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

The paper integrates the available data for contact binaries of the diskpopulation in a deep galactic field and in old open clusters. The two basicdata sets consist of 98 systems in the volume-limited 3 kpc sub-sample ofcontact binaries detected by the OGLE microlensing project toward Baade'sWindow (BW3) and of 63 members of 11 old open clusters (CL). Supplementary dataon the intrinsically bright, but spatially rare, long-period binaries areprovided by 238 systems in the BW sample to the distance of 5 kpc (BW5). Thebasic BW3 sample and the CL sample are remarkably similar in the period, color,luminosity and variability-amplitude distributions, in spite of very differentsample 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 arelocated; however, they are not concentrated at the respective TOP locationsbut, once the TOP happens to fall in the above color interval, they can appearanywhere within it. The luminosity function for the BW sample appears to bevery similar in shape to that for the solar neighborhood main-sequence starswhen corrections for the galactic disk structure are applied, implying a flatapparent 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 disklength scale h_R = 2.5 kpc and about 1/100 for h_R = 3.5 kpc. (abridged)Comment: submitted to AJ; 27 pages (aastex), 16 figures (.ps), 7 tables (aastex