HUBBLE SPACE TELESCOPE MEASURES OF MASS ACCRETION RATES IN THE ORION NEBULA CLUSTER

The present observational understanding of the evolution of the mass accretion rates () in pre-main-sequence stars is limited by the lack of accurate measurements of over homogeneous and large statistical samples of young stars. Such observational effort is needed to properly constrain the theory of star formation and disk evolution. Based on Hubble Space Telescope/WFPC2 observations, we present a study of for a sample of ∼700 sources in the Orion Nebula Cluster, ranging from the hydrogen-burning limit to M* ∼ 2 M☉. We derive from both the U-band excess and the Hα luminosity (LHα), after determining empirically both the shape of the typical accretion spectrum across the Balmer jump and the relation between the accretion luminosity (Lacc) and LHα, which is Lacc/L☉ = (1.31 ± 0.03) · LHα/L☉ + (2.63 ± 0.13). Given our large statistical sample, we are able to accurately investigate relations between and the parameters of the central star such as mass and age. We clearly find to increase with stellar mass and decrease over evolutionary time, but we also find strong evidence that the decay of with stellar age occurs over longer timescales for more massive PMS stars. Our best-fit relation between these parameters is given by log (/M☉ yr) = (−5.12 ± 0.86) − (0.46 ± 0.13) · log (t/yr) − (5.75 ± 1.47) · log (M*/M☉) + (1.17 ± 0.23) · log (t/yr) · log (M*/M☉). These results also suggest that the similarity solution model could be revised for sources with M* ≳ 0.5 M☉. Finally, we do not find a clear trend indicating environmental effects on the accretion properties of the sources.