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Cloud Atlas: Weak Color Modulations Due to Rotation in the Planetary-mass Companion GU Psc b and 11 Other Brown Dwarfs

2020; Institute of Physics; Volume: 159; Issue: 3 Linguagem: Inglês

10.3847/1538-3881/ab5f59

1538-3881

Ben W. P. Lew, Dániel Apai, Yifan Zhou, Jacqueline Radigan, Mark S. Marley, Glenn Schneider, Nicolas B. Cowan, Paulo A. Miles-Páez, Elena Manjavacas, Theodora Karalidi, L. R. Bedin, Patrick Lowrance, Adam J. Burgasser,

Astronomy and Astrophysical Research

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.