Space Velocities of L‐ and T‐Type Dwarfs

Abstract

We have obtained radial velocities of a sample of 18 ultracool dwarfs (M6.5YT8) using high-resolution, nearinfrared spectra obtained with NIRSPEC and the Keck II telescope. We have confirmed that the radial velocity of Gl 570D is coincident with that of the K-type primary star Gl 570A, thus providing additional support for their true companionship. The presence of planetary-mass companions around 2MASS J05591914À1404488 (T4.5 V ) has been analyzed using five NIRSPEC radial velocity measurements obtained over a period of 4.37 yr. We have computed UVW space motions for a total of 21 L and T dwarfs within 20 pc of the Sun. This 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 and tangential velocities of the L and T dwarfs appear to be smaller than those of G to M stars. A significant fraction ($40%) of the ultracool dwarfs lie near the Hyades moving group (0.4Y2 Gyr), which contrasts with the 10%Y12% found for earlier type stellar neighbors. In addition, the distributions of the UVW components and the distributions of the total Galactic and tangential velocities derived for the L and T 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 population of L-and T-type ultracool dwarfs (including brown dwarfs) is kinematically younger than solar-type to early M stars, with likely ages in the interval 0.5Y4 Gyr.

Physical parameters of T dwarfs derived from high-resolution near-infrared spectra

“…New atmospheric parameters were thus obtained consistently with the recalculated radial velocities. We note that our radial velocities are in agreement, within the errors, with those of Zapatero Osorio et al (2007).…”

“…This was done from a cross-correlation analysis. In a first iteration, we used the radial velocities published in Zapatero Osorio et al (2007). Then, we determined the atmospheric parameters for each T dwarf as explained later and used the corresponding synthetic spectra as templates to recompute the radial velocities.…”

Infrared radial velocities of vB 10

“…The 2001 observations were previously reported by us (Zapatero Osorio et al , 2007, and the two recent epochs (2007 and 2008) are presented here for the first time. We provide the log of the observations in Table 1, where the information related to the 2001 data is also included for completeness.…”

The distance to the cool T9 brown dwarf ULAS J003402.77-005206.7

“…Zapatero Osorio et al (2007), we conclude that ULAS 0034 has a radial velocity of +7±15 km s −1 . Combining this with the measured proper motions, parallax and the velocity of the Sun from Dehnen & Binney (1998) we find a space motion, in km/s, of U = 23 ± 3 (radially inwards), V = −8 ± 6 (in the direction of Galactic rotation), and W = −9 ± 13 (vertically upwards).…”