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

TL;DR

This study measures UV excess emission to determine accretion rates in young low-mass stars and brown dwarfs, revealing higher rates than previous H-alpha methods and correlations with line luminosities.

Contribution

It introduces UV excess as a new diagnostic for accretion in very low-mass stars and brown dwarfs, providing more accurate accretion rate estimates.

Findings

UV-based accretion rates are 4-7 times higher than H-alpha estimates.

Accretion luminosity correlates with multiple emission lines.

Accretion rate scales with stellar mass as M^{1.87}.

Abstract

Low-resolution spectra from 3000-9000 AA 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 2e-12 to 1e-8 Mo/yr. These results are compared with {\it HST}/STIS spectra of roughly solar-mass accretors with accretion rates that range from 2e-10 to 5e-8 Mo/yr. 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 H-alpha 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 H-alpha 10% width and line fluxes show a large amount of scatter. Our results and previous accretion rate measurements suggest that accretion rate is proportional to M^(1.87+/-0.26) for accretors in the Taurus Molecular Cloud.