Variability of the Lowest Mass Objects in the AB Doradus Moving Group

We present the detection of [$3.6\mu$m] photometric variability in two young, L/T transition brown dwarfs, WISE J004701.06+680352.1 (W0047) and 2MASS J2244316+204343 (2M2244) using the $\mathit{\text{Spitzer}}\mathit{\text{Space}}\mathit{\text{Telescope}}$. We find a period of $16.4\pm 0.2$hr and a peak-to-peak amplitude of $1.07\pm 0.04\mathrm{\%}$ for W0047, and a period of $11\pm 2$hr and amplitude of $0.8\pm 0.2\mathrm{\%}$ for 2M2244. This period is significantly longer than that measured previously during a shorter observation. {We additionally detect significant $J$-band variability in 2M2244 using the Wide-Field Camera on UKIRT. } We determine the radial and rotational velocities of both objects using Keck NIRSPEC data. We find a radial velocity of $-16.0_{-0.9}^{+0.8}$km s${}^{-1}$ for 2M2244, and confirm it as a bona fide member of the AB Doradus moving group. We find rotational velocities of $v\sin i=9.8\pm 0.3$km s${}^{-1}$and $14.3_{-1.5}^{+1.4}$km s${}^{-1}$ for W0047 and 2M2244, respectively. With inclination angles of $85_{-9}^{+5\circ }$ and $76_{-20}^{+14\circ }$, W0047 and 2M2244 are viewed roughly equator-on. Their remarkably similar colours, spectra and inclinations are consistent with the possibility that viewing angle may influence atmospheric appearance. We additionally present $\mathit{\text{Spitzer}}$ $[4.5\mu \mathrm{m}]$ monitoring of the young, T5.5 object SDSS111010+011613 (SDSS1110) where we detect no variability. For periods $<18$hr, we place an upper limit of $1.25\mathrm{\%}$ on the peak-to-peak variability amplitude of SDSS1110.