Eclipsing Binaries in the OGLE Variable Star Catalog.I. W UMa-type Systems as Distance and Population Tracers in Baade's Window

Abstract

An algorithm, based on Fourier decomposition of light curves, permitted to define a sample of 388 contact binaries with well observed light curves, periods shorter one day and with available V-I colors (the R-sample), from among 933 eclipsing binary systems in the OGLE variable-star catalog for 9 fields of Baade's Window. The sample of such systems which was visually classified by the OGLE project as EW-type binaries (the O-sample) is by 55% larger and consists of 604 stars. The algorithm prevents inclusion of RR Lyr and SX Phe stars which in visual classification might be mistakenly taken for contact binaries with periods equal to twice their pulsation periods. Determinations of distances for the contact systems, utilizing the M_I = M_I(log P, V-I) absolute-magnitude calibration and the map of reddening and extinction of Stanek (1996), indicate an approximately uniform distribution of contact binaries almost all the way to the Galactic Bulge, implying heights up to z = 420 - 450 pc. This distribution, as well as a tendency for the colors to be concentrated in the region normally occupied by old Turn-Off-Point stars, confirm the currently held opinion that contact binary systems belong to the old stellar population of the Galaxy. The apparent frequency estimated in the volume-limited sense, relative to nearby MS dwarfs to the distances of 2 and 3 kpc, is one contact system per about 250 - 300 stars; it is one contact system per 400 MS stars if M-dwarfs are included. The apparent density of contact systems is 7-10x10^-5 pc^-3.

latex, 44 pages, 19 figures; uses: aaspp.sty, psfig.sty; to appear in AJ, January 1997

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