Is WISEP J060738.65+242953.4 Really a Magnetically Active, Pole-on L Dwarf?

Abstract

The interplay of rotation and manifested magnetic activity on ultracool dwarfs (UCDs) is of key importance for gathering clues as to the operation of the dynamos within these objects. A number of magnetized UCDs host kG-strength magnetic fields. It was recently reported that the L8 dwarf WISEP J060738.65+242953.4 is a radio-emitting UCD that is likely observed pole-on, due to its lack of photometric variability and narrow spectral lines. Follow-up radio observations at Arecibo Observatory, together with careful analysis of previously published details, however, suggest that the scientific and statistical significance of the radio and spectroscopic data have been overstated. If the UCD is observed along its aligned spin/magnetic axis, the absence of observed Hα activity may present challenges to the auroral model of UCD magnetism, although short or long-term cyclic magnetic activity may explain this behavior. Monte Carlo simulations presented here suggest that the source probably rotates with v sin i=6-12 km s −1 , indicating that its inclination angle and rotational velocity are unexceptional and that its angular momentum has evolved as expected for brown dwarfs observed in ∼1 Myr-old clusters. The discovery and verification of the most rapidly and slowest rotating brown dwarfs place valuable constraints on the angular momentum evolution and magnetic activity history of these objects.

ROME. IV. An Arecibo Search for Substellar Magnetospheric Radio Emissions in Purported Exoplanet-hosting Systems at 5 GHz

“…Prior to analysis, the signal-to-noise ratio characteristics of the data were improved by rebinning the data from (frequency, time) resolutions of (20.9 kHz, 0.1 s) to (83.6 kHz, 0.9 s). Radio frequency interference (RFI) was mitigated by an iterative statistical process detailed in Route (2013).…”

“…The per-scan sensitivity ranged from 1σ ∼ 0.1-1.2 mJy with sensitivity distribution reported in Route (2017b). Comprehensive, frequency, and system-dependent sensitivity limits are available in Route (2013). These data products were then searched for bursts of 10% circularly polarized (Stokes V ) emission, which are readily apparent when they occur and are a telltale sign of ECM emission.…”

“…The rotational phase coverage of the brown dwarf targets HD 38529 c and HD 106252 b is probably significantly better. Magnetically active UCD rotation periods range from 0.288 hr (J1122+25) to ∼50 hr (SDSS J151643.01+305344.4), with a mean rotation period P ∼ 1.22 hr (Route 2017a(Route , 2017b. Since HD 38529 c and HD 106252 b were both observed for 1 hr, if they rotate at the mean UCD rotation period, then their rotational phase coverage is likely ∼80%, although given the spread in rotation periods, it could be as low as ∼2%.…”

“…Magnetic field evolutionary tracks that extrapolate from a scaling law derived from geodynamo models estimate that M dwarf, brown dwarf, and exoplanet magnetic field strengths peak at ∼4 kG, ∼3 kG, and ∼200 G, respectively, for objects of ages ∼ 1 Gyr (Christensen et al 2009;Reiners & Christensen 2010). During my first ∼5 GHz survey of UCDs conducted at Arecibo Observatory (AO), I detected three high brightness temperature (T B > 10 11 K), highly circular polarized (70%) flares from the first radio-emitting T dwarf, 2MASS J10475385 +2124234 (J1047+21 hereafter; Route & Wolszczan 2012, 2013. The flare properties indicated that they were caused by the same ECM mechanism that operates within the Jovian and solar magnetospheres.…”