Date of this Version
THE ASTROPHYSICAL JOURNAL, 360: 119-128,1990 September 1
Masses, temperatures, and absolute luminosities are proposed for a large sample of RRc stars in the globular cluster w Centauri. Our technique uses three observed quantities: period, relative luminosity, and Fourier phase ϕ31 interpreting them in the light of linear and hydrodynamic pulsation models. The masses we obtain agree with those of the RRd stars in other clusters, and our derived parameters pass a number of tests which are independent of the derivation: (1) we reproduce the observed slope, ∂ϕ31/∂ log P, of the ϕ31 -period diagram; (2) the values we obtain for M, T., and L locate the w Cen RRc stars in the first overtone instability strip, just where they belong; (3) our derived parameters are shown to be consistent with the fact that w Cen contains no RRd stars. However, our results not only disagree with current horizontal branch models, but also imply that the canonical slope of the log Te versus B - V relation (obtained from static model atmospheres) is too steep by a factor of 2, at least as applied to the RRc stars. Some additional results are presented which bear on the confrontation between evolution and pulsation theory: (1) it is shown that the disagreement between the RRd masses and those based on horizontal branch calculations cannot be reconciled by reasonable changes in metallicity or opacity; (2) we find essentially no correlation between the masses of the w Cen RRc stars and their metallicities, and we demonstrate that this circumstance cannot be due to late redward evolution by the RR Lyrae stars of lower mass; and (3) the mass-luminosity relation obtained by our method for three RRc field stars is shown to be consistent with the results of a Baade-Wesselink analysis, but not with horizontal branch models. Finally, we briefly discuss the question of globular cluster ages and calculate an age for w Centauri based upon its RRc stars.