Lew, Ben W. P.Ben W. P.LewApai, DánielDánielApaiZhou, YifanYifanZhouRadigan, JacquelineJacquelineRadiganMarley, MarkMarkMarleySchneider, GlennGlennSchneiderCowan, Nicolas B.Nicolas B.CowanMiles-Páez, Paulo A.Paulo A.Miles-PáezManjavacas, ElenaElenaManjavacasKaralidi, TheodoraTheodoraKaralidiBEDIN, LuigiLuigiBEDINLowrance, Patrick J.Patrick J.LowranceBurgasser, Adam J.Adam J.Burgasser2023-01-232023-01-2320200004-6256http://hdl.handle.net/20.500.12386/33016Among the greatest challenges in understanding ultracool 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 Hubble Space Telescope/WFC3 1.1-1.67 μm 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 8 h. The rotation-modulated spectral variations are weakly wavelength-dependent, or largely gray between 1.1 and 1.67 μm. 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.STAMPAenArticle10.3847/1538-3881/ab5f592-s2.0-85085116303https://iopscience.iop.org/article/10.3847/1538-3881/ab5f59https://api.elsevier.com/content/abstract/scopus_id/850851163032020AJ....159..125LFIS/05 - ASTRONOMIA E ASTROFISICA