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Published July 1, 2011 | Published + Submitted
Journal Article Open

The Magnetosphere of the Ultracool Dwarf DENIS 1048-3956


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 α = 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 corotation radius, RC , of the star. Adopting this model, we find R_C between 1.2 R* and 2.9 R*, and a non-thermal electron density and magnetic field strength between 10^5 and 10^(7.2) cm^–3 and between 70 and 260 G, respectively, at R_C . The model accounts for the violation of the Güdel-Benz relation between X-ray and radio luminosities of low-mass stars by DENIS-P J104814.9–395604.

Additional Information

© 2011 American Astronomical Society. Received 2010 September 21; accepted 2011 May 4; published 2011 June 3. We thank the ATCA staff and duty astronomers for their valuable assistance with the observations reported here. The Australia Telescope Compact Array is part of the Australia Telescope which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. G.H. is supported by an Australian Research Council QEII Fellowship (project no. DP0878388). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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