Spitzer Light Curves of the Young, Planetary-mass TW Hya Members 2MASS J11193254–1137466AB and WISEA J114724.10–204021.3

Abstract

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.

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