Cloud Atlas: Weak color modulations due to rotation in the planetary-mass companion GU Psc b and 11 other brown dwarfs

TL;DR

This study analyzes the rotational brightness variations of GU Psc b, a planetary-mass companion, revealing weak, gray cloud modulations and suggesting spectral-type dependent trends in brown dwarf atmospheric variability.

Contribution

It provides the first detailed rotational modulation analysis of GU Psc b, highlighting weak, gray cloud features and proposing a potential spectral-type dependence in brown dwarf brightness variations.

Findings

GU Psc b shows 2.7% flux variation with an 8-hour period.

Modulations are weakly wavelength-dependent, indicating gray clouds.

Mid-to-late T dwarfs may become redder with brightness, unlike L dwarfs.

Abstract

Among the greatest challenges in understanding ultra-cool brown dwarf and exoplanet atmospheres is the evolution of cloud structure as a function of temperature and gravity. In this study, we present the rotational modulations of GU Psc b -- a rare mid-T spectral type planetary-mass companion at the end of the L/T spectral type transition. Based on the HST/WFC3 1.1-1.67$\rm\, \mu m$ time-series spectra, we observe a quasi-sinusoidal light curve with a peak-to-trough flux variation of 2.7 % and a minimum period of eight hours. The rotation-modulated spectral variations are weakly wavelength-dependent, or largely gray between 1.1-1.67$\rm\,\mu$m. The gray modulations indicate that heterogeneous clouds are present in the photosphere of this low-gravity mid-T dwarf. We place the color and brightness variations of GU Psc b in the context of rotational modulations reported for mid-L to late-T dwarfs. Based on these observations, we report a tentative trend: mid-to-late T dwarfs become slightly redder in $J-H$ color with increasing $J$-band brightness, while L dwarfs become slightly bluer with increasing brightness. If this trend is verified with more T-dwarf samples, it suggests that in addition to the mostly gray modulations, there is a second-order spectral-type dependence on the nature of rotational modulations.