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J/A+A/333/479    NGC 1851 dynamical and evolutionary properties (Saviane+ 1998)
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The Galactic globular cluster NGC 1851:
its dynamical and evolutionary properties
       Saviane I., Piotto G., Fagotto F., Zaggia S., Capaccioli M., Aparicio A.
      <Astron. Astrophys. 333, 479 (1998)>
      =1998A&A...333..479S      (SIMBAD/NED BibCode)
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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 (<M_I_(RR)>=0.12+/-0.20mag at a metallicity [Fe/H]=-1.28).
    The mean absolute V magnitude is <M_V_(RR)>=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
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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
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Acknowledgements: Ivo Saviane <saviane@astrpd.pd.astro.it>
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(End)                                         Patricia Bauer [CDS]   16-Feb-1998