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 analy sis and derive a distance modulus to 47 Tuc of 13(m).23+/-0.17 and abs olute mean magnitudes of <M(V)>=0(m).32 and <M(K)>=-0(m).57 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 subgi ant branch. V9 turns out to be a highly evolved RR Lyrae star possessi ng an exceptionally long period for its color and metallicity. This le ads to a very low mass estimate of 0.46 M. In spite of these rather ex treme characteristics of V9 it still obeys the <M(K)> - log P relation , emphasizing the great robustness of this relation for distance estim ation. 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 diffe rences at the 0(m).1 level, although the cluster stars may be slightly brighter than the field stars. Systematic differences at this level a ffect age estimates of globular clusters by about 1 Gyr.