J/A+A/333/479 NGC 1851 dynamical and evolutionary properties (Saviane+ 1998) ================================================================================ The Galactic globular cluster NGC 1851: its dynamical and evolutionary properties Saviane I., Piotto G., Fagotto F., Zaggia S., Capaccioli M., Aparicio A. =1998A&A...333..479S (SIMBAD/NED BibCode) ================================================================================ ADC_Keywords: Clusters, globular ; Photometry, CCD Keywords: stars: evolution - Hertzsprung-Russell (HR) diagram - luminosity function, mass function - Population II - globular clusters: individual: NGC 1851 Abstract: We have completely mapped the Galactic globular cluster NGC 1851 with large-field, ground-based VI CCD photometry and pre-repair HST/WFPC1 data for the central region. The photometric data set has allowed a V vs. (V-I) colour-magnitude diagram for ~20500 stars to be constructed. From the apparent luminosity of the horizontal branch (HB) we derive a true distance modulus (m-M)_0_=15.44+/-0.20. An accurate inspection of the cluster's bright and blue objects confirms the presence of seven ``supra-HB'' stars, six of which are identified as evolved descendants from HB progenitors. The HB morphology is found to be clearly bimodal, showing both a red clump and a blue tail, which are not compatible with standard evolutionary models. Synthetic Hertzsprung-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 evidence that the radial distribution of the blue HB stars is different from that of the red HB and supgiant branch (SGB) stars. We give the first measurement of the mean absolute I magnitude for 22 known RR Lyr variables (=0.12+/-0.20mag at a metallicity [Fe/H]=-1.28). The mean absolute V magnitude is =0.58+/-0.20mag, and we confirm that these stars are brighter than those of the zero-age HB (ZAHB). Moreover, we found seven new RR Lyr candidates (six ab type 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 clusters a new calibration for {DELTA} V_bump_^HB^ as a function of cluster metallicity is derived. NGC 1851 follows this general trend fairly well. From a comparison with the theoretical models, we also find some evidence for an age-metallicity relation among globular clusters. We identify 13 blue straggler stars, which do not show any sign of variability. The blue stragglers are less concentrated than the subgiant branch stars with similar magnitudes for r>80arcsec. Finally, a radial dependence of the luminosity function, a sign of mass segregation, is found. Transforming the luminosity function into a mass function (MF) and correcting for mass segregation by means of multi-mass King-Michie models, we find a global MF exponent x_0_=0.2+/-0.3. Objects: ---------------------------------------------------------- RA (2000) DE Designation(s) ---------------------------------------------------------- 05 14 06.3 -40 02 50 NGC 1851 = C 0512-400 ---------------------------------------------------------- File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file appen.ps 102 11384 PostScript version of Appendix appen.tex 88 392 LaTeX version of Appendix cback.ps 102 1113 V-v_norm_ versus V-I diagram field1ms.ps 72 765 Completeness as a function of magnitude and radius for the central field xfig.ps 102 8473 Plot of the photometric zero-point differences -------------------------------------------------------------------------------- Acknowledgements: Ivo Saviane ================================================================================ (End) Patricia Bauer [CDS] 16-Feb-1998