We present an analysis , using Yale stellar evolution models , of the color-magnitude diagrams ( CMDs ) of three intermediate-age LMC clusters , namely NGC 2173 , SL 556 and NGC 2155 , obtained with the VLT .
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 CMD ’ s of all three clusters : unresolved binary stars fill in the expected core contraction gap , and make a unique sequence near the gap , which can not 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 \geq 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 \sim 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 .