The Radio Spectrum of TVLM 513‐46546: Constraints on the Coronal Properties of a Late M Dwarf

We explore the radio emission from the M9 dwarf, TVLM513-46546, at multipleradio frequencies, determining the flux spectrum of persistent radio emission,as well as constraining the levels of circular polarization. Detections at both3.6 and 6 cm provide spectral index measurement $\alpha$ (where S$_{\nu}\propto \nu^{\alpha}$) of $-0.4\pm0.1$. A detection at 20 cm suggests that thespectral peak is between 1.4 and 5 GHz. The most stringent upper limits oncircular polarization are at 3.6 and 6 cm, with $V/I <$15%. Thesecharacteristics agree well with those of typical parameters for early to mid Mdwarfs, confirming that magnetic activity is present at levels comparable withthose extrapolated from earlier M dwarfs. We apply analytic models toinvestigate the coronal properties under simple assumptions of dipole magneticfield geometry and radially varying nonthermal electron density distributions.Requiring the spectrum to be optically thin at frequencies higher than 5 GHzand reproducing the observed 3.6 cm fluxes constrains the magnetic field at thebase to be less than about 500 G. There is no statistically significantperiodicity in the 3.6 cm light curve, but it is consistent with low-levelvariability.Comment: 11 pages, 2 figures Accepted for publication in the Astrophysical Journa