HST measures of Mass Accretion Rates in the Orion Nebula Cluster
  The present observational understanding of the evolution of the mass
accretion rates (Macc) in pre-main sequence stars is limited by the lack of
accurate measurements of Macc 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 HST/WFPC2 observations,
we present a study of Macc for a sample of \sim 700 sources in the Orion Nebula
Cluster, ranging from the Hydrogen-burning limit to M\ast \sim 2M\odot. We
derive Macc from both the U-band excess and the H{\alpha} luminosity
(LH{\alpha}), after determining empirically both the shape of the typical
accretion spectrum across the Balmer jump and the relation between the
accretion luminosity (Lacc) and LH{\alpha}, that is Lacc/L\odot =
(1.31\pm0.03)\cdotLH{\alpha}/L\odot + (2.63\pm 0.13). Given our large
statistical sample, we are able to accurately investigate relations between
Macc and the parameters of the central star such as mass and age. We clearly
find Macc to increase with stellar mass, and decrease over evolutionary time,
but we also find strong evidence that the decay of Macc with stellar age occurs
over longer timescales for more massive PMS stars. Our best fit relation
between these parameters is given by: log(Macc/M\odot\cdotyr)=(-5.12 \pm 0.86)
-(0.46 \pm 0.13) \cdot log(t/yr) -(5.75 \pm 1.47)\cdot log(M\ast/M\odot) +
(1.17 \pm 0.23)\cdot log(t/yr) \cdot log(M\ast/M\odot). These results also
suggest that the similarity solution model could be revised for sources with
M\ast > 0.5M\odot. Finally, we do not find a clear trend indicating
environmental effects on the accretion properties of the sources.
