668 research outputs found
Discovery of very nearby ultracool dwarfs from DENIS
We report new spectroscopic results, obtained with UKIRT/CGS4, of a sample of
14 candidate ultracool dwarfs selected from the DENIS (Deep Near-Infrared
Survey of the Southern Sky) database. A further object, selected from the 2MASS
Second Incremental Release, was observed at a later epoch with the same
instrument. Six objects are already known in the literature; we re-derive their
properties. A further four prove to be very nearby (~10 pc) mid-to-late
L-dwarfs, three unknown hitherto, two of which are almost certainly substellar.
These findings increase the number of L-dwarfs known within ~10 pc by ~25%. The
remainder of the objects discussed here are early L or very late M-type dwarfs
lying between ~45 and 15 pc and are also new to the literature. Spectral types
have been derived by direct comparison with J-,H- and K- band spectra of known
template ultracool dwarfs given by Leggett et al.
(ftp://ftp.jach.hawaii.edu/pub/ukirt/skl/dL.spectra/) For the known objects, we
generally find agreement to within ~1 subclass with previously derived spectral
types. Distances are determined from the most recent M_J vs. spectral type
calibrations, and together with our derived proper motions yield kinematics for
most targets consistent with that expected for the disk population; for three
probable late M-dwarfs, membership of a dynamically older population is
postulated. The very nearby L-type objects discussed here are of great interest
for future studies of binarity and parallaxes.Comment: 4 pages, 2 figures, accepted in A&A Letter
Contextuality and Wigner function negativity in qubit quantum computation
We describe a scheme of quantum computation with magic states on qubits for
which contextuality is a necessary resource possessed by the magic states. More
generally, we establish contextuality as a necessary resource for all schemes
of quantum computation with magic states on qubits that satisfy three simple
postulates. Furthermore, we identify stringent consistency conditions on such
computational schemes, revealing the general structure by which negativity of
Wigner functions, hardness of classical simulation of the computation, and
contextuality are connected.Comment: published versio
Contextuality as a resource for models of quantum computation on qubits
A central question in quantum computation is to identify the resources that
are responsible for quantum speed-up. Quantum contextuality has been recently
shown to be a resource for quantum computation with magic states for odd-prime
dimensional qudits and two-dimensional systems with real wavefunctions. The
phenomenon of state-independent contextuality poses a priori an obstruction to
characterizing the case of regular qubits, the fundamental building block of
quantum computation. Here, we establish contextuality of magic states as a
necessary resource for a large class of quantum computation schemes on qubits.
We illustrate our result with a concrete scheme related to measurement-based
quantum computation.Comment: Published version. We have revised the title, introduction and
discussion, as well as slightly simplified the setting in this versio
Interferometric Astrometry of the Low-mass Binary Gl 791.2 (= HU Del) Using Hubble Space Telescope Fine Guidance Sensor 3: Parallax and Component Masses
With fourteen epochs of fringe tracking data spanning 1.7y from Fine Guidance
Sensor 3 we have obtained a parallax (pi_abs=113.1 +- 0.3 mas) and perturbation
orbit for Gl 791.2A. Contemporaneous fringe scanning observations yield only
three clear detections of the secondary on both interferometer axes. They
provide a mean component magnitude difference, Delta V = 3.27 +- 0.10. The
period (P = 1.4731 yr) from the perturbation orbit and the semi-major axis (a =
0.963 +- 0.007 AU) from the measured component separations with our parallax
provide a total system mass M_A + M_B = 0.412 +- 0.009 M_sun. Component masses
are M_A=0.286 +- 0.006 M_sun and M_B = 0.126 +- 0.003 M_sun. Gl 791.2A and B
are placed in a sparsely populated region of the lower main sequence
mass-luminosity relation where they help define the relation because the masses
have been determined to high accuracy, with errors of only 2%.Comment: 19 pages, 5 figures. The paper is to appear in August 2000 A
A possible third component in the L dwarf binary system DENIS-P J020529.0-115925 discovered with the Hubble Space Telescope
We present results showing that the multiple system DENIS-P J020529.0-115925
is likely to be a triple system of brown dwarfs. The secondary of this
previously known binary system shows a clear elongation on six images obtained
at six different epochs. Significant residuals remain after PSF subtraction on
these images, characteristic of multiplicity, and indicating that the secondary
is probably a double itself. Dual-PSF fitting shows that the shape of the
secondary is consistent with that of a binary system. These measurements show
that the probability that DENIS-P J020529.0-115925 is a triple system is very
high. The photometric and spectroscopic properties of the system are consistent
with the presence of three components with spectral types L5, L8 and T0.Comment: 15 pages, 3 tables, 6 figures, accepted for publication in AJ. High
resolution version available at
ftp://ftp.mpe.mpg.de/people/hbouy/publications/denis0205.ps.g
Spectroscopic identification of DENIS-selected brown dwarf candidates in the Upper Scorpius OB association
We present low-resolution (R=900) optical (576.1--1,051.1 nm) spectroscopic
observations of 40 candidate very low-mass members in the Upper Scorpius OB
association. These objects were selected using the , and photometry
available in the DENIS database. We have derived spectral types and we have
measured H and NaI doublet (at 818.3 and 819.5 nm) equivalent widths.
We assess the youth of the objects by comparing them to their older
counterparts of similar spectral type in the Pleiades cluster and the field.
Our analysis indicates that 28 of our targets are young very low-mass objects,
and thus they are strong candidate members of the OB association. The other 12
DENIS sources are foreground M dwarfs or background red giants. Our sample of
spectroscopic candidate members includes 18 objects with spectral types in the
range M6.5 and M9, which are likely young brown dwarfs. We classify these
candidates as accreting/non accreting using the scheme proposed by Barrado y
Navascu\'es & Mart\'\i n (2003). We find 5 substellar-mass candidate cluster
members that are still undergoing mass accretion, indicating that the timescale
for accretion onto brown dwarfs can be as long as 5 Myr in some cases.Comment: Accepted for publication in The Astronomical Journal, January 200
Multiplicity of Nearby Free-floating Ultra-cool Dwarfs: a HST-WFPC2 search for companions
We present HST/WFPC2 observations of a sample of 134 ultra-cool objects
(spectral types later than M7) coming from the DENIS, 2MASS and SDSS surveys,
with distances estimated to range from 7 pc to 105 pc. Fifteen new ultra-cool
binary candidates are reported here. Eleven known binaries are confirmed and
orbital motion is detected in some of them. We estimate that the closest binary
systems in this sample have periods between 5 and 20 years, and thus dynamical
masses will be derived in the near future. For the calculation of binary
frequency we restrict ourselves to systems with distances less than 20 pc.
After correction of the binaries bias, we find a ratio of visual binaries (at
the HST limit of detection) of around 10%, and that ~15% of the 26 objects
within 20 parsecs are binary systems with separations between 1 and 8 A.U. The
observed frequency of ultra-cool binaries is similar than that of binaries with
G-type primaries in the separation range from 2.1 A.U. to 140 A.U. There is
also a clear deficit of ultra-cool binaries with separations greater than 15
A.U., and a possible tendency for the binaries to have mass ratios near unity.
Most systems have indeed visual and near-infrared brightness ratios between 1
and 0.3. We discuss our results in the framework of current scenarios for the
formation and evolution of free-floating brown dwarfs.Comment: 67 pages, 14 figures, Accepted for publication in AJ, September 2003.
First submission to AJ: august 2002, 5 submission
Keck Imaging of Binary L Dwarfs
We present Keck near-infrared imaging of three binary L dwarf systems, all of
which are likely to be sub-stellar. Two are lithium dwarfs, and a third
exhibits an L7 spectral type, making it the coolest binary known to date. All
have component flux ratios near 1 and projected physical separations between 5
and 10 AU, assuming distances of 18 to 26 pc from recent measurements of
trigonometric parallax. These surprisingly similar binaries represent the sole
detections of companions in ten L dwarf systems which were analyzed in the
preliminary phase of a much larger dual-epoch imaging survey. The detection
rate prompts us to speculate that binary companions to L dwarfs are common,
that similar-mass systems predominate, and that their distribution peaks at
radial distances in accord both with M dwarf binaries and with the radial
location of Jovian planets in our own solar system. To fully establish these
conjectures against doubts raised by biases inherent in this small preliminary
survey, however, will require quantitative analysis of a larger volume-limited
sample which has been observed with high resolution and dynamic range.Comment: LaTex manuscript in 13 pages, 3 postscript figures, Accepted for
publication in the Letters of the Astrophysical Journal; Postscript pre-print
version available at: http://www.hep.upenn.edu/PORG/papers/koerner99a.p
Beyond the T Dwarfs: Theoretical Spectra, Colors, and Detectability of the Coolest Brown Dwarfs
We explore the spectral and atmospheric properties of brown dwarfs cooler
than the latest known T dwarfs. Our focus is on the yet-to-be-discovered
free-floating brown dwarfs in the \teff range from 800 K to 130 K
and with masses from 25 to 1 \mj. This study is in anticipation of the new
characterization capabilities enabled by the launch of SIRTF and the eventual
launch of JWST. We provide spectra from 0.4 \mic to 30 \mic, highlight
the evolution and mass dependence of the dominant HO, CH, and NH
molecular bands, consider the formation and effects of water-ice clouds, and
compare our theoretical flux densities with the sensitivities of the
instruments on board SIRTF and JWST. The latter can be used to determine the
detection ranges from space of cool brown dwarfs. In the process, we determine
the reversal point of the blueward trend in the near-infrared colors with
decreasing \teff, the \teffs at which water and ammonia clouds appear, the
strengths of gas-phase ammonia and methane bands, the masses and ages of the
objects for which the neutral alkali metal lines are muted, and the increasing
role as \teff decreases of the mid-infrared fluxes longward of 4 \mic. These
changes suggest physical reasons to expect the emergence of at least one new
stellar class beyond the T dwarfs. Our spectral models populate, with cooler
brown dwarfs having progressively more planet-like features, the theoretical
gap between the known T dwarfs and the known giant planets. Such objects likely
inhabit the galaxy, but their numbers are as yet unknown.Comment: Includes 14 figures, most in color; accepted to the Astrophysical
Journa
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