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Published May 10, 2009 | Published
Journal Article Open

Measuring Tiny Mass Accretion Rates Onto Young Brown Dwarfs


We present low-resolution Keck I/LRIS spectra spanning from 3200 to 9000 Å of nine young brown dwarfs and three low-mass stars in the TW Hya Association and in Upper Sco. The optical spectral types of the brown dwarfs range from M5.5 to M8.75, though two have near-IR spectral types of early L dwarfs. We report new accretion rates derived from excess Balmer continuum emission for the low-mass stars TW Hya and Hen 3-600A and the brown dwarfs 2MASS J12073347–3932540, UScoCTIO 128, SSSPM J1102–3431, USco J160606.29–233513.3, DENIS-P J160603.9–205644, and Oph J162225–240515B, and upper limits on accretion for the low-mass star Hen 3-600B and the brown dwarfs UScoCTIO 112, Oph J162225–240515A, and USco J160723.82–221102.0. For the six brown dwarfs in our sample that are faintest at short wavelengths, the accretion luminosity or upper limit is measurable only when the image is binned over large wavelength intervals. This method extends our sensitivity to accretion rate down to ~10^(–13) M_☉yr^(-1) for brown dwarfs. Since the ability to measure an accretion rate from excess Balmer continuum emission depends on the contrast between excess continuum emission and the underlying photosphere, for objects with earlier spectral types the upper limit on accretion rate is much higher. Absolute uncertainties in our accretion rate measurements of ~3-5 include uncertainty in accretion models, brown dwarf masses, and distance. The accretion rate of 2 × 10_(–12) M_☉ yr^(–1) onto 2MASS J12073347–3932540 is within 15% of two previous measurements, despite large changes in the Hα flux.

Additional Information

© 2009. The American Astronomical Society. Received 2008 October 7; accepted 2009 January 31; published 2009 April 27. We thank Adam Kraus for valuable discussion of the proposal and our results. We are grateful to Kevin Luhman for providing spectra for his spectral sequence of young late-M brown dwarfs and for classifying theTWABDs 2MASS J12073347−3932540 and SSSPM J1102−3431 from our data. G.J.H. thanks Leonardo Testi for discussion of the optical depth in hydrogen lines. We also thank Kevin Luhman, Nicolas Lodieu, and the anonymous referee for helpful comments. The data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. K.L.C. is supported by NASA through the Spitzer Space Telescope Fellowship Program, through a contract issued by the Jet Propulsion Laboratory, California Institute of Technology under a contract with National Aeronautics and Space Administration.

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