RR Lyrae stars as standard candles in the Gaia Data Release 2 Era

Tatiana Muraveva, Hector E. Delgado, Gisella Clementini, Luis M. Sarro, Alessia Garofalo

Published 2018-05-22Version 1

We present results from the analysis of 401 RR Lyrae stars (RRLs) belonging to the field of the Milky Way (MW), for which multiband (V, Ks, W1) photometry, metal abundances, extinction values and pulsation periods are available in the literature and accurate trigonometric parallaxes measured by the Gaia mission have become available with the Gaia second data release (DR2) on 2018 April 25. Using a Bayesian fitting approach we derive new near- and mid-infrared period-absolute magnitude-metallicity (PMZ) relations and new absolute visual magnitude-metallicity (Mv - [Fe/H]) relations based on the Gaia DR2 parallaxes of these 401 RRLs. We find the dependence of luminosity on metallicity to be higher than usually found in the literature, irrespective of the passband considered. Running the adopted Bayesian model on a simulated dataset we show that the high metallicity dependence is not caused by the method, but likely arises from the actual distribution of the data and the presence of a zero-point offset in the Gaia parallaxes. We infer a zero-point offset of -0.056 mas, with the Gaia DR2 parallaxes being systematically smaller. This value is in excellent agreement with the offset found by the Gaia validation team using a different sample of RRLs. In order to gauge our newly derived relations we use them to infer the distance to the Large Magellanic Cloud, which turns out to be in a good agreement with values currently adopted in the literature.