A Spectroscopic Study of the RR Lyrae Stars.

The possibility that the RR Lyrae stars do not constitute a homogeneous spectroscopic group has been investigated by surveying the spectra of more than one hundred RR Lyrae stars at very low dispersion (430 A/mm at H ). During the quarter of the light-cycle preceding minimum light, all the Bailey type a variables have hydrogen lines of similar strength, the spectral types ranging from F4 to F6. However, the K line of Ca II differs in strength from star to star by an amount that corresponds to about one spectral class, the spectra ranging in appearance from those of normal F-type stars to those of extreme F-type subdwarfs. Intermediate cases are common, and no separation into discrete spectroscopic groups is indicated by the material of this study. The Bailey type c variables have systematically earlier spectral types at minimum light, but they show qualitatively the same spread in spectroscopic peculiarity. The difference, AS, between the spectral types derived from the hydrogen lines and from the K line of Ca ii at minimum light has been adopted as a parameter to describe the extent of the weak-line characteristic; high-dispersion studies indicate that this characteristic is due to low metal abundances. AS increases systematically with increasing period, P, for both the Bailey type c and the Bailey type a variables with P 0d.75, the spread in P for a given AS among the latter corresponding approximately to the spreads observed in individual globular clusters. A third P versus AS sequence appears to exist among the variables with P > 0d.75. The period-frequency distributions of the strong- and weak-line variables indicate that the RR Lyrae star population near the sun differs from those found in globular clusters and far from the galactic plane, in that it possesses a strong-line (small AS) component. The concentration of strong-line variables to the plane is confirmed by the intercomparison of the period-frequency distributions of various regions of the Galaxy. In addition, the strong-line variables appear to be relatively less concentrated toward the galactic center. The solar motion and mean peculiar radial velocity of the weak-line variables resemble those derived for the globular clusters; those for the strong-line stars are intermediate between the values derived for halo and spiral-arm objects.