TESS Observations of the Luhman 16 AB Brown Dwarf System: Rotational Periods, Lightcurve Evolution, and Zonal Circulation* * Based on observations collected with the NASA TESS.

Abstract

Brown dwarfs were recently found to display rotational modulations, commonly attributed to cloud cover of varying thickness, possibly modulated by planetary-scale waves. However, the long-term, continuous, high-precision monitoring data to test this hypothesis for more objects is lacking. By applying our novel photometric approach to Transiting Exoplanet Survey Satellite data, we extract a high-precision lightcurve of the closest brown dwarfs, which form the binary system Luhman 16 AB. Our observations, which cover about 100 rotations of Luhman 16 B, display continuous lightcurve evolution. The periodogram analysis shows that the rotational period of the component that dominates the lightcurve is 5.28 hr. We also find evidence for periods of 2.5, 6.94, and 90.8 hr. We show that the 2.5 and 5.28 hr periods emerge from Luhman 16 B and that they consist of multiple, slightly shifted peaks, revealing the presence of high-speed jets and zonal circulation in this object. We find that the lightcurve evolution is well fit by the planetary-scale waves model, further supporting this interpretation. We argue that the 6.94 hr peak is likely the rotation period of Luhman 16 A. By comparing the rotational periods to observed v sin(i) measurements, we show that the two brown dwarfs are viewed at angles close to their equatorial planes. We also describe a long-period (P ∼ 91 hr) evolution in the lightcurve, which we propose emerges from the vortex-dominated polar regions. Our study paves the way toward direct comparisons of the predictions of global circulation models to observations via periodogram analysis.