Contact Binaries of the Galactic Disk: Comparison of the Baade's Window and Open Cluster Samples

Abstract

The paper integrates the available data for contact binaries of the disk population in a deep galactic field and in old open clusters. The two basic data sets consist of 98 systems in the volume-limited 3 kpc sub-sample of contact binaries detected by the OGLE microlensing project toward Baade's Window (BW3) and of 63 members of 11 old open clusters (CL). Supplementary data on the intrinsically bright, but spatially rare, long-period binaries are provided by 238 systems in the BW sample to the distance of 5 kpc (BW5). The basic BW3 sample and the CL sample are remarkably similar in the period, color, luminosity and variability-amplitude distributions, in spite of very different sample selections. The contact systems are found in the color interval 0.3 < (B-V)0 < 1.2 where the turn-off points (TOP) of the considered clusters are located; however, they are not concentrated at the respective TOP locations but, once the TOP happens to fall in the above color interval, they can appear anywhere within it. The luminosity function for the BW sample appears to be very similar in shape to that for the solar neighborhood main-sequence stars when corrections for the galactic disk structure are applied, implying a flat apparent frequency-of-occurrence distribution. In the interval 2.5 < M_V < 7.5, the frequency relative to MS stars equals about 1/130 for the exponential disk length scale h_R = 2.5 kpc and about 1/100 for h_R = 3.5 kpc. (abridged)