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

Abstract

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