Quantitative Evidence for an Intrinsic Age Spread in the Orion Nebula Cluster

TL;DR

This study uses precise HST photometry and advanced modeling to demonstrate that the Orion Nebula Cluster has a significant intrinsic age spread of 1.5 to 3.5 million years, indicating ongoing star formation.

Contribution

It provides the first detailed quantitative analysis of the age distribution in the ONC, accounting for uncertainties and supporting the presence of an intrinsic age spread.

Findings

Mean cluster age is 2.2 Myr with a few Myrs of scatter.

Observed age spread is inconsistent with a coeval population.

Evidence suggests an age gradient with spatial distribution.

Abstract

Aims. We present a study of the distribution of stellar ages in the Orion Nebula Cluster (ONC) based on accurate HST photometry taken from the HST Treasury Program observations of the ONC utilizing the most recent estimate of the cluster's distance (Menten et al. 2007). We investigate the presence of an intrinsic age spread in the region and a possible trend of age with 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 (Da Rio et al. 2010) using the package Chorizos (Maiz-Apellaniz 2004). The position of the sources in the HR-diagram is compared with different theoretical isochrones to estimate the mean cluster age and age dispersion. Through Monte Carlo simulations we quantify the amount of intrinsic age spread in the region, taking into account uncertainties on the distance, spectral type, extinction, unresolved binaries, accretion and photometric variability. Results. According to Siess et al. (2000) evolutionary models the mean age of the Cluster is 2.2 Myr with a scatter of few Myrs. With Monte Carlo simulations we find that the observed age spread is inconsistent with a coeval stellar population, but is in agreement with a star formation activity between 1.5 and 3.5 Myrs. We also observe light evidence for a trend of ages with spatial distribution.