HST PHOTOMETRY OF 47-TUC AND ANALYSIS OF THE STELLAR LUMINOSITY FUNCTION IN MILKY-WAY CLUSTERS

We present V and I photometry for over 1000 stars in a region 5 arcmin from the centre of the globular cluster 47 Tuc. The field was imaged with the Wide Field and Planetary Camera 2 (WFPC-2) as part of the Hub ble Space Telescope's Medium Deep Survey key project. The luminosity f unction (LF) continually rises in the domain 5 less than or similar to M(I) less than or similar to 9 with a slope Delta log Phi(M)/Delta(M) similar to 0.15 and then drdps off sharply. We compare our LF with tha t derived by De Marchi & Paresce for a neighbouring HST field. The two independent LFs are remarkably similar in the entire range of luminos ities probed (M(I) less than or similar to 10). Comparisons are also m ade to other HST LFs derived by several authors for both globular and open clusters in the Galaxy. We use the KS test to assess the signific ance of the differences found. The luminosity distributions obtained w ith HST are consistent with being derived from the same population dow n to M(I) similar to 9.0. Beyond that, statistically significant varia tions arise. Globular cluster LFs also differ according to the promine nce of a plateau in the bright end (5 less than or similar to M(I) les s than or similar to 6.5). The mass functions are rather uncertain and sensitive to the mass-luminosity relation. Different approaches to de riving the 47 Tuc mass function from its LF lead to markedly different results at the low-mass end. For M greater than or similar to 0.4 M., the 47 Tuc mass function is significantly different from that of omeg a Cen. The calibrated HST M(V)(V-I) colour-magnitude diagrams (CMDs) s how a trend with metallicity in the expected sense of systems that are more metal rich having redder CMDs for a fixed absolute magnitude. Th e main-sequence slope becomes shallower with increasing metallicity. T he CMDs derived from HST are in general agreement with previous ground -based studies, especially for metal-rich stars. However, the CMDs of metal-poor subdwarfs observed from the ground are shallower than those of globular clusters observed with HST. This discrepancy may either r esult from calibration problems of HST data or reflect real difference s between the CMDs of globular cluster and field halo stars.