Absolute-magnitude Calibration for W UMa-type Systems Based on Gaia Data

Tycho-Gaia Astrometric Solution (TGAS) parallax data are used to determine absolute magnitudes M-V for 318 W. UMa-type (EW) contact binary stars. A very steep (slope similar or equal to -9), single-parameter (log P), linear calibration can be used to predict MV to about 0.1-0.3 mag over the whole range of accessible orbital period, 0.22 < P < 0.88 days. A similar calibration for the most common systems with 0.275 < P < 0.575 days predicts MV values to about 0.06-0.16 mag. For orbital period values both shorter and longer than the central range, the period dependence is respectively steeper and shallower, i. e., the binaries are fainter in M-V than predicted by the whole-range linear law. The steepness of the relation for short-period systems implies important consequences for the detectability of the faintest binaries, defining the short-period cut-off of the period distribution. Although the scatter around the linear log P-fit is fairly large (0.2-0.4 mag), the current data do not support the inclusion of a B - V color term in the calibration.