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

TL;DR

This study reports the discovery of the shortest known rotation periods in ultra-cool dwarfs, confirmed through photometry and spectroscopy, revealing their rapid rotation and potential for auroral emissions.

Contribution

It presents the first measurement of ultra-cool dwarfs with rotation periods around 1 hour, establishing the fastest known spins for such objects and analyzing their rotational velocities and physical properties.

Findings

Shortest photometric periods for ultra-cool dwarfs are around 1 hour.

All three objects have equatorial velocities exceeding 100 km/s.

These dwarfs are likely the most oblate and rapidly spinning known field ultra-cool dwarfs.

Abstract

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: ${1.080}^{+0.004}_{-0.005}$ h, ${1.14}^{+0.03}_{-0.01}$ h, and ${1.23}^{+0.01}_{-0.01}$ h, 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 $\gtrsim$100 km s$^{-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 h suggests that brown dwarfs are unlikely to spin much faster.