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

The purpose of the paper is to predict the temperature at the fundamentalblue edge (FBE) of the instability strip for RR Lyrae (RRL) variables from thepulsation equation that relates temperature to period, luminosity, and mass.Modern data for the correlations between period, luminosity, and metallicity atthe FBE for field and cluster RRL are used for the temperature calculation. Thepredicted temperatures are changed to B-V colors using an adopted colortransformation. 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 isat the level of 0.03 mag in B-V. The predictions are compared with the observedRRL colors at the FBE for both the field and cluster variables, showing generalagreement at the level of 0.02 mag in (B-V)$_o$, which, however, is theuncertainty of the reddening corrections. The focus of the problem is then reversed by fitting a better envelope to theobserved FBE relation between color and metallicity for metallicities smallerthan -1.8 which, when inserted in the pulsation equation, gives a non-linearcalibration ....Comment: 34 pages, 11 figures, 2 tables. To appear in March 2006 A