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

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

a r X i v : 0 8 0 1 .3 5 2 5 v 1 [ a s t r o - p h ] 2 3 J a n 2 0 0 8 Draft version February 2, 2008 Preprint typeset using LATEX style emulateapj v. 03/07/07 UV EXCESS MEASURES OF ACCRETION ONTO YOUNG VERY LOW-MASS STARS AND BROWN DWARFS Gregory J. Herczeg1 Lynne A. Hillenbrand1 Draft version February 2, 2008 ABSTRACT Low-resolution spectra from 3000–9000 A? 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 A? 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? ∝ M1.87±0.26 for accretors in the Taurus Molecular Cloud. Subject headings: stars: pre-main sequence — stars: planetary systems: protoplanetary disks 1. INTRODUCTION In the magnetospheric accretion model for accretion onto young stars, the stellar magnetosphere truncates the disk at a few stellar radii. Gas from the disk accretes onto the star along magnetic field lines and shocks at the stellar surface. The ～ 104 K optically-thick post- shock and optically-thin pre-shock gas produce emission in the Balmer and Paschen continuum and in many lines, including the Balmer an