. The inclusion of helium diffusion in the models provides an age reduction (compared with nondiffusive models) that is consistent with the age of the universe being 13.7 ± 0.2 Gyr.