A Baade-Wesselink analysis of the RR Lyrae star V9 in 47 Tucanae.

We present an infrared K-band light curve and a radial velocity curve for the RR Lyrae star V9 in the metal rich globular cluster 47 Tucanae ([Fe/H]=-0.71). Combining these new data with the optical photometry presented in Carney et al. (1993), we perform a Baade-Wesselink analysis and derive a distance modulus to 47 Tuc of 13.23+/-0.17mag and absolute mean magnitudes of <M_V_>=0.32mag and <M_K_>=-0.57mag for V9. We also present a well populated visual-infrared color-magnitude diagram of the cluster, reaching from the upper red giant branch to the subgiant branch. V9 turns out to be a highly evolved RR Lyrae star possessing an exceptionally long period for its color and metallicity. This leads to a very low mass estimate of 0.46{vec}(M)_sun_. In spite of these rather extreme characteristics of V9 it still obeys the <M_K_>-logP relation, emphasizing the great robustness of this relation for distance estimation. Comparing the currently available mean absolute magnitudes for cluster RR Lyrae stars from Baade-Wesselink analyses with similar data and relations for field stars reveals no significant systematic differences at the 0.1mag level, although the cluster stars may be slightly brighter than the field stars. Systematic differences at this level affect age estimates of globular clusters by about 1Gyr.