A PRELIMINARY CALIBRATION OF THE RR LYRAE PERIOD-LUMINOSITY RELATION AT MID-INFRARED WAVELENGTHS: WISE DATA

Using time-resolved, mid-infrared data from the Wide-field Infrared Survey Explorer (WISE) and geometric parallaxes from the Hubble Space Telescope for four Galactic RR Lyrae variables, we derive the following Population II period-luminosity (PL) relations for the WISE [W1], [W2], and [W3] bands at 3.4, 4.6, and 12 mu m, respectively: M-[W1] = -2.44(+/- 0.95) x log(P) - 1.26(+/- 0.25) sigma = 0.10 M-[W2] = -2.55(+/- 0.89) x log(P) - 1.29(+/- 0.23) sigma = 0.10 M-[W3] = -2.58(+/- 0.97) x log(P) - 1.32(+/- 0.25) sigma = 0.10. The slopes and the scatter around the fits are consistent with a smooth extrapolation of those same quantities from previously published K-band observations at 2.2 mu m, where the asymptotic (long-wavelength) behavior is consistent with a period-radius relation with a slope of 0.5. No obvious correlation with metallicity (spanning 0.4 dex in [Fe/H]) is found in the residuals of the four calibrating RR Lyrae stars about the mean PL regression line.