Accretion signatures in the X-shooter spectrum of the substellar companion to SR12

TL;DR

This study presents spectroscopic analysis of the young brown dwarf SR 12 C, revealing its physical properties and accretion rate, and compares these with isolated brown dwarfs to infer possible formation mechanisms.

Contribution

First detailed accretion rate measurement of a substellar companion at large separation, providing insights into its formation and evolution.

Findings

SR 12 C is an L0 brown dwarf with T_eff ~2600 K.

Accretion rate of SR 12 C is approximately 10^{-10} solar masses per year.

No significant difference in age and accretion rates between substellar companions and isolated brown dwarfs.

Abstract

About a dozen substellar companions orbiting young stellar objects or pre-main sequence stars at several hundred au have been identified in the last decade. These objects are interesting both due to the uncertainties surrounding their formation, and because their large separation from the host star offers the potential to study the atmospheres of young giant planets and brown dwarfs. Here, we present X-shooter spectroscopy of SR 12 C, a $\sim\,2\,$Myrs young brown dwarf orbiting SR 12 at an orbital separation of 1083 au. We determine the spectral type, gravity, and effective temperature via comparison with models and observational templates of young brown dwarfs. In addition, we detect and characterize accretion using several accretion tracers. We find SR 12 C to be a brown dwarf of spectral type L0 $\pm$ 1, $\log$ g = 4 $\pm$ 0.5, an effective temperature of 2600 $\pm$ 100 K. Our spectra provide clear evidence for accretion at a rate of $\sim$ $10^{-10}$ $M_{\odot}yr^{-1}$. This makes SR 12 one of the few sub-stellar companions with a reliable estimate for its accretion rate. A comparison of the ages and accretion rates of sub-stellar companions with young isolated brown dwarfs does not reveal any significant differences. If further accretion rate measurements of a large number of substellar companions can confirm this trend, this would hint towards a similar formation mechanism for substellar companions at large separations and isolated brown dwarfs.