RR Lyraes, Galactic and extragalactic distances, and the age of the oldest globular clusters

The usual, direct, M(V)-[Fe/H] relation for RR Lyrae variables is subject to bias and the best relation to adopt is the inverse one. It is then found that the RR Lyrae distance modulus of the Large Magellanic Cloud (LMC) (18.53) agrees well with the Cepheid modulus (18.57) or the commonly adopted value of 18.50, Statistical parallax results are consistent with the (Baade-Wesselink) zero-point adopted for the M(V)-[Fe/H] relation. Using the revised (inverse) relation results in a considerable reduction in the estimated age of the oldest Galactic globular clusters. This is found to be 12.6 Gyr with a 95 per cent confidence lower limit of 10.4 Gyr. Given current uncertainties in the value of the Hubble constant and the formation time of the oldest clusters, the observations provide no good grounds for rejecting a standard low-density Friedmann universe, It is shown that the best estimate of the distance to the Galactic Centre from an RR Lyrae M(K)-log P-0 relation is 8.1 +/- 0.4 kpc. A comparison is made with other recent work on the M(V)-[Fe/H] relation.