The magnetosphere of the ultracool dwarf DENIS 1048-3956

TL;DR

This study presents wide-band radio observations of the ultracool dwarf DENIS 1048-3956, revealing gyrosynchrotron emission characteristics and estimating magnetic field properties beyond the star's co-rotation radius.

Contribution

It provides the first wide-band radio spectrum of an ultracool dwarf and models its magnetic environment, explaining its radio emission and X-ray/radio luminosity relation.

Findings

Detected a stable power-law radio spectrum with spectral index 1.71.

Found circular polarization fractions of 0.25 to 0.4 at low frequencies.

Estimated magnetic field strength between 70 and 260 G at the co-rotation radius.

Abstract

Ultracool dwarfs, the least-massive contributors to the stellar mass function, exhibit striking magnetic properties that are inconsistent with trends for more massive stars. Here, we present the widest-band radio observations to date of an ultracool dwarf, DENIS-P J104814.9-395604, in four 2 GHz bandwidths between wavelengths of 1 cm and 10 cm. These data were obtained with the Australia Telescope Compact Array using the new Compact Array Broadband Backend instrument. We detected a stable negatively-sloped power-law spectrum in total intensity, with spectral index alpha=1.71+-0.09. Circular polarization fractions between 0.25 and 0.4 were found at the low-frequency end of our detection band. We interpret these results as indicative of gyrosynchrotron emission. We suggest that the radio emission originates from beyond the co-rotation radius, R_C, of the star. Adopting this model, we find R_C between 1.2-2.9 R_*, and a non-thermal electron density and magnetic field strength between 10^(5)-10^(7.2) cm^(-3) and 70-260 G respectively at R_C. The model accounts for the violation of the Guedel-Benz relation between X-ray and radio luminosities of low-mass stars by DENIS-P J104814.9-395604.