THE GALACTIC GLOBULAR-CLUSTER NGC-1851 - ITS DYNAMICAL AND EVOLUTIONARY PROPERTIES

We have completely mapped the Galactic globular cluster NGC 1851 with large-field, ground-based VI CCD photometry and pre-repair HST/WFPC1 d ata for the central region. The photometric data set has allowed a V v s. (V-I) colour-magnitude diagram for similar to 20500 stars to be con structed. From the apparent luminosity of the horizontal branch (HB) w e derive a true distance modulus (m - M)(0) = 15.44 +/- 0.20. An accur ate inspection of the cluster's bright and blue objects confirms the p resence of seven ''supra-HB'' stars, six of which are identified as ev olved descendants from HE progenitors. The HB morphology is found to b e clearly bimodal, showing both a red clump and a blue tail, which are not compatible with standard evolutionary models. Synthetic Hertzspru ng-Russell (HR) diagrams demonstrate that the problem could be solved by assuming a bimodal efficiency of the mass loss along the red giant branch (RGB). With the aid of Kolmogorov-Smirnov statistics we find ev idence that the radial distribution of the blue HE stars is different from that of the red HB and supgiant branch (SGB) stars. We give the f irst measurement of the mean absolute I magnitude for 22 known RR Lyr variables (< M-I(RR) >= 0.12 +/- 0.20 mag at a metallicity [Fe/H] = -1 .28). The mean absolute V magnitude is < M-V(RR) >= 0.58 +/- 0.20 mag, and we confirm that these stars are brighter than those of the zero-a ge HE (ZAHB). Moreover, we found seven new RR Lyr candidates (six ab t ype and one c type). With these additional variables the ratio of the two types is now N-c/N-ab = 0.38. From a sample of 25 globular cluster s a new calibration for Delta V-bump(HB) function of cluster metallici ty is derived. NGC 1851 follows this general trend fairly well. From a comparison with the theoretical models, we also find some evidence fo r an age-metallicity relation among globular clusters. We identify 13 blue straggler stars, which do not show any sign of variability. The b lue stragglers are less concentrated than the subgiant branch stars wi th similar magnitudes for r > 80 arcsec. Finally, a radial dependence of the luminosity function, a sign of mass segregation, is found. Tran sforming the luminosity function into a mass function (MF) and correct ing for mass segregation by means of multi-mass King-Michie models, we find a global MF exponent x(0) = 0.2 +/-0.3.