Gaia DR2 RR Lyrae stars as standard candles

For a fraction of them 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 alongside Gaia G-band time-series photometry have become available with the Gaia Data Release 2 (DR2) on 2018 April 25.

Using a Bayesian fitting approach, we derive new near-, mid-infrared period-absolute magnitude-metallicity (PMZ) relations and new absolute magnitude-metallicity relations in the visual (M_V_-[Fe/H]) and G bands (M_G_-[Fe/H]), based on the Gaia DR2 parallaxes. 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 data set 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.