THE DETERMINATION OF GLOBULAR-CLUSTER AGES USING STROMGREN PHOTOMETRY- NGC-6752

Images of the Galactic globular cluster NGC 6752 have been obtained in the uv by filters of the Stromgren system using 1- and 4-m telescopes . A (V, b - y) colour-magnitude diagram and (m(1), b - y) and (c(1), b - y) colour-colour diagrams have been constructed. The high precision of the photometry leads to the (m(1), b - y), (c(1), b - y) and (V, b - y) sequences being well delineated down to V similar or equal to 20 mag, some 2.5 mag below the turn-off. Thus, for the first time, the S tromgren system can be used for an accurate determination of a cluster age. In principle, this method improves on use of BV observations for determining ages and metallicities. By comparing theoretical isochron es with the (c(1), b - y) diagram, we derive an age for this cluster o f 16 Gyr with an internal accuracy of +/- 1.5 Gyr, thus supporting the previous estimates of the age of this cluster. The age is reduced by similar to 2 Gyr if oxygen enhancement or alpha-element enhancement ef fects are included. From the (m(1), b - y) diagram we deduce a metalli city for the cluster in the range - 1.7 <[Fe/H] < - 1.0 with a possibl e alpha-element enhancement of a factor of 3 over the scaled solar val ues. Main-sequence matching to the (V, b - y) diagram leads to an esti mate of the distance modulus of 13.23 +/- 0.10.