Auroral radio emission from ultracool dwarfs: a Jovian model

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 magnetosphereionosphere 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 −1 , 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 −1 and a plasma mass outflow rate of ∼ 1000 kg s −1 . 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.

The Strongest Magnetic Fields on the Coolest Brown Dwarfs

“…Indeed, all of the examples of planetary aurorae in our Solar System demonstrate that the ECM emission can originate from a dipole component of our targets' magnetic fields. Similarly, models of the co-rotation breakdown mechanism that occurs in the Jovian auroral system assuming dipolar magnetic fields show close agreement between modeled and observed auroral radio luminosities for TVLM 513-46546 (M9), LSR J1835+3259 (M8.5), and 2MASS J00361617+1821104 (L3.5) (Nichols et al 2012;Turnpenney et al 2017). This model also predicted rotation periods between ∼2.1-2.8 hr for 2M1047, which is not inconsistent with the rotation period measured by Williams & Berger (2015).…”

A Deep Radio Limit for the TRAPPIST-1 System

“…Williams et al (2014) attributed the X-ray bright objects to the population with axisymmetric dipolar large-scale fields and the radio load objects to those with predominantly multi-polar fields. We posit that it is more likely to be the reverse, as the mechanisms theorized to produce the radio emission require strong dipolar large-scale field topologies (Pineda et al 2017;Turnpenney et al 2017), and in light of the Shulyak et al (2017) results, that the X-ray emission alone cannot be used to distinguish the likely topology, the radio measurements are necessary to classify the X-ray bright and radio loud populations. Moreover, if the presence of the radio emission depends on additional factors, not just the field topology or stellar properties (see Section 5), the populations of X-ray and radio emitting objects may not be totally mutually exclusive.…”

“…Although this estimate is smaller than the strength of some of the observed highly circularly polarized radio bursts from UCDs (see Table 1 of Pineda et al 2017), it is subject to many unknown quantities, including the radio emission efficiency of the ECM instability (∼0.01), and the beaming solid angle (∼1.6 sr), in addition to other system properties like the plasma environment. For example, while a beaming solid angle of ∼1.6 sr is commonly used as a basis for estimating ECM radio fluxes from UCDs (Nichols et al 2012;Turnpenney et al 2017), it could be as low as ∼0.16 sr (Queinnec & Zarka 2001;Zarka et al 2004), which would increase the predicted flux by a factor of 10. Our estimates based on Equation 1 are linear in the magnetic field strength, so a factor of 10 weaker field (500 G), with the same rapid rotation could still produce detectable emission (∼30 µJy ), however, if the rotation is slightly slower or the plasma environment is less dense, the prospects for currently detectable emission become marginal.…”

“…It remains to be seen whether the strong ECM and quiescent radio emissions of the few radio UCDs is related to the presence of planets, or are predominantly driven without them (Turnpenney et al 2017). While the magnetic field strength and rotation rate of TRAPPIST-1 are consistent with the non-detection of radio emission within a Jupiter-Io flux tube paradigm, it is notable that the same planetary configuration orbiting a rapidly rotating dwarf with a large-scale dipolar field can account for the observed radio luminosities of radio emitting UCDs.…”

Constraints on magnetospheric radio emission from Y dwarfs

“…Instead, brown dwarf magnetic activity or a component of it may be more analogous to what has been observed in Jupiter. In the planetary case, tracers of magnetic activity such as optical, infrared, and UV aurora (Bhardwaj & Gladstone 2000;Clarke et al 2004;Grodent et al 2009;Maillard & Miller 2011;Dyudina et al 2016;Moore et al 2017) are powered by an external source, the outer magnetosphere, via auroral current systems, such as magnetosphere-ionosphere coupling currents that give rise to auroral activity (Schrijver 2009;Nichols et al 2012;Bagenal et al 2014;Turnpenney et al 2017).…”