Abstract
A number of fast-rotating ultra cool dwarfs (UCDs) emit pulsed coherent radiation, attributed to the electron cyclotron maser instability, a phenomenon that occurs in the solar system at planets with strong auroral emission. In this paper we examine magnetosphere-ionosphere coupling currents in UCDs, adopting processes used in models of Jovian emission. We consider the angular velocity gradient arising from a steady outward flux of angular momentum from an internal plasma source, as analogous to the jovian main oval current system, as well as the interaction of a rotating magnetosphere with the external medium. Both of these mechanisms are seen in the solar system to be responsible for the production of radio emission. We present the results of an investigation over a range of relevant plasma and magnetosphere-ionosphere coupling parameters to determine regimes consistent with observed UCD radio luminosities. Both processes are able to explain observed UCD luminosities with ionospheric Pedersen conductances of ~1-2 mho, either for a closed magnetosphere with a plasma mass outflow rate of ~10$^5$ kg/s, i.e. a factor of ~100 larger than that observed at Jupiter's moon Io, or for a dwarf with an open magnetosphere moving through the interstellar medium at ~50 km/s and a plasma mass outflow rate of ~1000 kg/s. The radio luminosity resulting from these mechanisms have opposing dependencies on the magnetic field strength, a point which may be used to discriminate between the two models as more data become available.
Highlights
Ultracool dwarfs (UCDs) are objects with spectral type M7 and later, a class encompassing brown dwarfs along with the lowest mass stars (Kirkpatrick et al 1997)
The plasma populations at UCDs are unknown, we can examine a range of values for the plasma and MI coupling parameters, observing how the angular velocity profile, current system, and radio emission vary in response, in order to estimate the likely parameters at observed UCDs
The field-aligned current density per unit magnetic field strength ( j /B), a quantity that is constant along a field line, is shown in Fig. 3(d) and is found to rise to positive peak, i.e. upwardflowing current associated with auroral emission, of ∼0.037 pAm−2 nT−1 at 440 RUCD before transitioning to large negative values in the outer region of the magnetosphere, peaking at -0.266 pAm−2 nT−1 at the magnetopause
Summary
Ultracool dwarfs (UCDs) are objects with spectral type M7 and later, a class encompassing brown dwarfs along with the lowest mass stars (Kirkpatrick et al 1997). Quiescent and flaring radio emission was first reported by Berger et al (2001), with a luminosity approximately four orders of magnitude greater than predicted by the Gudel-Benz relation (Guedel & Benz 1993), an empirical relation between the observed X-ray and radio luminosities of stellar objects over a wide range of spectral classes. As UCDs are fully convective, this mechanism is precluded and other dynamo models are suggested (Dobler et al 2006; Browning 2008)
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