R. Deshpande

Spectroscopic Rotational Velocities of Brown Dwarfs

We have obtained projected rotation velocities (vrot sin i) of a sample of 19 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 young brown dwarfs, two likely field stars, and 15 likely brown dwarfs (30-72 MJup) in the solar neighborhood. Our results indicate that the T-type dwarfs are fast rotators in marked contrast to M-type stars. We have derived vrot sin i velocities between ≤15 and 40 km s-1 for them and have found no clear evidence for T dwarfs rotating strongly faster than L dwarfs. However, there is a hint for an increasing lower envelope on moving from mid-M to L spectral types in the vrot sin i-spectral-type diagram that was previously reported in the literature; our vrot sin i results extend it to even cooler types. Assuming that field brown dwarfs have a size of 0.08-0.1 R☉, we can place an upper limit of 12.5 hr on the equatorial rotation period of T-type brown dwarfs. In addition, we have compared our vrot sin i measurements to spectroscopic rotational velocities of very young brown dwarfs of similar mass available in the literature. The comparison, although model dependent, suggests that brown dwarfs lose some angular momentum during their contraction; however, their spin-down time seems to be significantly longer than that of solar-type to early M stars.

Space Velocities of L- and T-type Dwarfs

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 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 J05591914-1404488 (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 (0.4-2 Gyr), which contrasts with the 10%-12% 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 0.5-4 Gyr.

The Volatile Composition of Comet 17P/Holmes after Its Extraordinary Outburst

The volatile abundances in comet 17P/Holmes were measured on three dates (UT 2007 October 27.6 and 31.3 and November 2.3) using high-dispersion (λ/Δ λ ~ 2.5 × 104) infrared spectroscopy with NIRSPEC at the W. M. Keck Observatory and CSHELL at the NASA Infrared Telescope Facility. Compared to other comets, the relative gas production rates in the coma show an enhancement of C2H6, HCN, and C2H2 with respect to H2O, by factors of ~2-3. CH3OH was also detected with an abundance relative to H2O that is similar to or perhaps slightly enhanced compared to the values observed in other comets. The apparent enrichment of some volatiles in the coma of 17P/Holmes does not necessarily imply an unusual composition for its nucleus because fractionation effects may be important at the relatively large heliocentric distance (Rh = 2.45 AU) at which our observations were performed. Rotational temperatures were determined for H2O, HCN, C2H6, and C2H2 in the coma on UT October 27.6 and found to be between 60 and 80 K. We used lines in both the ν5 and ν7 bands to obtain the best constraints yet achieved for the rotational temperature of C2H6. The spatial distributions of all measured volatiles in the coma are consistent with each other and suggest at most only a minor contribution from sublimating icy grains within our aperture. The overall gas production rate declined by approximately a factor of 7 between UT October 27.6 and November 2.3 with no significant change measured in the relative production rates of C2H6 and H2O during this time.

Resolved Hubble space spectroscopy of ultracool binary systems

Using the low-resolution mode of the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST), we obtain spectra from 20xa0spatially-resolved, ultracool dwarfs, eighteen of which belong to nine very low-mass binary systems with known angular separations in the range 0

Infrared radial velocities of vB 10

farcs

The Least Massive (Sub)Stellar Component of the Milky Way

37–0

A High-Resolution Infrared Study of the Chemistry of Comet 17P/Holmes After Its Outburst

farcs

High precision radial velocities in the near-infrared domain: Status and prospects

098. For these dwarfs, we derive their spectral types to be in the rangexa0dM7.5 toxa0dL6 by either obtaining their PC3xa0index value or by comparing our STISxa0spectra with ground-based spectra of similar spectral resolution from Martin etxa0al. (1999b). We have searched for H α xa0emission in each object, but the emission is clearly detected in only two of them. We find that the distribution of H α xa0emission in our sample is statistically different from that of single field dwarfs, suggesting anxa0intriguing anti-correlation between chromospheric activity and binarity for M7–M9.5xa0dwarfs. We provide strength measuments of the main photospheric features. We derive calibrations of spectral subclasses versusxa0

Physical parameters of a sample of M dwarfs from high-resolution near-infrared spectra

it F814W

Radial velocity observations of VB10

and K -band absolute magnitudes for axa0subset of 10xa0dwarfs in 5xa0binaries that have known trigonometric parallaxes.