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[85.04] Photometry and Spectroscopy of Short-Period Binary Stars in Four Old Open Clusters

R. M. Blake (Pisgah Astronomical Research Institute), S. M. Rucinski (David Dunlap Observatory)

We have performed a spectroscopic and photometric study of six contact binary stars in four old open clusters, M67, Praesepe, NGC 6791 and NGC 752, in order to evaluate their suitability for measuring the distance to their respective clusters. The technique being tested uses the cosine Fourier coefficients of the light curves of the binary stars, and the mass ratios obtained spectroscopically, to provide distances to the binaries. The contact binary TX Cnc was used to obtain the distance to Praesepe, which we find to be (V - M_V)_o = 6.30 ±0.08, which is in good agreement with the values of V - M_V = 6.20 - 6.35 found in color-magnitude diagram (CMD) studies. Our spectroscopic study of QX And in NGC 752 provided a distance modulus of (V - M_V)_o = 8.30±0.07 for this cluster. This compares to a value of (V - M_V)_o = 7.9 ±0.1 obtained by Milone et al. (1995) using the same star, but is in good agreement with V - M_V = 8.25 ±0.10 obtained by Daniel et al. (1994) from the CMD. We obtained a distance modulus of (V - M_V)_o = 12.71 ± 0.44 for V7 in NGC 6791, the oldest cluster in our survey. This agrees within ~1\sigma the values of 13.3 \leq (V - M_V) \leq 13.42 obtained by isochrone fitting of the cluster CMD. EV Cnc in M67 did not yield a distance to the cluster from our procedure because of faintness of the system resulting in poor spectroscopic data.

The distances to the clusters as determined from the contact binaries using our procedure do not seem to give systematically smaller or larger distances as compared to CMD fitting. However, we have measured the distances to only three clusters, and only one object per cluster, and so establishing any systematic differences may require a larger survey.

R. M. Blake acknowledges the support of the Natural Science and Engineering Research Council of Canada through grants to S. M. Rucinski and C. T. Bolton.

The author(s) of this abstract have provided an email address for comments about the abstract: mblake@pari.edu

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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.