The effect of livermore opal opacities on the evolutionary masses of RR Lyrae stars

We have investigated the effect of the new Livermore OPAL opacities on the evolution of horizontal-branch (HB) stars. This work was motivated by the recent stellar pulsation calculations using the new Livermore opacities, which suggest that the masses of double-mode RR Lyrae stars are 0.1-0.2 M_⊙ larger than those based on earlier opacities. Unlike the pulsation calculations, we find that the effect of opacity change on the evolution of HB stars is not significant. In particular, the effect of the mean masses of RR Lyrae stars (<M_RR>) is very small, showing a decrease of only 0.01-0.02 M_⊙ compared to the models based on old Cox-Stewart opacities. Consequently, with the new Livermore OPAL opacities, both the stellar pulsation and evolution models now predict approximately the same masses for the RR Lyrae stars. Our evolutionary models suggest that the mean masses of the RR Lyrae stars are ∼0.76 and ∼0.71 M_⊙ for M15 (Oosterhoff group II) and M3 (group I), respectively. If [α/Fe] = 0.4, these values are decreased by ∼0.03 M_⊙. Variations of <M_RR> with HB morphology and metallicity are also presented.