Contact Binaries as Viable Distance Indicators: New, Competitive (V)JHKs Period-Luminosity Relations

Abstract: Based on the largest catalogs currently available, comprising 6090 contact binaries (CBs) and 2167 open clusters, we determine the near-infrared $JHK_{\rm s}$ CB period--luminosity (PL) relations, for the first time achieving the low levels of intrinsic scatter that make these relations viable as competitive distance calibrators. To firmly establish our distance calibration on the basis of open cluster CBs, we require that (i) the CB of interest must be located inside the core radius of its host cluster; (ii) the CB's proper motion must be located within the $2\sigma$ distribution of that of its host open cluster; and (iii) the CB's age, $t$, must be comparable to that of its host cluster, i.e., $\Delta \log (t\mbox{ yr}^{-1}) <0.3$. We thus select a calibration sample of 66 CBs with either open cluster distances or accurate space-based parallaxes. The resulting near-infrared PL relations, for both late-type (i.e., W Ursae Majoris-type) and---for the first time---early-type CBs, are as accurate as the well-established $JHK_{\rm s}$ Cepheid PL relations, (characterized by single-band statistical uncertainties of $\sigma < 0.10$ mag). We show that CBs can be used as viable distance tracers, yielding distances with uncertainties of better than 5\% for 90\% of the 6090 CBs in our full sample. By combining the full $JHK_{\rm s}$ photometric data set, CBs can trace distances with an accuracy, $\sigma=0.05 \mbox{ (statistical)} \pm0.03 \mbox{ (systematic)}$ mag. The 102 CBs in the Large Magellanic Cloud are used to determine a distance modulus to the galaxy of $(m-M_V)_0^{\rm LMC}=18.41\pm0.20$ mag.