Cloud Atlas: Rotational Modulations in the L/T Transition Brown Dwarf Companion HN Peg B

TL;DR

This study uses Hubble observations to analyze rotational modulations in the brown dwarf HN Peg B, revealing cloud properties and atmospheric features, with a new method for contamination removal in high-contrast data.

Contribution

It presents the first detailed rotational modulation analysis of HN Peg B, including a novel empirical method for spectral contamination correction in high-contrast observations.

Findings

Modulation amplitude of 1.206% with a 15.4 hr period.

Large condensate particles (>1μm) inferred from wavelength dependence.

Spectral modulation similar to early T brown dwarfs.

Abstract

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 HN Peg B, an L/T transition brown dwarf companion to a G0V type star. The best-fit sine wave to the $1.1-1.7\mu$m broadband light curve has the amplitude of $1.206\pm0.025\%$ and period of $15.4\pm0.5$ hr. The modulation amplitude has no detectable wavelength dependence except in the 1.4 $\mu$m water absorption band, indicating that the characteristic condensate particle sizes are large ($>1\mu$m). We detect significantly ($4.4\sigma$) lower modulation amplitude in the 1.4$\mu$m water absorption band, and find that 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.