Abstract
Context. Observations of auroral emissions are powerful means to remotely sense the space plasma environment around planetary bodies and ultracool dwarfs. Therefore successful searches and characterization of aurorae outside the Solar System will open new avenues in the area of extrasolar space physics. Aims. We aim to demonstrate that brown dwarfs are ideal objects to search for UV aurora outside the Solar System. We specifically search for UV aurora on the late-type T6.5 brown dwarf 2MASS J12373919+6526148 (in the following 2MASS J1237+6526). Methods. Introducing a parameter referred to as auroral power potential, we derive scaling models for auroral powers for rotationally driven aurora applicable to a broad range of wavelengths. We also analyze Hubble Space Telescope observations obtained with the STIS camera at near-UV, far-UV, and Ly-α wavelengths of 2MASS J1237+6526. Results. We show that brown dwarfs, due to their typically strong surface magnetic fields and fast rotation, can produce auroral UV powers on the order of 1019 watt or more. Considering their negligible thermal UV emission, their potentially powerful auroral emissions make brown dwarfs ideal candidates for detecting extrasolar aurorae. We find possible emission from 2MASS J1237+6526, but cannot conclusively attribute it to the brown dwarf due to low signal-to-noise values in combination with nonsystematic trends in the background fluxes. The observations provide upper limits for the emission at various UV wavelength bands. The upper limits for the emission correspond to a UV luminosity of ~1 × 1019 watt, which lies in the range of the theoretically expected values. Conclusions. The possible auroral emission from the dwarf could be produced by a close-in companion and/or magnetospheric transport processes.
Highlights
Brown dwarfs are objects in the mass range between low mass stars and planets (13–80 MJ Jupiter masses; see, e.g., Kumar 1962; Hayashi & Nakano 1963)
The brown dwarf 2MASS J1237+6526, for which we present Hubble Space Telescope (HST) observations, is a dwarf of spectral type T6.5 in the constellation Draco
We show that brown dwarfs are ideal objects to search for UV aurorae outside the Solar System
Summary
Brown dwarfs are objects in the mass range between low mass stars and planets (13–80 MJ Jupiter masses; see, e.g., Kumar 1962; Hayashi & Nakano 1963). The magnetic fields have been determined with various methods, such as in situ measurements for the Solar System targets, observations of radio emission near the electron cyclotron frequency for brown dwarfs and τ Boötis b, or with methods based on the Zeeman-effect for the stars (see last column in Table A.1 for the explicit method applied for each target) Some of these values might be lower limits as radio emission, for example, often stems from magnetospheric layers above the objects’ surfaces. The analysis shows that the low S/N feature between 3065–3110 Å within the inhomogeneous background flux needs to be taken with caution Keeping this uncertainty in mind, we compare the NUV spectrum of the T-type brown dwarf 2MASS J1237+6526 with slightly more massive dwarfs, such as the ultracool M8.5 dwarf LSR J1835+3259 (Saur et al 2018a) and the M4.5V star GJ 1214 (France et al 2016). The flux in the same band is lower for LSR J1835+3259, while it appears negligible for GJ 1214
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