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.
<Astron. Astrophys. 333, 479 (1998)>
=1998A&A...333..479S 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 (<MI(RR)≥0.12±0.20mag at a metallicity [Fe/H]=-1.28).
The mean absolute V magnitude is <MV(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 Nc/Nab=0.38. From a sample of 25 globular clusters a
new calibration for {DELTA} VbumpHB 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
x0=0.2±0.3.
Objects:
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RA (2000) DE Designation(s)
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05 14 06.3 -40 02 50 NGC 1851 = C 0512-400
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File Summary:
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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-vnorm 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
(End) Patricia Bauer [CDS] 16-Feb-1998