359 research outputs found
The Brown Dwarf Kinematics Project I. Proper motions and tangential velocities for a large sample of late-type M, L, and T dwarfs
We report proper-motion measurements for 427 late-type M, L, and T dwarfs, 332 of which have been measured for the first time. Combining these new proper motions with previously published measurements yields a sample of 841 M7-T8 dwarfs. We combined parallax measurements or calculated spectrophotometric distances, and computed tangential velocities for the entire sample. We find that kinematics for the full and volume-limited 20 pc samples are consistent with those expected for the Galactic thin disk, with no significant differences between late-type M, L, and T dwarfs. Applying an age-velocity relation we conclude that the average kinematic age of the 20 pc sample of ultracool dwarfs is older than recent kinematic estimates and more consistent with age results calculated with population synthesis models. There is a statistically distinct population of high tangential velocity sources (V tan > 100 km s^–1) whose kinematics suggest an even older population of ultracool dwarfs belonging to either the Galactic thick disk or halo. We isolate subsets of the entire sample, including low surface gravity dwarfs, unusually blue L dwarfs, and photometric outliers in J – Ks color and investigate their kinematics. We find that the spectroscopically distinct class of unusually blue L dwarfs has kinematics clearly consistent with old age, implying that high surface gravity and/or low metallicity may be relevant to their spectral properties. The low surface gravity dwarfs are kinematically younger than the overall population, and the kinematics of the red and blue ultracool dwarfs suggest ages that are younger and older than the full sample, respectively. We also present a reduced proper-motion diagram at 2MASS (Two Micron All Sky Survey) Ks for the entire population and find that a limit of HKs > 18 excludes M dwarfs from the L and T dwarf population regardless of near-infrared color, potentially enabling the identification of the coldest brown dwarfs in the absence of color information
The Brown Dwarf Kinematics Project (BDKP). II. Details on Nine Wide Common Proper Motion Very Low-Mass Companions to Nearby Stars
We report on nine wide common proper motion systems containing late-type M,
L, or T companions. We confirm six previously reported companions, and identify
three new systems. The ages of these systems are determined using diagnostics
for both stellar primaries and low--mass secondaries and masses for the
secondaries are inferred using evolutionary models. Of our three new
discoveries, the M3+T6.5 pair G 204-39 and SDSS J1758+4633 has an age
constrained to 0.5-1.5 Gyr making the secondary a potentially useful brown
dwarf benchmark. The G5+L4 pair G 200-28 and SDSS J1416+5006 has a projected
separation of ~25,000 AU making it one of the widest and lowest binding energy
systems known to date. The system containing NLTT 2274 and SDSS J0041+1341 is
an older M4+L0 (>4.5 Gyr) pair which shows Halpha activity in the secondary but
not the primary making it a useful tracer of age/mass/activity trends. We find
a resolved binary frequency for widely-separated (> 100 AU) low--mass
companions (i.e. at least a triple system) which is at least twice the
frequency found for the field ultracool dwarf population. The ratio of triples
to binaries and quadruples to binaries is also high for this sample: 3:5 and
1:4, respectively, compared to 8-parsec sample values of 1:4 and 1:26. The
additional components in these wide companion systems indicates a formation
mechanism that requires a third or fourth component to maintain gravitational
stability or facilitate the exchange of angular momentum. The binding energies
for the nine multiples discussed in this text are among the lowest known for
wide low-mass systems, suggesting that weakly bound, low--to--intermediate mass
(0.2M_sun < M_tot <1.0M_sun) multiples can form and survive to exist in the
field (1-8 Gyr).Comment: 62 pages, 12 figures, 11 Tables, AJ accepted for publicatio
2MASS J06164006-6407194: The First Outer Halo L Subdwarf
We present the serendipitous discovery of an L subdwarf, 2MASS
J06164006-6407194, in a search of the Two Micron All Sky Survey for T dwarfs.
Its spectrum exhibits features indicative of both a cool and metal poor
atmosphere including a heavily pressured-broadened K I resonant doublet, Cs I
and Rb I lines, molecular bands of CaH, TiO, CrH, FeH, and H2O, and enhanced
collision induced absorption of H2. We assign 2MASS 0616-6407 a spectral type
of sdL5 based on a comparison of its red optical spectrum to that of near
solar-metallicity L dwarfs. Its high proper motion (mu =1.405+-0.008 arcsec
yr-1), large radial velocity (Vrad = 454+-15 km s-1), estimated uvw velocities
(94, -573, 125) km s-1 and Galactic orbit with an apogalacticon at ~29 kpc are
indicative of membership in the outer halo making 2MASS 0616-6407 the first
ultracool member of this population.Comment: Accepted for publication in Ap
Deep search for companions to probable young brown dwarfs
We have obtained high contrast images of four nearby, faint, and very low
mass objects 2MASSJ04351455-1414468, SDSSJ044337.61+000205.1,
2MASSJ06085283-2753583 and 2MASSJ06524851-5741376 (here after 2MASS0435-14,
SDSS0443+00, 2MASS0608-27 and 2MASS0652-57), identified in the field as
probable isolated young brown dwarfs. Our goal was to search for binary
companions down to the planetary mass regime. We used the NAOS-CONICA adaptive
optics instrument (NACO) and its unique capability to sense the wavefront in
the near-infrared to acquire sharp images of the four systems in Ks, with a
field of view of 28"*28". Additional J and L' imaging and follow-up
observations at a second epoch were obtained for 2MASS0652-57. With a typical
contrast DKs= 4.0-7.0 mag, our observations are sensitive down to the planetary
mass regime considering a minimum age of 10 to 120 Myr for these systems. No
additional point sources are detected in the environment of 2MASS0435-14,
SDSS0443+00 and 2MASS0608-27 between 0.1-12" (i.e about 2 to 250 AU at 20 pc).
2MASS0652-57 is resolved as a \sim230 mas binary. Follow-up observations reject
a background contaminate, resolve the orbital motion of the pair, and confirm
with high confidence that the system is physically bound. The J, Ks and L'
photometry suggest a q\sim0.7-0.8 mass ratio binary with a probable semi-major
axis of 5-6 AU. Among the four systems, 2MASS0652-57 is probably the less
constrained in terms of age determination. Further analysis would be necessary
to confirm its youth. It would then be interesting to determine its orbital and
physical properties to derive the system's dynamical mass and to test
evolutionary model predictions.Comment: Research note, 5 pages, 2 tables and 3 figures, accepted to A&
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