WISE J064336.71-022315.4: A Thick Disk L8 Gaia DR2-Discovered Brown Dwarf at 13.9 Parsecs

While spectroscopically characterizing nearby ultracool dwarfs discovered in the Gaia Second Data Release with the TripleSpec spectrograph on the Palomar 200'' telescope, we encountered a particularly cool, nearby, new member of the solar neighborhood: Gaia DR2 3106548406384807680 = WISE J064336.71-022315.4 = 2MASS J06433670-0223130. The $\it{Gaia}$ parallax corresponds to a distance of 13.9 $\pm$ 0.3 pc. Using our TripleSpec spectrum we classify W0643 as spectral type L8, and measured a heliocentric radial velocity of 142 $\pm$ 12 km s$^{-1}$. When combined with $\it{Gaia}$ astrometry, we determine a Galactic velocity (heliocentric; $U$ towards Galactic center) of $U, V, W$ = -109, -91, -12 ($\pm$10, 5, 3) km s$^{-1}$. We estimate that W0643 passed within $\sim$1.4 pc away from the Sun $\sim$100,000 years ago.