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Published July 1, 2008 | Published
Journal Article Open

UV excess measures of accretion onto young very low mass stars and brown dwarfs


Low-resolution spectra from 3000 to 9000 Å of young low-mass stars and brown dwarfs were obtained with LRIS on Keck I. The excess UV and optical emission arising in the Balmer and Paschen continua yields mass accretion rates ranging from 2 × 10^(−12) to 10^−8 M☉ yr^−1. These results are compared with HST STIS spectra of roughly solar-mass accretors with accretion rates that range from 2 × 10^(−10) to 5 × 10^−8 M☉ yr^−1. The weak photospheric emission from M dwarfs at <4000 Å leads to a higher contrast between the accretion and photospheric emission relative to higher mass counterparts. The mass accretion rates measured here are systematically ~4-7 times larger than those from Hα emission line profiles, with a difference that is consistent with but unlikely to be explained by the uncertainty in both methods. The accretion luminosity correlates well with many line luminosities, including high Balmer and many He I lines. Correlations of the accretion rate with Hα 10% width and line fluxes show a large amount of scatter. Our results and previous accretion rate measurements suggest that M ∝ M^(1.87 ± 0.26) for accretors in the Taurus molecular cloud.

Additional Information

© 2008 The American Astronomical Society. Print publication: Issue 1 (2008 July 1) Received 2007 November 6, accepted for publication 2008 January 16. We thank Jeff Valenti and Chris Johns-Krull for use of their plane-parallel slab model codes that they developed in Valenti et al. (1993). We thank Russel White for discussion of the proposal and Nuria Calvet for discussion of the line emission. We thank the anonymous referee for helpful comments, which served to improve the structure and clarity of the paper. Most of 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. Some of the data presented in this paper were obtained from the Multimission Archive at the Space Telescope Science Institute (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NAG5-7584 and by other grants and contracts.

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