Rotational Modulations of Brown Dwarfs and Planetary-Mass Companions

Rotation introduces brightness and spectroscopic variability in brown dwarfs and giant exoplanets. Because the substellar objects have patchy condensate clouds and heterogeneous atmospheres, the disk-integrated light from the visible hemisphere modulates as they rotate. Using time-resolved observations, we can learn about the condensate cloud properties, atmospheric circulation patterns, and rotation rates of substellar objects. Such information is critical for characterizing exoplanets.

Time-resolved observations have been extremely successful in unveiling the condensate cloud structures of brown dwarfs. One focus of my thesis study is to apply this technique to high-contrast observations of planetary-mass companions. In Zhou et al. (2016), I led the first discovery of rotational modulations in a high-contrast planetary-mass companion. With the modulation amplitudes observed in multi-band Hubble Space Telescope (HST) light curves, we determined that condensate clouds in the four Jupiter-mass substellar companion 2M1207b are similar to those in higher mass brown dwarfs. In the Cloud Atlas project (e.g., Zhou et al. 2018, 2019, 2020a), we expanded the application of high-contrast time-resolved observations to additional planetary-mass companions. These observations helped us obtain crucial knowledge about the rotations and cloud structures of substellar companions.

One of my current research focus is to study the light curves of an extremely red L-dwarf companion VHS1256b. It has one of the greatest modulation amplitudes observed so far. It is also a JWST/ERS program target. At UT Austin, I am collaborating with Prof. Brendan Bowler and Prof. Caroline Morley to study this object with HST and Spitzer observations. We found that our observations agree with the predictions from patchy cloud models, supporting the theory of the dissipation of clouds in photospheres as the rotational modulation source. Our work will also offer crucial information that helps interpret its JWST spectrum.