A mid-infrared study of RR Lyrae stars with the Wide-field Infrared Survey Explorer all-sky data release

We present a group of 3740 previously identified RR Lyrae variables well observed with the Wide-field Infrared Survey Explorer (WISE). We explore how the shape of the generic RR Lyrae mid-infrared light curve varies over period-space, comparing light curves in mid-infrared and optical bands. We find that optical light curves exhibit high amplitudes and a large spectrum of light-curve shapes, while mid-infrared light curves have low amplitudes and uniform light-curve shapes. From the period-space analysis, we hope to improve the classification methods of RR Lyrae variables and enable reliable discovery of these pulsators in the WISE catalogue and future mid-infrared surveys such as the James Webb Space Telescope. We provide mid-infrared templates for typical RR Lyrae stars and demonstrate how these templates can be applied to improve estimates of mid-infrared RR Lyrae mean magnitude, which is used for distance measurement. This method of template fitting is particularly beneficial for improving observational efficiency. For example, using light curves with observational noise of 0.05 mag, we obtain the same level of accuracy in mean magnitude estimates for light curves randomly sampled at 12 data points with template fitting as with light curves randomly sampled at 20 data points with harmonic modelling.