Time-resolved observations of brown dwarfs ’ rotational modulations provide powerful insights into the properties of condensate clouds in ultra-cool atmospheres .
Multi-wavelength light curves reveal cloud vertical structures , condensate particle sizes , and cloud morphology , which directly constrain condensate cloud and atmospheric circulation models .
We report results from Hubble Space Telescope/Wide Field Camera 3 near-infrared G141 taken in six consecutive orbits observations of \object HN Peg B , an L/T transition brown dwarf companion to a G0V type star .
The best-fit sine wave to the 1.1 - 1.7 \micron broadband light curve has the amplitude of 1.206 \pm 0.025 \% and period of 15.4 \pm 0.5 hr .
The modulation amplitude has no detectable wavelength dependence except in the 1.4 \micron water absorption band , indicating that the characteristic condensate particle sizes are large ( > 1 \micron ) .
We detect significantly ( 4.4 \sigma ) lower modulation amplitude in the 1.4 \micron water absorption band , and find that \object HN Peg B ’ s spectral modulation resembles those of early T type brown dwarfs .
We also describe a new empirical interpolation method to remove spectral contamination from the bright host star .
This method may be applied in other high-contrast time-resolved observations with WFC3 .