2018AJ....155..238S

We present Spitzer Space Telescope time-series photometry at 3.6 and 4.5 µm of 2MASS J11193254-1137466AB and WISEA J114724.10-204021.3, two planetary-mass, late-type (∼L7) brown dwarf members of the ∼10 Myr old TW Hya Association. These observations were taken in order to investigate whether or not a tentative trend of increasing variability amplitude with decreasing surface gravity seen for L3-L5.5 dwarfs extends to later-L spectral types and to explore the angular momentum evolution of low-mass objects. We examine each light curve for variability and find a rotation period of 19.39^+0.33_–0.28 hr and semi-amplitudes of 0.798^+0.081_–0.083% at 3.6 µm and 1.108^+0.093_–0.094% at 4.5 µm for WISEA J114724.10-204021.3. For 2MASS J11193254-1137466AB, we find a single period of 3.02^+0.04_–0.03 hr with semi-amplitudes of 0.230^+0.036_–0.035% at 3.6 µm and 0.453 ± 0.037% at 4.5 µm, which we find is possibly due to the rotation of one component of the binary. Combining our results with 12 other late-type L dwarfs observed with Spitzer from the literature, we find no significant differences between the 3.6 µm amplitudes of low surface gravity and field gravity late-type L brown dwarfs at Spitzer wavelengths, and find tentative evidence (75% confidence) of higher amplitude variability at 4.5 µm for young, late-type Ls. We also find a median rotation period of young brown dwarfs (10-300 Myr) of ∼10 hr, more than twice the value of the median rotation period of field-age brown dwarfs (∼4 hr), a clear signature of brown dwarf rotational evolution.