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.
Highlights
Planetary mass or brown dwarf companions which orbit their host stars at separations exceeding 100 au are intriguing objects that may provide the potential to perform detailed investigations of the atmospheres of young giant planets and brown dwarfs
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
We find SR 12 C to be a brown dwarf of spectral type L0 ± 1, log g = 4 ± 0.5, an effective temperature of 2600 ± 100 K
Summary
Planetary mass or brown dwarf companions which orbit their host stars at separations exceeding 100 au are intriguing objects that may provide the potential to perform detailed investigations of the atmospheres of young giant planets and brown dwarfs. How these interesting sub-stellar companions (SSCs) formed remains a mystery. At separations of several hundred au (i.e. much larger than most known exoplanets or brown dwarf companions), the currently most accepted planet formation scenario of in situ core accretion is unlikely to occur. We find that SSCs accrete at a rate similar to isolated objects and that accretion might perhaps cease at about the same age as in isolated low-mass objects
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