The luminosity scale of RR Lyrae stars with the Baade-Wesselink method. II. The absolute magnitudes of 13 field RR Lyrae stars

The surface brightness version of the Baade-Wesselink method has been applied to 13 field RR Lyrae stars-SW And, TV Boo, RR Cet, SU Dra, RX Eri, RR Gem, RR Leo, TF Lyn, AV Peg, AR Per, T Sex, TU UMa, and UU Vir-to derive their absolute magnitudes from the photometry and radial velocities presented in a previous paper. Using static model atmospheres to provide metallicity- and gravity-dependent color-temperature transformations, we find that the RR Lyrae temperature scale can be relatively well defined using the infrared color index V-K except for a small phase interval where strong atmospheric acceleration and shock waves are present, while the temperatures determined from bluer color indices are systematically overestimated. The use of infrared photometry reduces uncertainties because the sensitivity to gravity and metallicity is small. Complications in pulsational velocity determination also have small effects on the [M-V] determinations if the analysis is restricted to the phase interval where the temperature change is minimal. Our results show a clear correlation between the luminosities of the RR Lyraes and their metallicities such that: [M-V] = 0.20 (+/- 0.06) [Fe/H] + 1.05 (+/- 0.14), with a metal-poor RR Lyrae star being more luminous. Taking into account possible systematic uncertainties as well as (random) measurement errors, the typical accuracy of the derived [M-V] for a single star is 0.20 mag. These RR Lyraes also follow a period-infrared luminosity relation, a potentially important distance calibrator.