We have obtained radial velocities of a sample of 18 ultracool dwarfs with spectral types in the interval M6.5–T8 using high-resolution , near-infrared spectra obtained with NIRSPEC and the Keck II telescope .
Among our targets there are two likely field stars of type late M , one M6.5 Pleiades brown dwarf , and fifteen L and T likely brown dwarfs of the solar neighborhood with estimated masses in the range 30–75 M _ { Jup } .
Two dwarfs , vB 10 ( M8V ) and Gl 570 D ( T7.5V/T8V ) , are known wide companions to low-mass stars .
We have confirmed that the radial velocity of Gl 570 D is coincident with that of the K-type primary star Gl 570 A , thus providing additional support for their true companionship .
The presence of planetary-mass companions around 2MASS J05591914 - 1404488 ( T4.5V ) has been analyzed using five NIRSPEC radial velocity measurements obtained over a period of 4.37 yr .
Using our radial velocity data and the radial velocities of an additional set of eight L-type dwarfs compiled from the literature in combination with their proper motions and trigonometric parallaxes , we have computed UVW space motions for the complete expanded sample , which comprises a total of 21 L and T dwarfs within 20 pc of the Sun .
This ultracool dwarf population shows UVW velocities that nicely overlap the typical kinematics of solar to M-type stars within the same spatial volume .
However , the mean Galactic ( v _ { tot } = 44.2 km s ^ { -1 } ) and tangential ( v _ { t } = 36.5 km s ^ { -1 } ) velocities of the L and T dwarfs appear to be smaller than those of G to M stars .
A significant fraction ( \sim 40 % ) of the L and T dwarfs 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 of all three UVW components ( \sigma _ { UVW } = 30.2 , 16.5 , 15.8 km s ^ { -1 } ) and the distributions of the total Galactic ( \sigma _ { v _ { tot } } = 19.1 km s ^ { -1 } ) and tangential ( \sigma _ { v _ { t } } = 17.6 km s ^ { -1 } ) velocities derived for the L and T dwarf sample are narrower than those measured for nearby G , K , and M-type stars , but similar to the dispersions 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 kinematically younger than solar-type to early M stars with likely ages in the interval 0.5–4 Gyr .