Quantitative evidence of an intrinsic luminosity spread in the Orion nebula cluster

¹ Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
e-mail: reggiani@phys.ethz.ch
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany

Received: 23 March 2011
Accepted: 2 August 2011

Abstract

Aims. We study the distribution of stellar ages in the Orion nebula cluster (ONC) using accurate HST photometry taken from HST Treasury Program observations of the ONC utilizing the cluster distance estimated by Menten and collaborators. We investigate whether there is an intrinsic age spread in the region and whether the age depends on the spatial distribution.

Methods. We estimate the extinction and accretion luminosity towards each source by performing synthetic photometry on an empirical calibration of atmospheric models using the package Chorizos of Maiz-Apellaniz. The position of the sources in the HR-diagram is compared with different theoretical isochrones to estimate the mean cluster age and age dispersion. On the basis of Monte Carlo simulations, we quantify the amount of intrinsic age spread in the region, taking into account uncertainties in the distance, spectral type, extinction, unresolved binaries, accretion, and photometric variability.

Results. According to the evolutionary models of Siess and collaborators, the mean age of the Cluster is 2.2 Myr with a scatter of few Myr. With Monte Carlo simulations, we find that the observed age spread is inconsistent with that of a coeval stellar population, but in agreement with a star formation activity between 1.5 and 3.5 Myr. We also observe some evidence that ages depends on the spatial distribution.