Wisep J060738.65+242953.4: A Nearby Pole-on L8 Brown Dwarf With Radio Emission

Abstract

We present a simultaneous, multi-wavelength campaign targeting the nearby (7.2 pc) L8/L9 (optical/near-infrared) dwarf WISEP J060738.65+242953.4 in the mid-infrared, radio, and optical. Spitzer Space Telescope observations show no variability at the 0.2% level over 10 hours each in the 3.6 and 4.5 micron bands. Kepler K2 monitoring over 36 days in Campaign 0 rules out stable periodic signals in the optical with amplitudes great than 1.5% and periods between 1.5 hours and 2 days. Non-simultaneous Gemini optical spectroscopy detects lithium, constraining this L dwarf to be less than ∼ 2 Gyr old, but no Balmer emission is observed. The low measured projected rotation velocity (v sin i < 6 km s −1 ) and lack of variability are very unusual compared to other brown dwarfs, and we argue that this substellar object is likely viewed pole-on. We detect quiescent (non-bursting) radio emission with the VLA. Amongst radio detected L and T dwarfs, it has the lowest observed L ν and the lowest v sin i. We discuss the implications of a pole-on detection for various proposed radio emission scenarios.

A Panchromatic View of Brown Dwarf Aurorae

“…If this is the case, and the fraction of auroral objects is driven largely by the proportion of objects with physical conditions amenable to the generation of auroral magnetospheric currents, then generically, the magnetic axes of brown dwarfs are likely to be misaligned or at least a substantial component of the large-scale field is misaligned. Given these considerations, the detection of quiescent radio emission from a pole-on L-dwarf is unlikely to generate pulsed radio emission unless the magnetic axis is totally misaligned (Gizis et al 2016). Similarly, the detection of variable circularly polarized emission in the data of WISEP J112254.73+255021.5 Figure 12.…”

“…h Although no period has yet been observed in the radio, a period from photometric variability yields 1.86±0.02 hr (Harding et al 2013a). i Although no period has yet been observed in the radio, a period from photometric variability yields 8.9 hr (Gizis et al 2016). j Objects have been observed to display multiple highly circularly polarized pulses, but a periodicity has not yet been determined.…”

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

“…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.…”

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

“…However, several cautionary remarks should be noted regarding the quiescent radio emission from J0607+24 described in Gizis et al (2016). I note that the source was reported to be detected with a "total flux density of 15.6±6.3 µJy at a mean frequency of 6.05 GHz", yielding a "3.5σ source" detection (Gizis et al 2016).…”

“…The study also presented a preliminary parallax measurement for the source as 139 ± 2 mas, yielding a revised distance estimate of 7.19 +0.11 −0.10 pc -the value adopted to compute physical quantities throughout the remainder of this paper. Optical spectroscopy of J0607+24 with the Gemini-North telescope and GMOS spectrograph on 2012 October 15 failed to detect any Hα emission with a limiting equivalent width of <0.5Å (Gizis et al 2016). However, lithium absorption was detected, which along with the measurement of L bol /L ⊙ = −4.66 ± 0.02 (T ef f = 1250 K), constrained J0607+24's mass to be M≤0.055M ⊙ , with age t 2 Gyr.…”