The absolute magnitudes of RR Lyrae stars from Hipparcos parallaxes

Abstract

Using the method of ``reduced parallaxes'' for the Halo RR Lyrae stars in the Hipparcos catalogue we derive a zero point of 0.77 $\pm$ 0.26 mag for an assumed slope of 0.18 in the $M_{\rm V}$-[Fe/H] relation. This is 0.28 magnitude brighter than the value Fernley et al. (1998a) derived by employing the method of statistical parallax for the {\it identical} sample and using the same slope. We point out that a similar difference exists between the ``reduced parallaxes'' method and the statistical parallax method for the Cepheids in the Hipparcos catalogue. We also determine the zero point for the $M_{\rm K}$-$\log P_{0}$ relation, and obtain a value of -1.16 $\pm$ 0.27 mag (for a slope of -2.33). The distance moduli to the Hipparcos RR Lyrae stars derived from the two relations agree well. The derived distance scale is in good agreement with the results from the Main Sequence fitting distances of Galactic globular clusters and with the results of theoretical Horizontal Branch models, and implies a distance modulus to the LMC of 18.61 $\pm$ 0.28 mag.