Bibcode
DOI
Woo, Jong-Hak; Gallart, Carme; Demarque, Pierre; Yi, Sukyoung; Zoccali, Manuela
Referencia bibliográfica
The Astronomical Journal, Volume 125, Issue 2, pp. 754-769.
Fecha de publicación:
2
2003
Número de citas
34
Número de citas referidas
33
Descripción
Using Yale stellar evolution models, we present an analysis of the
color-magnitude diagrams (CMDs) of three intermediate-age LMC clusters,
namely, NGC 2173, SL 556, and NGC 2155, obtained with the Very Large
Telescope. The main goal of our project is to investigate the amount of
convective core overshoot necessary to reproduce the CMDs of relatively
metal-poor, intermediate-age stellar populations, to check whether the
extrapolation that is usually made from solar metallicity is valid. In
the process, we obtained values for the binary fraction of each cluster,
together with refined age estimates. Our method involved the comparison
of the observed CMDs with synthetic CMDs computed using various values
of the overshoot parameter and binary fraction. We conclude that a
moderate amount of overshoot and some fraction of binary stars are
essential for reproducing the observed shapes around the turnoff in the
CMDs of all three clusters: unresolved binary stars fill in the expected
core contraction gap and make a unique sequence near the gap, which
cannot be reproduced by single stars alone, even with a larger amount of
overshoot. We utilize ratios of the number of stars in different areas
around the core contraction gap to constrain the binary fraction, which
is around 10%-20% (for primary-to-secondary mass ratio >=0.7) in all
three clusters. Even if binary stars contaminate the core contraction
gap, it is shown that the overshoot parameter can be inferred from the
color dispersion of the stars around the contraction gap, regardless of
the assumed binary fraction. From our overall analysis such as shape of
isochrones, star counts, color distribution, and synthetic CMD
comparisons, we conclude that overshoot ~20% of the local pressure scale
height best reproduces the CMD properties of all three clusters. The
best age estimates are 1.5, 2.1, and 2.9 Gyr for NGC 2173, SL 556, and
NGC 2155, respectively.
Based on observations collected at the European Southern Observatory,
Paranal, Chile (ESO 64.L-0385).