7,967 research outputs found
Metallurgy and properties of plasma spray formed materials
Understanding the fundamental metallurgy of vacuum plasma spray formed materials is the key to enhancing and developing full material properties. Investigations have shown that the microstructure of plasma sprayed materials must evolve from a powder splat morphology to a recrystallized grain structure to assure high strength and ductility. A fully, or near fully, dense material that exhibits a powder splat morphology will perform as a brittle material compared to a recrystallized grain structure for the same amount of porosity. Metallurgy and material properties of nickel, iron, and copper base alloys will be presented and correlated to microstructure
Shaped Pupil Lyot Coronagraphs: High-Contrast Solutions for Restricted Focal Planes
Coronagraphs of the apodized pupil and shaped pupil varieties use the
Fraunhofer diffraction properties of amplitude masks to create regions of high
contrast in the vicinity of a target star. Here we present a hybrid coronagraph
architecture in which a binary, hard-edged shaped pupil mask replaces the gray,
smooth apodizer of the apodized pupil Lyot coronagraph (APLC). For any contrast
and bandwidth goal in this configuration, as long as the prescribed region of
contrast is restricted to a finite area in the image, a shaped pupil is the
apodizer with the highest transmission. We relate the starlight cancellation
mechanism to that of the conventional APLC. We introduce a new class of
solutions in which the amplitude profile of the Lyot stop, instead of being
fixed as a padded replica of the telescope aperture, is jointly optimized with
the apodizer. Finally, we describe shaped pupil Lyot coronagraph (SPLC) designs
for the baseline architecture of the Wide-Field Infrared Survey
Telescope-Astrophysics Focused Telescope Assets (WFIRST-AFTA) coronagraph.
These SPLCs help to enable two scientific objectives of the WFIRST-AFTA
mission: (1) broadband spectroscopy to characterize exoplanet atmospheres in
reflected starlight and (2) debris disk imaging.Comment: 41 pages, 15 figures; published in the JATIS special section on
WFIRST-AFTA coronagraph
A search for 183-GHz emission from water in late-type stars
A search was made for 183 GHz line emission from water vapor in the direction of twelve Mira and two semiregular variables. Upper limits to the emission are in the range of 2000 to 5000 Jy. It is estimated that thermal emission from the inner regions of late type stellar envelopes will be on the order of ten Jy. Maser emission, according to one model, would be an order of magnitude stronger. From the limited set sampled, the possibility of very strong maser emission at 183 GHz cannot yet be ruled out
Perturbation study on the spin and charge susceptibilities of the two-dimensional Hubbard model
We investigate the spin and charge susceptibilities of the two-dimensional
Hubbard model based upon the perturbative calculation in the strength of
correlation . For comparable to a bare bandwidth, the charge
susceptibility decreases near the half-filling as hole-doping approaches zero.
This behavior suggesting the precursor of the Mott-Hubbard gap formation cannot
be obtained without the vertex corrections beyond the random phase
approximation. In the low-temperature region, the spin susceptibility deviates
from the Curie-Weiss-like law and finally turns to decrease with the decrease
of temperature. This spin-gap-like behavior is originating from the van Hove
singularity in the density of states.Comment: Revtex file + 11 figures, to appear in Phys. Rev.
Finding the Needles in the Haystacks: High-Fidelity Models of the Modern and Archean Solar System for Simulating Exoplanet Observations
We present two state-of-the-art models of the solar system, one corresponding
to the present day and one to the Archean Eon 3.5 billion years ago. Each model
contains spatial and spectral information for the star, the planets, and the
interplanetary dust, extending to 50 AU from the sun and covering the
wavelength range 0.3 to 2.5 micron. In addition, we created a spectral image
cube representative of the astronomical backgrounds that will be seen behind
deep observations of extrasolar planetary systems, including galaxies and Milky
Way stars. These models are intended as inputs to high-fidelity simulations of
direct observations of exoplanetary systems using telescopes equipped with
high-contrast capability. They will help improve the realism of observation and
instrument parameters that are required inputs to statistical observatory yield
calculations, as well as guide development of post-processing algorithms for
telescopes capable of directly imaging Earth-like planets.Comment: Accepted for publication in PAS
The ESPRI project: astrometric exoplanet search with PRIMA I. Instrument description and performance of first light observations
The ESPRI project relies on the astrometric capabilities offered by the PRIMA
facility of the Very Large Telescope Interferometer for the discovery and study
of planetary systems. Our survey consists of obtaining high-precision
astrometry for a large sample of stars over several years and to detect their
barycentric motions due to orbiting planets. We present the operation
principle, the instrument's implementation, and the results of a first series
of test observations. A comprehensive overview of the instrument infrastructure
is given and the observation strategy for dual-field relative astrometry is
presented. The differential delay lines, a key component of the PRIMA facility
which was delivered by the ESPRI consortium, are described and their
performance within the facility is discussed. Observations of bright visual
binaries are used to test the observation procedures and to establish the
instrument's astrometric precision and accuracy. The data reduction strategy
for astrometry and the necessary corrections to the raw data are presented.
Adaptive optics observations with NACO are used as an independent verification
of PRIMA astrometric observations. The PRIMA facility was used to carry out
tests of astrometric observations. The astrometric performance in terms of
precision is limited by the atmospheric turbulence at a level close to the
theoretical expectations and a precision of 30 micro-arcseconds was achieved.
In contrast, the astrometric accuracy is insufficient for the goals of the
ESPRI project and is currently limited by systematic errors that originate in
the part of the interferometer beamtrain which is not monitored by the internal
metrology system. Our observations led to the definition of corrective actions
required to make the facility ready for carrying out the ESPRI search for
extrasolar planets.Comment: 32 pages, 39 figures, Accepted for publication in Astronomy and
Astrophysic
Characterization of the gaseous companion {\kappa} Andromedae b: New Keck and LBTI high-contrast observations
We previously reported the direct detection of a low mass companion at a
projected separation of 55+-2 AU around the B9 type star {\kappa} Andromedae.
The properties of the system (mass ratio, separation) make it a benchmark for
the understanding of the formation and evolution of gas giant planets and brown
dwarfs on wide-orbits. We present new angular differential imaging (ADI) images
of the Kappa Andromedae system at 2.146 (Ks), 3.776 (L'), 4.052 (NB 4.05) and
4.78 {\mu}m (M') obtained with Keck/NIRC2 and LBTI/LMIRCam, as well as more
accurate near-infrared photometry of the star with the MIMIR instrument. We
derive a more accurate J = 15.86 +- 0.21, H = 14.95 +- 0.13, Ks = 14.32 +- 0.09
mag for {\kappa} And b. We redetect the companion in all our high contrast
observations. We confirm previous contrasts obtained at Ks and L' band. We
derive NB 4.05 = 13.0 +- 0.2 and M' = 13.3 +- 0.3 mag and estimate
Log10(L/Lsun) = -3.76 +- 0.06. We build the 1-5 microns spectral energy
distribution of the companion and compare it to seven PHOENIX-based atmospheric
models in order to derive Teff = 1900+100-200 K. Models do not set constrains
on the surface gravity. ``Hot-start" evolutionary models predict masses of
14+25-2 MJup based on the luminosity and temperature estimates, and considering
a conservative age range for the system (30+120-10 Myr). ``warm-start"
evolutionary tracks constrain the mass to M >= 11 MJup. Therefore, the mass of
{\kappa} Andromedae b mostly falls in the brown-dwarf regime, due to remaining
uncertainties in age and mass-luminosity models. According to the formation
models, disk instability in a primordial disk could account for the position
and a wide range of plausible masses of {\kappa} And b.Comment: 20 pages, 16 figures, accepted for publication in Astronomy and
Astrophysics on August 6, 201
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