ECLIPSING BINARIES IN THE OGLE VARIABLE-STAR CATALOG .1. W-UMA-TYPE SYSTEMS AS DISTANCE AND POPULATION TRACERS IN BAADE WINDOW

The paper demonstrates that the W UMa-type systems have a potential of playing an important role in studies of the Galactic structure due to their high spatial frequency of occurrence, ease of detection and rea sonably tight absolute-magnitude (period, color) calibration. An algor ithm, based on Fourier decomposition of light curves, permitted to def ine a sample of 388 contact binaries with well observed light curves, periods shorter than one day and with available V-I colors (the R-samp le), from among 933 eclipsing binary systems in the OGLE variable-star catalog for 9 fields of Baade's Window. The sample of such systems wh ich were visually classified by the OGLE project as EW-type binaries ( the O-sample) is by 55% larger and consists of 604 stars. The algorith m prevents inclusion of RR Lyr and SX Phe stars which in visual classi fication might be mistakenly taken for contact binaries with periods e qual to twice their pulsation periods. Determinations of distances for the contact systems, utilizing the M(I)=M(I)(log P,V-I) absolute-magn itude calibration and the map of reddening and extinction of Stanek (1 996), indicate an approximately uniform distribution of contact binari es almost all the way to the Galactic Bulge (there is a hint of a gap at 6.5 to 8 kpc), with 9 well observed systems identified in the Bulge . The distances have been derived assuming two hypotheses: (1) extinct ion extends uniformly all the way to the Bulge at d(0)=8 kpc or (2) ex tinction is truncated at d(0) = 2 kpc. Analysis of the period-color di agram favors the latter hypothesis which has been assumed throughout t he paper. The uniform distribution of the contact systems with distanc e, implying heights up to z less than or equal to 420-450 pc, as well as a tendency for their colors to be concentrated in the region normal ly occupied by old turn-off-point stars, confirm the currently held op inion that contact binary systems belong to the old stellar population of the Galaxy. A first attempt to construct the luminosity function f or contact binaries has been made for samples defined by distances of 2 and 3 kpc. The apparent density of contact systems is about (7-10) X 10(-5) systems per pc(3), with the main uncertainty coming from the d efinitions of the R- and O-samples. If the spatial density (corrected for undetected low-inclination systems) is two times higher than the l ower limit of the above range, and equals 1.5 X 10(-4) systems per pc( 3), then one star among 400 main sequence stars is a contact system; h owever, this number includes M-dwarfs among which contact binaries do not occur. An independent estimate of the apparent frequency, relative to nearby dwarfs with colors similar to those of the contact binaries , obtained in the volume-limited sense to 2 and 3 kpc, is one contact system per about 250-300 main sequence stars, which agrees well with t he recent estimates for old open clusters and their background/foregro und fields. (C) 1997 American Astronomical Society.