SCYON Abstract

Received on December 1 2010

A New Method for the Assessment of Age and Age Spread of Pre-main-sequence Stars in Young Stellar Associations of the Magellanic Clouds

Authors	Da Rio Nicola, Gouliermis Dimitrios A., and Gennaro Mario
Affiliation	Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
To appear in	Published in the Astrophysical Journal, Volume 723, Issue 1, pp. 166-183 (2010)
Contact	dgoulier@mpia.de
URL	http://iopscience.iop.org/0004-637X/723/1/166/
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Abstract

We present a new method for the evaluation of the age and age spread among pre--main-sequence (PMS) stars in star-forming regions in the Magellanic Clouds, accounting simultaneously for photometric errors, unresolved binarity, differential extinction, stellar variability, accretion, and crowding. The application of the method is performed with the statistical construction of synthetic color-magnitude diagrams (CMDs) using isochrones from two families of PMS evolutionary models. We convert each isochrone into two-dimensional probability distributions of artificial PMS stars in the CMD by applying the aforementioned biases that dislocate these stars from their original CMD positions. A maximum-likelihood technique is then applied to derive the probability for each observed star to have a certain age as well as the best age for the entire cluster. We apply our method to the photometric catalog of ~2000 PMS stars in the young association LH 95 in the Large Magellanic Cloud, based on the deepest HST/ACS imaging ever performed toward this galaxy, with a detection limit of V ~ 28, corresponding to M ~ 0.2 M(solar). We assume the initial mass function and reddening distribution for the system, as they have been previously derived by us. Our treatment shows that the age determination is very sensitive to the considered grid of evolutionary models and the assumed binary fraction. The age of LH 95 is found to vary from 2.8 Myr to 4.4 Myr, depending on these factors. We evaluate the accuracy of our age estimation and find that the method is fairly accurate in the PMS regime, while the precision of the measurement of the age is lower at higher luminosities. Our analysis allows us to disentangle a real age spread from the apparent CMD broadening caused by the physical and observational biases. We find that LH 95 hosts an age spread that is represented well by a Gaussian distribution with an FWHM of the order of 2.8-4.4 Myr depending on the model and binary fraction. We detect a dependence of the average age of the system with the stellar mass. This dependence does not appear to have any physical meaning, being rather due to imperfections of the PMS evolutionary models, which tend to predict lower ages for the intermediate-mass and higher ages for the low-mass stars.