TESS Observations of the Luhman 16AB Brown Dwarf System: Rotational Periods, Lightcurve Evolution, and Zonal Circulation

TL;DR

This study uses high-precision TESS data to analyze the rotational periods, lightcurve evolution, and atmospheric circulation of the brown dwarf binary system Luhman 16AB, revealing complex zonal flows and planetary-scale waves.

Contribution

It provides the first long-term, continuous high-precision lightcurve analysis of Luhman 16AB, demonstrating the presence of high-speed jets, zonal circulation, and planetary-scale waves in brown dwarf atmospheres.

Findings

Rotational period of Luhman 16B is 5.28 hours.

Detected multiple periods indicating zonal flows and jets.

Evidence for a long-period vortex in polar regions.

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 TESS data, we extract a high-precision lightcurve of the closest brown dwarfs, which form the binary system Luhman 16AB. Our observations, that cover about 100 rotations of Luhman 16B, display continuous lightcurve evolution. The periodogram analysis shows that the rotational period of the component that dominates the lightcurve is 5.28 h. We also find evidence for periods of 2.5 h, 6.94 h, and 90.8 h. We show that the 2.5 h and 5.28 h periods emerge from Luhman 16B 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 h peak is likely the rotation period of Luhman 16A. 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 h) 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.