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

Abstract

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 subsample 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 subsample of all contact systems discovered by the OGLE project, and for CL—as a collection of contact systems discovered in open clusters that 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 5.5, but the available data are consistent with a continuation of the high apparent frequency beyond MV = 7.5, i.e., past the current short-period, low-luminosity end, delineated by the shortest period field system CC Com at MV = 6.7. The current data indicate that the sky-field sample of contact binaries starts showing discovery-selection effects at a level as high as V 10–11.