New absolute magnitude calibrations for W Ursa Majoris type binaries

Parallaxes of W UMa stars in the Hipparcos catalogue have been analyzed. 31 W UMa stars, which have the most accurate parallaxes ($\sigma_{\pi}/\pi<0.15$) which are neither associated with a photometric tertiary nor with evidence of a visual companion, were selected for re-calibrating the Period--Luminosity--Color (PLC) relation of W UMa stars. Using the Lutz--Kelker (LK) bias corrected (most probable) parallaxes, periods ($0.26< P(day)< 0.87$), and colors (0.04<$(B-V)_{0}$<1.28) of the 31 selected W UMa, the PLC relation have been revised and re-calibrated. The difference between the old (revised but not bias corrected) and the new (LK bias corrected) relations are almost negligible in predicting the distances of W UMa stars up to about 100 parsecs. But, it increases and may become intolerable as distances of stars increase. Additionally, using $(J-H)_{0}$ and $(H-K_{s})_{0}$ colors from 2MASS (Two Micron All Sky Survey) data, a PLC relation working with infrared data was derived. It can be used with infrared colors in the range $-0.01<(J-H)_{0}<0.58$, and $-0.10<(H-K_{s})_{0}<0.18$. Despite {\em 2MASS} data are single epoch observations, which are not guaranteed at maximum brightness of the W UMa stars, the established relation has been found surprisingly consistent and reliable in predicting LK corrected distances of W UMa stars.Comment: 9 pages, 8 figures and 2 tables, accepted for publication in Astronomische Nachrichte

Contact binary stars of the W UMa type as distance tracers

Contact binaries can be used for distance determinations of stellar systems. They are easy to discover and identify and are very abundant among solar-type stars, particularly for M_V > +3. The period - luminosity - colour (PLC) relations have similar properties to those for pulsating stars and can currently predict individual values of M_V to about +/-0.25 mag.Comment: IAU Sydney 2003, JD13 Extragalactic Binaries, ed.I.Ribas, uses elsart3.cl

Eclipsing Binaries in the OGLE Variable Star Catalog.II. Light Curves of the W UMa-type Systems in Baade's Window

Light curves of the contact systems visible in the direction of Baade's Window have been analyzed using the first coefficients of the Fourier representation. The results confirm that the geometric contact between components is usually weak. Systems showing significant differences in the depths of eclipses are very rare in the volume-limited sample to 3 kpc: only 2 among 98 contact systems show the difference larger than 0.065 mag; for most systems the difference is <0.04 mag. If this relative frequency of 1/50 is representative, then one among 12,500 - 15,000 Main Sequence F-K spectral-type stars is either a semi-detached or poor-thermal-contact system. Below the orbital period of 0.37 day, no systems with appreciable differences in the eclipse depths have been discovered. Since large depth differences are expected to be associated with the "broken-contact" phase of the Thermal Relaxation Oscillations, this phase must be very short for orbital periods above 0.37 day and possibly entirely absent for shorter periods. In the full sample, which is dominated by intrinsically bright, distant, long-period systems, larger eclipse-depth differences are more common with about 9% of binaries showing this effect. Sizes of these differences correlate with the sense of light-curve asymmetries (differing heights of maxima) for systems with orbital periods longer than 0.4 day suggesting an admixture of semi-detached systems with accretion hot spots on cooler components. The light-curve amplitudes in the full sample as well as in its volume-limited sub-sample are surprisingly small and strongly suggest a mass-ratio distribution steeply rising toward more dissimilar components. Many low mass-ratio systems remain to be discovered in the sky field.Comment: latex 14 pages of text and 14 figures (aastex40 and psfig), submitted to AJ; the first paper astro-ph/9607009 will appear in AJ, Jan.199

Luminosity function of contact binaries based on the ASAS survey

The luminosity function for contact binary stars of the W~UMa-type is evaluated on the basis of the ASAS photometric project covering all stars south of delta= +28 within a magnitude range 8<V<13. Lack of colour indices enforced a limitation to 3373 systems with P<0.562 days (i.e. 73% of all systems with P<1 day) where a simplified Mv(log P) calibration could be used. The spatial density relative to the main sequence FGK stars of 0.2%, as established previously from the Hipparcos sample to V=7.5, is confirmed. While the numbers of contact binaries in the ASAS survey are large and thus the statistical uncertainties small, derivation of the luminosity function required a correction for missed systems with small amplitudes and with orbital periods longer than 0.562 days; the correction, by a factor of 3 times, carries an uncertainty of about 30%.Comment: Submitted to MNRA