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

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 using PMS evolutionary models. We convert each isochrone into 2D 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 LMC, based on the deepest HST/ACS imaging ever performed toward this galaxy, with a detection limit of V~28, corresponding to M~0.2 Msun. 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. 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 well represented by a gaussian distribution with a FWHM of the order of 2.8 Myr to 4.2 Myr depending on the model and binary fraction. We detect a dependence of the average age of the system with 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 masses, and higher ages for low-mass stars.