A universal period-infrared luminosity relation for RR Lyraes?

Abstract

From the observed optical (V) properties of RR Lyraes in globular clusters it is straightforward to show that in a given cluster these stars should show a Period-Infrared (K) Luminosity relation with a slope of −2.2 and scatter =0.05 mags. We have obtained infrared data on RR Lyraes in several clusters and this data is satisfactorily consistent with the above expectation. Providing the total spread in RR Lyrae optical (V) absolute magnitudes is ≲0.5 mags it is reasonable, from the point of view of distance determination, to consider a universal Period-Infrared Luminosity relation.

We have also obtained VJHK light curves for three field RR Lyraes. Using this data, Plus radial velocity curves either obtained ourselves or from the literature, we have derived Baade-Wesselink radii, and hence absolute magnitudes, for these three stars. The infrared, because of its reduced sensitivity to temperature, is particularly suitable for Baade-Wesselink analysis. Using these results to calibrate the zero-point we find M_k = −2.2(logP + 0.2) −0.53