The Baade-Wesselink method applied to RR Lyrae variables in the globular clusters M5 and M92

Abstract

An infrared Baade-Wesselink analysis is given for two RR Lyrae variables in the extremely metal-poor globular cluster M92 and four in the intermediate metallicity cluster M5. The method of analysis relies on the theoretical model atmospheres of Kurucz and on the absolute flux calibration of observed V- and K-magnitudes. The best results are obtained with V-K colors to obtain T_(eff) and K-magnitudes to deduce absolute fluxes. We obtain a distance to M5 of 6.9 ± 0.45 kpc and to M92 of 7.85 ± 0.45 kpc, with a 0.3 kpc additional systematic uncertainty. These results are in excellent agreement with the previously published distances, based on matching the globular cluster main sequence to that defined by the field subdwarfs of known distance. With these distances we obtain an absolute mean magnitude corrected for reddening, M^0_v, of 0.86 ± 0.14 mag for the M5 RR Lyrae variables, while M^0_v= 0.62 ± 0.11 for the M92 RR Lyrae variables. The globular cluster RR Lyrae stars appear to behave just like the field RR Lyrae stars. They obey the same〈M_v〉- [Fe/H] relationship as do the field stars, and the same〈M_K〉-period relationship. Previous work which assumed the two groups of stars to behave identically is thus shown to be correct.