The ages of the globular clusters M 71 and 47 Tuc from Strömgrenuvbyphotometry

Abstract

New uvby CCD photometry for the fairly metal-rich globular clusters M71 (NGC 6838) and 47 Tuc (NGC 104) is presented. We derive the cluster distances using a sample of field subdwarfs with metallicities determined from uvby photometry and accurate parallaxes from the Hipparcos mission. The biases associated with the main-sequence fitting technique are discussed and only that due to metallicity is found to be significant, corresponding to a -0.05 mag change in distance modulus. Our main results are that: 1) The distance moduli of 47 Tuc and M71 are somewhat shorter than that derived by Reid (1998, AJ 115, 204). For M71 and 47Tuc we find (metallicity corrected) (m - M) v = 13.71 ′ 0.04 ′ 0.1 and (m - M) v = 13.33 ′ 0.04 ′ 0.1, for adopted reddenings of E(B - V) = 0.28 and E(B - V) = 0.04 respectively (first errorbar denotes random errors and the second systematic errors). The main source of difference with Reid is the selection of subdwarfs with this study having more intrinsically faint field subdwarfs; 2) These values lead to ages of nearly 12 Gyr when using the isochrones of VandenBerg et al. (2000, ApJ, 532, 430); this estimate does not include the effects of He diffusion. 3) A differential comparison of the cluster colour-magnitude diagrams show that the age difference between the two is very small - less than one billion years. 4) The observed scatter in the c 1 index (due to star-to-star nitrogen variations) among main-sequence stars does not allow us to use the [(v - y) 0 , c 0 ] diagram for a distance-independent age determination.