BV CCD PHOTOMETRY OF THE OLD OPEN CLUSTER NGC 2243
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1991
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Abstract
The color-magnitude diagram (CMD) and luminosity function (LF) of two overlapping fields in NGC 2243, reaching approximately 4 mag below the main-sequence turnoff point, are presented. The photometry has been calibrated using stars from the lists of Landolt and Graham, and is independent of previous work on this cluster. A small clump of horizontal branch stars is evident in the CMD, as is a noticeable gap at V = 16.1 near the turnoff point, but the most striking feature of the CMD is a well defined and populous binary star sequence. Previous estimates of the cluster's metal abundance suggest that it is slightly lower than that of the globular cluster 47 Tuc. A direct comparison of our CMD with the fiducial sequence of 47 Tuc from Hesser et al. [PASP, 99, 739 (1987)], taking into account the slight difference in reddening between the two clusters, produces a good match for the horizontal branch and the unevolved portion of the main sequence. However, because the giant branch of NGC 2243 is redder than that of 47 Tuc, we are led to believe that the open cluster has a slightly higher metal abundance. Assuming the two clusters to have the same abundance parameters, the distance modulus is estimated at (m - M) v = 13.05 from fits to a semiempirical main sequence, and by requiring the two horizontal branches to have the same luminosity. Comparisons with isochrones in the metallicity range - 0.78 less-than-or-equal-to [Fe/H] less-than-or-equal-to 0.47, computed from the latest oxygen enhanced evolutionary sequences, imply a cluster age of 5 +/- 1 Gyr. While all of the isochrones in this metallicity range provide similar fits through the turnoff region, only the [Fe/H] = - 0.47, [O/Fe] = + 0.23 isochrones reproduce the location of the giant branch. The observed LF has a rather flat mass spectrum, with x almost-equal-to - 0.5 in model LFs providing a reasonable fit to the observations. The small gap at V = 16.1 may be associated with the development of the convective hook which follows central hydrogen exhaustion in standard evolutionary models. However, the isochrones neither predict its location correctly nor do they reproduce the morphology of the turnoff region. These discrepancies cannot adequately be explained as the result of the convergence of the single and binary star sequences near the turnoff. Moreover, structure in the observed LF near the turnoff cannot be reproduced even when the binary sequence contribution is included. We suggest that convective overshooting in the cores of moderate mass stars is the probable cause.