On the Predicted and Observed Color Boundaries of the RR Lyrae Instability Strip as a Function of Metallicity

Abstract

The purpose of the paper is to predict the temperature at the fundamental blue edge (FBE) of the instability strip for RR Lyrae (RRL) variables from the pulsation equation that relates temperature to period, luminosity, and mass. Modern data for the correlations between period, luminosity, and metallicity at the FBE for field and cluster RRL are used for the temperature calculation. The predicted temperatures are changed to B-V colors using an adopted color transformation. The predicted temperatures at the FBE become hotter as [Fe/H] changes from 0 to -1.5, and thereafter cooler as the metallicity decreases to -2.5 and beyond. The temperature range over this interval of metallicity is $\Delta$log $T_e$ = 0.04, or 640 K at 6900K. The predicted color variation is at the level of 0.03 mag in B-V. The predictions are compared with the observed RRL colors at the FBE for both the field and cluster variables, showing general agreement at the level of 0.02 mag in (B-V)$_o$, which, however, is the uncertainty of the reddening corrections. The focus of the problem is then reversed by fitting a better envelope to the observed FBE relation between color and metallicity for metallicities smaller than -1.8 which, when inserted in the pulsation equation, gives a non-linear calibration ....