A Provisional RR Lyrae Distance Scale

Hydrodynamic pulsation models are matched with observations of globular cluster RRc stars to derive masses, luminosities and temperatures as functions of two observables: pulsation period, P1, and Fourier phase parameter, phi_31. We find that mean RRc masses and luminosities increase, and mean temperatures fall, with decreasing cluster metallicity. The Oosterhoff dichotomy is explained as mainly a temperature effect, while hints of a new dichotomy (in RRc mass and luminosity) are found among certain Oo II clusters. A provisional RR Lyrae distance scale emerges for the RRc stars in the form of a relation between luminosity and the two observables, P1 and phi_31. Finally, the scheme we propose passes three independent tests: (1) it reproduces the observed hierarchy of relative luminosity among a large sample of RRc stars in Omega Centauri; (2) it yields mean RRc masses which are fully consistent with the RRd masses derived for the clusters M68 and M15; and (3) it gives an LMC distance modulus in agreement with that obtained by other methods.