X-shooter observations of the accreting brown dwarf J053825.4-024241

TL;DR

This study presents the first X-shooter spectroscopic observations of a young, accreting brown dwarf in sigma Orionis, revealing detailed accretion diagnostics, emission line properties, and physical conditions of the accretion region.

Contribution

First simultaneous UV to NIR spectrum of a young brown dwarf showing rich emission lines and detailed accretion diagnostics, expanding understanding of substellar accretion processes.

Findings

Large spread in accretion rate measurements, partly intrinsic.

Hydrogen Balmer lines indicate cold, dense, LTE emitting region.

Mean accretion rate logMacc ~ -9.86 with significant uncertainties.

Abstract

We present the first observations of a probable brown dwarf, obtained with the new spectrograph X-shooter mounted on the UT2@VLT. The target (2MASS J053825.4-024241) is a 0.06 Msun object in the star-formation region sigma Orionis. The X-shooter spectrum covers simultaneously the whole range from UV to NIR (300-2500 nm). The J053825.4-024241 spectrum is rich in emission lines that are typical of accreting young object and clearly shows the Balmer jump. Moreover, many photospheric atomic and molecular absorption lines yield the spectral type and confirm that the object is young. We compute the mass accretion rate from all available observed accretion diagnostics. We find that there is a large spread in the Macc values (up to a factor 40) that is not caused by variability; some of this spread may be intrinsic, i.e., owing to different physical conditions of the emitting region for the same Macc. However, within the large error bars all Macc measurements agree, and the mean value is logMacc ~ -9.86 +- 0.45 Myr. The hydrogen Balmer lines are clearly detected up to n=25. Their ratios suggest that the emitting region is cold (T~2000-3000 K), dense and in thermal equilibrium (LTE), and that the lines are optically thick up to n~21. We briefly discuss the implications of this result for magnetospheric accretion models.