The Absolute Magnitude of RR Lyrae Stars Derived from the Hipparcos Catalogue

Abstract

The present determination of the absolute magnitude M_V(RR) of RR Lyrae stars is twofold, relying upon Hipparcos proper motions and trigonometric parallaxes separately. First, applying the statistical parallax method to the proper motions, we find V(RR)>=0.69+/-0.10 for 99 halo RR Lyrae stars with <[Fe/H>=-1.58. Second, applying the Lutz-Kelker correction to the RR Lyrae HIP 95497 with the most accurately measured parallax, we obtain M_V(RR)=(0.58-0.68)^+0.28_-0.31 at [Fe/H=-1.6. Furthermore, allowing full use of low-accuracy and negative parallaxes, as well 125 RR Lyrae stars with -2.49<=[Fe/H<=0.07, the maximum likelihood estimation yields the relation M_V(RR) = (0.59 +/- 0.37) + (0.20 +/- 0.63)([Fe/H] + 1.60), which formally agrees with the recent preferred relation. The same estimation again yields V(RR)>=0.65+/-0.33 for the 99 halo RR Lyrae stars. Although the formal errors in the latter three parallax estimates are rather large, all four results suggest the fainter absolute magnitude, M_V(RR)~0.6-0.7 at [Fe/H=-1.6. The present results still provide the lower limit on the age of the universe that is inconsistent with a flat, matter-dominated universe and current estimates of the Hubble constant.