A measurement of the wind speed on a brown dwarf

Abstract

Zonal (latitudinal) winds dominate the bulk flow of planetary atmospheres. For gas giant planets such as Jupiter, the motion of clouds can be compared with radio emissions from the magnetosphere, which is connected to the planet’s interior, to determine the wind speed. In principle, this technique can be applied to brown dwarfs and/or directly imaged exoplanets if periods can be determined for both the infrared and radio emissions. We apply this method to measure the wind speeds on the brown dwarf 2MASS J10475385+2124234. The difference between the radio period of 1.751 to 1.765 hours and infrared period of 1.741 ± 0.007 hours implies a strong wind (+650 ± 310 meters per second) proceeding eastward. This could be due to atmospheric jet streams and/or low frictional drag at the bottom of the atmosphere.

Atmospheric circulation of brown dwarfs and directly imaged exoplanets driven by cloud radiative feedback: global and equatorial dynamics

“…They can cause short-term evolution of lightcurves via wave beating effects, qualitatively similar to the observed lightcurves of some field BDs as summarized in Apai et al (2017). The eastward Kelvin waves can cause the equatorial flux inhomogeneity rotating faster than the underlying planetary rotation, which may help to explain the observed shorter rotation period of atmospheric features than that of the interior (Allers et al 2020). Isotropic storms and vortices at mid-to-high latitudes also contribute to the lightcurve variability.…”

“…The differential propagation of equatorially trapped waves induces short-term evolution of simulated lightcurves, analogous to the wave beating effects described in Apai et al (2017). Eastward propagating equatorial Kelvin waves are sometimes dominant, causing a slightly shorter rotation period of the atmospheric features relative to the underlying planetary rotation period, which agrees well with observational results by Allers et al (2020). When the bottom dissipation is weak, strong and broad zonal jets develop and modify wave propagation and lightcurve variability.…”

“…Both dynamical features are driven by the cloud radiative feedback, providing an essential physical mechanism to explain several types of time evolution of lightcurve variability (see a summary and analysis in Apai et al 2017). Eastward propagating Kelvin waves with phase speeds of a few hundred m s −1 are sometimes dominant in our simulations, and the existence of these waves in atmospheres of BDs may explain the shorter rotation period of the atmosphere than that of the interior observed for a nearby BD (Allers et al 2020). Our models predict that different equatorial waves that may travel in different zonal velocities could influence the lightcurves differently at different times.…”

“…The interesting consequence is that these waves can cause differences between the "observed" lightcurve periodicity and the underlying rotation period, and the degree of this deviation may vary with time. It will be interesting that the same observations performed by Allers et al (2020) could be repeated for the same system in the future to examine its long-term variability. Recently, Vos et al (2017), Vos et al (2018) and Vos et al (2020) suggested a viewing angle dependence of the observed near-IR colors and variability amplitude for a large sample of field BDs.…”

“…• Simultaneous tracking of near-IR and radio variability, in which the former measures the period at which atmospheric features rotate in and out of the view and the latter likely reflects the rotation period of the interior, could track possible differential rotation between the atmosphere and the interior. Recently, Allers et al (2020) applied this technique to a nearby T6 brown dwarf and showed that the period of the near-IR variability is slightly shorter than that of the radio emission, suggesting that the dominant atmospheric features travel eastward relative to the interior with a zonal speed of a few hundred m s −1 .…”