Constraints on magnetospheric radio emission from Y dwarfs

Abstract

As a pilot study of magnetism in Y dwarfs, we have observed the three known infrared variable Y dwarfs WISE J085510.83−071442.5, WISE J140518.40+553421.4, and WISEP J173835.53+273258.9 with the NSF’s Karl G. Jansky Very Large Array in the 4–8 GHz frequency range. The aim was to investigate the presence of non-bursting quiescent radio emission as a proxy for highly circularly polarized radio emission associated with large-scale auroral currents. Measurements of magnetic fields on Y dwarfs may be possible by observing auroral radio emission, and such measurements are essential for constraining fully convective magnetic dynamo models. We do not detect any pulsed or quiescent radio emission, down to rms noise levels of 7.2 µJy for WISE J085510.83−071442.5, 2.2 µJy for WISE J140518.40+553421.4, and 3.2 µJy for WISEP J173835.53+273258.9. The fractional detection rate of radio emission from T dwarfs is ∼10 per cent suggesting that a much larger sample of deep observations of Y dwarfs is needed to rule out radio emission in the Y dwarf population. We discuss a framework that uses an empirical relationship between the auroral tracer Hα emission and quiescent radio emission to identify brown-dwarf auroral candidates. Finally, we discuss the implications that Y dwarf radio detections and non-detections can have for developing a picture of brown dwarf magnetism and auroral activity.

O(3P) + CO2 scattering cross-sections at superthermal collision energies for planetary aeronomy

“…Thus, the cross sections and derived physical quantities were constructed from first principles without any external empirical parameters. Although computationally intensive, the ab initio approach has several major advantages over classical theory, such as evaluation of all stateto-state transitions induced by collisions and more complete treatment of purely quantum effects, expected to play a significant role in improving the predictive power and precisions of models (Kao et al 2019). Our computed elastic cross sections are smaller than the elastic cross sections estimated by mass-scaling from welldescribed atom-molecule systems (Lewkow & Kharchenko 2014;Fox & Hać 2018), but larger than a Lennard-Jones cross section reported by Yeung et al (2012).…”

“…Here, we present the results of a theoretical study of O( 3 P) + CO 2 ( j) → O( 3 P) + CO 2 ( j ) elastic and inelastic quantum-mechanical scattering cross sections at collision energies between 0.03 and 5 eV. In addition, we present differential and momentum transfer cross sections, of interest for hot atomic oxygen transport and energy transfer at nonthermal-equilibrium conditions in middle and upper plane-tary atmospheres (Kao et al 2019). Our newly constructed cross sections are of particular interest to Mars aeronomy missions, such as NASA's MAVEN mission (Jakosky et al 2015) and ESA's Trace Gas Orbiter (TGO), as well as to future Venus missions (Kallio et al 2011).…”

Direct Radio Discovery of a Cold Brown Dwarf

“…Brown dwarfs (BDs), with masses between that of stars and planets, display optical aurorae (Hallinan et al 2015) and the associated auroral radio emission (Nichols et al 2012;Hallinan et al 2015;Pineda et al 2017;Kao et al 2018) powered by the electron cyclotron maser instability (Wu & Lee 1979;Treumann 2006;Hallinan et al 2008). In addition, because there appears to be no clear demarcation between the atmospheres and magnetospheres of the smallest coldest brown dwarfs and the largest planets (Deeg & Belmonte 2018), radio observations at the end of the BD sequence are expected to provide a tantalizing glimpse into magnetospheric properties of exoplanets (Kao et al 2018(Kao et al , 2019. Christensen et al (2009) have argued that the magnetic fields of planets, brown dwarfs, and low-mass stars of sufficiently rapid rotation are dipolar and that the field strength scales with the heat flux from the bodies' interior.…”

On the Correlation between L Dwarf Optical and Infrared Variability and Radio Aurorae

“…For any target with a rotation period longer than 2 hr, we were not able to observe full coverage of the brown dwarf and thus may have missed when the pulsed emission was beamed toward Earth. However, since quiescent radio emission at 4-8 GHz (Kao et al 2019) accompanies all known examples of ECM emission from ultracool dwarfs at GHz frequencies. Since we do not detect such emission, these objects likely do not have time-variable ECM emission at our observed frequencies.…”

“…Detectable levels of quiescent radio emission have been observed in all previous observations of periodically pulsed auroral emission, and is therefore considered a proxy for auroral activity. While the source of the quiescent emission is unconfirmed, it has been speculated that it may trace extrasolar analogs to the Jovian radiation belts, where high-energy electrons are trapped by the magnetosphere (Hallinan et al 2006;Pineda et al 2017;Kao et al 2019). The large (kG) magnetic fields of brown dwarfs and surrounding plasma radiation belts may provide the necessary powerhouse and electron reservoir for both the quiescent emission and auroral ECMI emission Kao et al 2019).…”