Astronomy and Astrophysics – Astronomy
Scientific paper
May 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...462..255d&link_type=abstract
Astrophysical Journal v.462, p.255
Astronomy and Astrophysics
Astronomy
14
Galaxy: Globular Clusters: Individual Messier Number: M22, Infrared: Stars, Stars: Evolution, Stars: Luminosity Function, Mass Function
Scientific paper
Moderately deep J and K' images, recorded during subarcsecond seeing conditions, are used to investigate the near-infrared color-magnitude diagram (CMD) of the heavily reddened metal-poor globular cluster M22 (NGC 6656). The CMD constructed from these data extends from K = 7 to K = 18, and includes the upper portions of the main sequence. The main-sequence turnoff (MSTO) occurs at K = 15.1 and J - K = 0.41 values that are well matched by metal-poor 12 Gyr isochrones from Bergbusch & VandenBerg and Straniero & Chieffi after adopting canonical values for the cluster distance and reddening. For comparison, 16 Gyr isochrones are unable to match the brightness and color of the MSTO, although we emphasize that uncertainties in our current knowledge of stellar structure and evolution make any estimate of absolute ages tentative. We also compare the M22 CMD with that obtained for M13 by Davidge & Harris. After correcting for differences in distance, reddening, and [Fe/H], we find that the brightnesses of the subgiant branch (SGB) and MSTO in M22 and M13 are not significantly different. However, the M22 giant branch is steeper than that in M13, a finding contrary to the relative metallicities of these clusters, but consistent with earlier optical observations by Hesser, Hart- wick, & McClure. In particular, the lower giant branch of M22 falls blueward of M13, although the upper giant branches coincide. We suggest that the overall giant branch slope at near-infrared wavelengths may be sensitive to parameters other than age and [Fe/H]. Finally, we investigate the mass function of stars with masses between 0.5 and 0.8 Msun. The data are consistent with a relatively flat mass function above 0.6 Msun, a result broadly consistent with what would be predicted based on the mean metallicity and location of M22 in the Galaxy.
Davidge Tim J.
Harris William E.
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