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

The paper attempts to integrate 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 selections, for BW3 – as a volume-limited sub-sample of all contact systems discovered by the OGLE project, and for CL – as a collection of contact systems discovered in open clusters which had been subject to searches differing in limiting magnitudes, cluster area coverage and photometric errors. 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 (MS) stars when corrections for the galactic disk structure are applied, implying a flat apparent frequency-of-occurrence distribution. In the accessible interval 2.5 < MV < 7.5, the frequency of contact binaries relative to MS stars equals about 1/130 for the exponential disk length scale hR = 2.5 kpc and about 1/100 for hR = 3.5 kpc. The high frequency cannot continue for MV < 2.5 as the predicted numbers of bright systems would then become inconsistent with the numbers of known systems to Vlim = 7.5 in the sky sample. The previous estimate of the frequency from the BW sample of 1/250 − 1/300 did not correctly relate the numbers of the contact binaries to the numbers of MS stars. The magnitude limit of the OGLE survey limits the accuracy of the current luminosity function determination for MV > 5.5, but the available data are consistent with a