First spectropolarimetric measurement of a brown dwarf magnetic field in molecular bands

TL;DR

This study reports the first spectropolarimetric measurement of a magnetic field in a brown dwarf using molecular bands, revealing a strong, possibly variable magnetic field of about 5 kG.

Contribution

It introduces the first direct measurement of a brown dwarf’s magnetic field through full-Stokes spectropolarimetry in molecular bands, employing synthetic spectra and statistical analysis.

Findings

Magnetic field strength of approximately 5 kG determined.

Evidence of small-scale magnetic field coverage on the surface.

Indications of rapid magnetic field changes associated with flare activity.

Abstract

We present the first measurement of the surface magnetic field of a late-M dwarf, LSR J1835+3259, with the help of the full-Stokes spectropolarimetry in the bands of diatomic molecules. Our magnetic field measurements at different rotational phases of the dwarf yielded one 5$\sigma$ and two 3$\sigma$ detections. The observational data have been obtained with the LRISp polarimeter at the Keck observatory on August 22nd and 23rd, 2012. These data have been compared against synthetic full-Stokes spectra in the bands of the molecules CrH, FeH and TiO, which have been calculated for a range of the stellar parameters and magnetic field strengths. Making use of $\chi^2$-minimization and maximum likelihood estimation, we determine the net magnetic field strength $B$ (and not flux $Bf$) of LSR J1835+3259 to $\sim5$ kG, mainly with the help of the Paschen-Back effect in the CrH lines. Our measurements at different rotational phases suggest that the dwarf's surface might be covered with strong small-scale magnetic fields. In addition, recent findings of the hydrogen emission and the Stokes V signal in the Na I near-infrared doublet indicate that the dwarf's surface magnetic field might be changing rapidly giving rise to flare activity, similar to young dMe dwarfs. We substantiate the substellar origin of LSR J1835+3259 by making use of our own data as well as the photometric data from the all-sky surveys 2MASS and WISE.