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 held and in old open clusters. The t wo basic data sets consist of 98 systems in the volume-limited 3 kpc subsam ple of contact binaries detected by the OGLE microlensing project toward Ba ade's window (BW3) and of 63 members of 11 old open clusters (CL). Suppleme ntary data on the intrinsically bright, but spatially rare, long-period bin aries 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 th e period, color, luminosity, and variability-amplitude distributions, in sp ite 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 collect ion of contact systems discovered in open clusters that had been subject to searches differing in limiting magnitudes, cluster area coverage, and phot ometric 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 a re located; however, they are not concentrated at the respective TOP locati ons, but, once the TOP happens to fall in the above color interval, they ca n appear anywhere within it. The luminosity function for the BW sample appe ars to be very similar in shape to that for the solar neighborhood main-seq uence (MS) stars when corrections for the Galactic disk structure are appli ed, which implies a flat apparent frequency-of-occurrence distribution. In the accessible interval 2.5 < M-V < 7.5, the frequency of contact binaries relative to MS stars equals about 1/130 for the exponential disk length sca le h(R) = 2.5 kpc and about 1/100 for h(R) = 3.5 kpc. The high frequency ca nnot continue for M-V < 2.5 as the predicted numbers of bright systems woul d then become inconsistent with the numbers of known systems to V-lim = 7.5 in the sky sample. The previous estimate of the frequency from the BW samp le of 1/250-1/300 did not correctly relate the numbers of the contact binar ies to the numbers of MS stars. The magnitude limit of the OGLE survey limi ts the accuracy of the current luminosity function determination for M-V > 5.5, but the available data are consistent with a continuation of the high apparent frequency beyond M-V = 7.5, i.e., past the current short-period, l ow-luminosity end, delineated by the shortest period field system CC Com at M-V = 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 similar or equal to 10-11.