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Published August 20, 2012 | Published
Journal Article Open

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 (Ṁ_acc) in pre-main-sequence stars is limited by the lack of accurate measurements of Ṁ_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 Ṁ_acc for a sample of ~700 sources in the Orion Nebula Cluster, ranging from the hydrogen-burning limit to M_* ~ 2 M_☉. We derive Ṁ_acc from both the U-band excess and the Hα luminosity (L_Hα), after determining empirically both the shape of the typical accretion spectrum across the Balmer jump and the relation between the accretion luminosity (L acc) and L_Hα, which is L_acc/L_☉ = (1.31 ± 0.03) ⋅ L_Hα/L_☉ + (2.63 ± 0.13). Given our large statistical sample, we are able to accurately investigate relations between Ṁ_acc and the parameters of the central star such as mass and age. We clearly find Ṁ_acc to increase with stellar mass and decrease over evolutionary time, but we also find strong evidence that the decay of Ṁ_acc with stellar age occurs over longer timescales for more massive PMS stars. Our best-fit relation between these parameters is given by log(Ṁ_acc)/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.

Additional Information

© 2012 American Astronomical Society. Received 2012 May 3; accepted 2012 June 19; published 2012 August 6. This work was made possible by GO program 10246 of the Hubble Space Telescope, which is operated by the Space Telescope Science Institute. C.F.M. thanks James Pringle and Philip J. Armitage for insightful discussions. Facility: HST (WFPC2)

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