Accretion and outflow activity in proto-brown dwarfs

Abstract

We present a near-infrared study of accretion and outflow activity in six Class 0/I proto-brown dwarfs (proto-BDs) using VLT/SINFONI spectroscopy and spectroimaging observations. The spectra show emission in several [Fe II] and H₂ lines associated with jet/outflow activity, and in the accretion diagnostics of Pa β and Br γ lines. The peak velocities of the [Fe II] lines (>100 km s^-1) are higher than the H₂ lines. The Class 0 proto-BDs show strong emission in the H₂ lines but the [Fe II] lines are undetected, while the Class I objects show emission in both [Fe II] and H₂ lines, suggesting an evolutionary trend in the jets from a molecular to an ionic composition. Extended emission with knots is seen in the [Fe II] and H₂ spectroimages for three proto-BDs, while the rest show compact morphologies with a peak on-source. The accretion rates for the proto-BDs span the range of (2 × 10^-6-2 × 10^-8) M_⊙ yr^-1, while the mass-loss rates are in the range of (4 × 10^-8-5 × 10^-9) M_⊙ yr^-1. These rates are within the range measured for low-mass protostars and higher than Class II brown dwarfs. We find a similar range in the jet efficiency for proto-BDs as measured in protostars. We have performed a study of the Brackett decrement from the Br 7-19 lines detected in the proto-BDs. The upper Brackett lines of Br 13-19 are only detected in the earlier stage systems. The ratios of the different Brackett lines with respect to the Br γ line intensity are consistent with the ratios expected from Case B recombination.