Absolute Magnitude Calibration for the W UMa-type Systems Based on Hipparcos Data

Abstract

Hipparcos parallax data for 40 contact binary stars of the W UMa-type (with errors in M_V < 0.5) are used to derive a new, (B-V)-based absolute-magnitude calibration of the form M_V = M_V(logP,B-V). The calibration covers the ranges 0.26 < (B-V)_0 < 1.14, 0.24 < P < 1.15 day, and 1.4 < M_V < 6.1; it is based on a solution weighted by relative errors in the parallaxes (2.7% to 24%). Previous calibrations have not been based on such a wide period and color space, and while they have been able to predict M_V with sufficient accuracy for systems closely following the well-known period-color relation, the new calibration should be able to give also good predictions for more exotic &#34;outlying&#34; contact binary systems. The main limitations of this calibration are the inadequate quality of the ground-based photometric data, and the restriction to the (B-V) index, which is more sensitive to metallicity effects than the (V-I) index; metallicities are, however, basically unknown for the local W UMa-type systems.