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 μ m 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 μ m and 1.108 +0.093 −0.094 % at 4.5 μ m 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 μ m and 0.453 ± 0.037% at 4.5 μ m, 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 μ m 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 μ m 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.