Weather On Other Worlds. V. The Three Most Rapidly Rotating Ultra-Cool Dwarfs

We present the discovery of rapid photometric variability in three ultra-cool dwarfs from long-duration monitoring with the Spitzer Space Telescope. The T7, L3.5, and L8 dwarfs have the shortest photometric periods known to date: 1.080(-0.005)(+0.004) hr, 1.14(-0.01)(+0.03) hr, and 1.23(-0.01)(+0.01) hr, respectively. We confirm the rapid rotation through moderate-resolution infrared spectroscopy, which reveals projected rotational velocities between 79 and 104 km s(-1). We compare the near-infrared spectra to photospheric models to determine the objects' fundamental parameters and radial velocities. We find that the equatorial rotational velocities for all three objects are greater than or similar to 100 km s(-1). The three L and T dwarfs reported here are the most rapidly spinning and likely the most oblate field ultra-cool dwarfs known to date. Correspondingly, all three are excellent candidates for seeking auroral radio emission and net optical/infrared polarization. As of this writing, 78 L-, T-, and Y-dwarf rotation periods have now been measured. The clustering of the shortest rotation periods near 1 hr suggests that brown dwarfs are unlikely to spin much faster.