Absolute Magnitude Calibration for W UMa-Type Systems Based on Gaia Data

TL;DR

This study uses Gaia TGAS parallaxes to establish a simple, linear calibration of absolute magnitudes for W UMa-type contact binaries based solely on their orbital period, enabling more accurate distance estimates.

Contribution

It provides the first Gaia-based, single-parameter period calibration for W UMa stars' absolute magnitudes, improving previous methods by demonstrating a steep, linear relation with minimal color dependence.

Findings

Calibration predicts $M_V$ with 0.1-0.3 mag accuracy across period range.

Short-period systems are fainter than the linear model suggests, affecting detectability.

Scatter around the fit is 0.2-0.4 mag, with no need for color correction.

Abstract

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 $\simeq -9$), single-parameter ($\log{P}$), linear calibration can be used to predict $M_V$ 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 $M_V$ 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.