878 research outputs found
A large spectroscopic sample of L and T dwarfs from UKIDSS LAS : peculiar objects, binaries, and space density
This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record [F. Marocco, et al, 'A large spectroscopic sample of L and T dwarfs from UKIDSS LAS: peculiar objects, binaries, and space density', MNRAS, Vol. 449)4): 3651-3692, April 2015] is available online at: https://doi.org/10.1093/mnras/stv530.We present the spectroscopic analysis of a large sample of late-M, L, and T dwarfs from UKIDSS. Using the YJHK photometry from ULAS and the red-optical photometry from SDSS we selected a sample of 262 brown dwarf candidates and we followed-up 196 of them using X-shooter on the VLT. The large wavelength coverage (0.30-2.48 m) and moderate resolution (R~5000-9000) of X-shooter allowed us to identify peculiar objects including 22 blue L dwarfs, 2 blue T dwarfs, and 2 low gravity M dwarfs. Using a spectral indices-based technique we identified 27 unresolved binary candidates, for which we determined the spectral type of the potential components via spectral deconvolution. The spectra allowed us to measure the equivalent width of the prominent absorption features and to compare them to atmospheric models. Cross-correlating the spectra with a radial velocity standard, we measured the radial velocity for our targets, and we determined the distribution of the sample, which is centred at -1.71.2 km s with a dispersion of 31.5 km s. Using our results we estimated the space density of field brown dwarfs and compared it with the results of numerical simulations. Depending on the binary fraction, we found that there are (0.850.55) x 10 to (1.000.64) x 10 objects per cubic parsec in the L4-L6.5 range, (0.730.47) x 10 to (0.850.55) x 10 objects per cubic parsec in the L7-T0.5 range, and (0.740.48) x 10 to (0.880.56) x 10 objects per cubic parsec in the T1-T4.5 range. There seem to be an excess of objects in the L/T transition with respect to the late T dwarfs, a discrepancy that could be explained assuming a higher binary fraction than expected for the L/T transition, or that objects in the high-mass end and low-mass end of this regime form in different environments, i.e. following different IMFs.Peer reviewe
A Near-Infrared Spectroscopic Study of Young Field Ultracool Dwarfs
We present a near-infrared (0.9-2.4 microns) spectroscopic study of 73 field
ultracool dwarfs having spectroscopic and/or kinematic evidence of youth
(~10-300 Myr). Our sample is composed of 48 low-resolution (R~100) spectra and
41 moderate-resolution spectra (R>~750-2000). First, we establish a method for
spectral typing M5-L7 dwarfs at near-IR wavelengths that is independent of
gravity. We find that both visual and index-based classification in the near-IR
provide consistent spectral types with optical spectral types, though with a
small systematic offset in the case of visual classification at J and K band.
Second, we examine features in the spectra of ~10 Myr ultracool dwarfs to
define a set of gravity-sensitive indices based on FeH, VO, K, Na and H-band
continuum shape. We then create an index-based method for classifying the
gravities of M6-L5 dwarfs that provides consistent results with gravity
classifications from optical spectroscopy. Our index-based classification can
distinguish between young and dusty objects. Guided by the resulting
classifications, we propose a set of low-gravity spectral standards for the
near-IR. Finally, we estimate the ages corresponding to our gravity
classifications.Comment: Published in ApJ. IDL program for calculating indices
(allers13_index.pro) included in the source gzipped ta
The sub-stellar birth rate from UKIDSS
We present a new sample of mid-L to mid-T dwarfs with effective temperatures of 11001700 K selected from the UKIDSS Large Area Survey (LAS) and confirmed with infrared spectra from X-shooter/Very Large Telescope. This effective temperature range is especially sensitive to the formation history of Galactic brown dwarfs and allows us to constrain the form of the sub-stellar birth rate, with sensitivity to differentiate between a flat (stellar like) birth rate and an exponentially declining form. We present the discovery of 63 new L and T dwarfs from the UKIDSS LAS DR7, including the identification of 12 likely unresolved binaries, which form the first complete sub-set from our programme, covering 495 square degrees of sky, complete to J = 18.1. We compare our results for this sub-sample with simulations of differing birth rates for objects of masses 0.10-0.03 M-circle dot and ages 1-10 Gyr. We find that the more extreme birth rates (e. g. a halo type form) can likely be excluded as the true form of the birth rate. In addition, we find that although there is substantial scatter we find a preference for a mass function, with a power-law index a in the range -1 <alpha <0 that is consistent (within the errors) with the studies of late T dwarfs.Peer reviewe
A Stereotaxic Atlas of the Brain of the Chick (Gallus domesticus)
Nine brains taken from chicks two weeks of age were used for the development of this atlas. Each chick was first anesthetized with an intravenous (IV) injection of Chloropent2 (1.8 ml/kg). Chicks were then perfused via the heart with 90 ml physiological saline followed by 90 ml Heidenhain\u27s3 solution. Each head was then positioned in a stereotaxic instrument as described in the previous section and three stainless steel (SS) insect pins (#2) were implanted in each brain at known coordinates. In the case of the brains used to construct the cross- sectional atlas plates, two pins were implanted horizontal to the base of the stereotaxic instrument. Each was inserted into the forebrain and the pins exited either the brainstem or the cerebellum. The third pin was inserted vertical to the base of the stereotaxic instrument. The brains used to construct the sagittal atlas plates had two pins inserted horizontally and one pin inserted vertically to the stereotaxic instrument. The horizontal pins entered the right side of the brain and exited the left side. The brains used for the horizontal plates had two pins inserted vertically and one pin horizontally. The latter entered the forebrain and exited the brainstem
On the kinematic age of brown dwarfs: Radial velocities and space motions of 43 nearby L dwarfs
We present radial velocity measurements of a sample of L0-L8 dwarfs observed
with VLT/UVES and Keck/HIRES. We combine these measurements with distance and
proper motion from the literature to determine space motions for 43 of our
targets. We identify nine candidate members of young moving groups, which have
ages of 50-600 Myr according to their space motion. From the total velocity
dispersion of the 43 L dwarfs, we calculate a kinematic age of ~5 Gyr for our
sample. This age is significantly higher than the ~3 Gyr age known for late M
dwarfs in the solar neighbourhood. We find that the distributions of the U and
V velocity components of our sample are clearly non-Gaussian, placing the age
estimate inferred from the full space motion vector into question. The
W-component exhibits a distribution more consistent with a normal distribution,
and from W alone we derive an age of ~3 Gyr, which is the same age found for
late-M dwarf samples. Our brightness-limited sample is probably contaminated by
a number of outliers that predominantly bias the U and V velocity components.
The origin of the outliers remain unclear, but we suggest that these brown
dwarfs may have gained their high velocities by means of ejection from multiple
systems during their formation.Comment: 8 pages, accepted for publication in A&
The Brown Dwarf Kinematics Project (BDKP). III. Parallaxes for 70 Ultracool Dwarfs
We report parallax measurements for 70 ultracool dwarfs (UCDs). Using both
literature values and our sample, we report new polynomial relations between
spectral type and M. Including resolved L/T transition binaries in the
relations, we find no reason to differentiate between a "bright" (unresolved
binary) and "faint" (single source) sample across the L/T boundary. Isolating
early T dwarfs, we find that the brightening of T0-T4 sources is prominent in
M where there is a [1.2 - 1.4] magnitude difference. A similar yet
dampened brightening of [0.3 - 0.5] magnitude happens at M and a plateau
or dimming of [-0.2 - -0.3] magnitude is seen in M. Comparing with
evolutionary models that vary gravity, metallicity, and cloud thickness we find
that a near constant temperature of 1200 100 K along a narrow spectral
subtype of T0-T4 is required to account for the brightening and color magnitude
diagram of the L-dwarf/T-dwarf transition. Furthermore, there is a significant
population of both L and T dwarfs which are red or potentially "ultra-cloudy"
compared to the models, many of which are known to be young indicating a
correlation between enhanced photospheric dust and youth. For the low
surface-gravity or young companion L dwarfs we find that 8 out of 10 are at
least [0.2-1.0] magnitude underluminous in M and/or M compared to
equivalent spectral type objects. We speculate that this is a consequence of
increased dust opacity and conclude that low-surface gravity L dwarfs require a
completely new spectral-type/absolute magnitude polynomial for analysis.Comment: 65 pages, Accepted for publication to Ap
- …