[Abridged] We have obtained radial velocities of a sample of 18 ultracool dwarfs (M6.5-T8)
using high-resolution, near-infrared spectra obtained with NIRSPEC and the Keck
II telescope. We have confirmed that the radial velocity of Gl 570 D is coincide
nt with that of the K-type primary star Gl 570 A, thus providing additional supp
ort for their true companionship. The presence of planetary-mass companions arou
nd 2MASS J05591914-1404488 (T4.5V) has been analyzed using five NIRSPEC radial v
elocity measurements obtained over a period of 4.37 yr. We have computed UVW spa
ce motions for a total of 21 L and T dwarfs within 20 pc of the Sun. This popula
tion shows UVW velocities that nicely overlap the typical kinematics of solar to
M-type stars within the same spatial volume. However, the mean Galactic (44.2 k
m/s) and tangential (36.5 km/s) velocities of the L and T dwarfs appear to be sm
aller than those of G to M stars. A significant fraction (~40%) of the L and T d
warfs lies near the Hyades moving group (0.4-2 Gyr), which contrasts with the 10
-12% found for earlier-type stellar neighbors. Additionally, the distributions o
f all three UVW components (sigma_{UVW} = 30.2, 16.5, 15.8 km/s) and the distrib
utions of the total Galactic (sigma_{v_tot} = 19.1 km/s) and tangential (sigma_{
v_t} = 17.6 km/s) velocities derived for the L and T dwarf sample are narrower t
han those measured for nearby G, K, and M-type stars, but similar to the dispers
ions obtained for F stars. This suggests that, in the solar neighborhood, the L-
and T-type ultracool dwarfs in our sample (including brown dwarfs) is kinematic
ally younger than solar-type to early M stars with likely ages in the interval 0
.5-4 Gyr.