Auroral Radio Emission From Late L and T Dwarfs: A New Constraint on Dynamo Theory in the Substellar Regime

Abstract

We have observed 6 late-L and T dwarfs with the Karl G. Jansky Very Large Array (VLA) to investigate the presence of highly circularly polarized radio emission, associated with large-scale auroral currents. Previous surveys encompassing ∼60 L6 or later targets in this spectral range have yielded only one detection. Our sample includes the previously detected T6.5 dwarf 2MASS 10475385+2124234 as well as 5 new targets selected for the presence of Hα emission or optical/infrared photometric variability, which are possible manifestations of auroral activity. We detect 2MASS 10475385+2124234, as well as 4 of the 5 targets in our biased sample, including the strong IR variable SIMP J01365662+0933473 and bright Hα emitter 2MASS 12373919+6526148, reinforcing the possibility that activity at these disparate wavelengths is related. The radio emission frequency corresponds to a precise determination of the lower-bound magnetic field strength near the surface of each dwarf and this new sample provides robust constraints on dynamo theory in the low mass brown dwarf regime. Magnetic fields 2.5 kG are confirmed for 5/6 targets. Our results provide tentative evidence that the dynamo operating in this mass regime may be inconsistent with predicted values from a recently proposed model. Further observations at higher radio frequencies are essential for verifying this assertion. Subject headings: brown dwarfs -planets and satellites: aurorae -planets and satellites: magnetic fields -radio continuum: stars -stars: individual (

A Panchromatic View of Brown Dwarf Aurorae

“…Within the past 15 years, the discovery and follow-up observations of radio emission from brown dwarfs (e.g., Berger et al 2001;Hallinan et al 2007;Route & Wolszczan 2012;Hallinan et al 2015;Kao et al 2016) have heralded a shift in our understanding of magnetic activity in low-mass stars and ultracool dwarfs (UCDs; spectral type M7). The accumulating evidence now suggest that there may be a transition at the end of the main sequence away from coronal/chromospheric solar-like magnetic activity toward auroral-planet-like phenomena, from current systems driven by local photospheric plasma motions to those driven by a global electrodynamic interaction in the large-scale magnetosphere.…”

Probing the magnetosphere of the M8.5 dwarf TVLM 513−46546 by modelling its auroral radio emission. Hint of star exoplanet interaction?

“…These fields can be generated by other kinds of dynamo processes working in fully convective stars, such as the α 2 -type dynamo (Kuker & Rudiger 1999;Chabrier & Kuker 2006), or a process similar to the geodynamo operating on Jupiter and Earth (Christensen, Holzwarth & Reiners 2009). The detection of fully polarised radio pulses at the microwave regime (a signature of stellar auroral radio emission) is further evidence that this kind of late-type dwarfs is characterised by magnetic fields at kGauss level (Kao et al 2016), being the emission frequency directly related to the local value of the magnetic field (ν ≈ νB ∝ B). The detection of the auroral radio emission requires the existence of fast electron beams precipitating toward the stellar surface, which has a role as an ionising factor of the UCD atmospheres, as shown by Vorgul & Helling (2016).…”

“…The order of magnitude of the luminosity of a typical UCD detected at radio wavelengths is roughly in the range 10 12 -10 13 erg s −1 Hz −1 or less (Berger et al 2010;McLean et al 2012;Williams et al 2014;Kao et al 2016;Gizis et al 2016). Taking into account also coherent events (Berger et al 2008a), this magnitude can exceed 10 14 erg s −1 Hz −1 , which anyway makes only a small sample observable.…”

“…In these kinds of objects, the classical magnetic-activity indicators (Hα and X-ray emission) fade away (Schmidt et al 2007), even though a number of these UCDs have been detected as non-thermal radio source (Berger et al 2001;Berger 2002Berger , 2006McLean, Berger & Reiners 2012;Antonova et al 2013;Route & Wolszczan 2013;Kao et al 2016;Lynch et al 2016;Williams, Gizis & Berger 2017). If analysed in the context of the late-type stellar magnetic activity, the radio emission from UCDs is peculiar.…”

“…Their magnetic dipole axis is tilted with respect to the rotational axis (oblique rotator model) (Babcock 1949). Furthermore, at the bottom of the main sequence, the signature of the auroral radio emission has been clearly identified in many very low mass stars and brown dwarfs, with spectral type ranging from M8 to T6.5 (Berger 2002;Burgasser & Putman 2005;Antonova et al 2008;Hallinan et al 2008;Route & Wolszczan 2012, 2016Williams et al 2015a;Burgasser et al 2015;Kao et al 2016).…”