2021AJ....162..179M

Young brown dwarfs are analogs to giant exoplanets, as they share effective temperatures, near-infrared colors, and surface gravities. Thus, the detailed characterization of young brown dwarfs might shed light on the study of giant exoplanets that we are currently unable to observe with a sufficient signal-to-noise to allow a precise characterization of their atmospheres. 2MASS J22081363+2921215 is a young L3 brown dwarf, and a member of the β-Pictoris young moving group (23 ± 3 Myr), which shares its effective temperature and mass with the β Pictoris b giant exoplanet. We performed a ∼2.5 hr spectrophotometric J-band monitoring of 2MASS J22081363+2921215 with the MOSFIRE multi-object spectrograph, installed at the Keck I telescope. We measured a minimum variability amplitude of 3.22 ± 0.42% for its J-band light curve. The ratio between the maximum and the minimum flux spectra of 2MASS J22081363+2921215 shows a weak wavelength dependence, and a potentially enhanced variability amplitude in its alkali lines. Further analysis suggests that the variability amplitudes of the alkali lines are higher than its overall variability amplitude (4.5%-11%, depending on the lines). The variability amplitudes of these lines are lower if we degrade the resolution of the original MOSFIRE spectra to R ∼ 100, which explains why this potentially enhanced variability of the alkali lines had not been found previously in Hubble Space Telescope (HST)/WFC3 light curves. Using radiative-transfer models, we obtained the different cloud layers that might be introducing the spectrophotometric variability we observe for 2MASS J22081363+2921215, which further supports the measured enhanced variability amplitudes of the alkali lines. We provide an artistic recreation of the vertical cloud structure of this β-Pictoris b analog.