913 research outputs found
Separating inner and outer contributions in gravitational lenses using the perturbative method
This paper presents a reconstruction of the gravitational lens SL2S02176-0513
using the singular perturbative method presented in Alard 2007, MNRAS Letters,
382, 58 and Alard, C., 2008, MNRAS, 388, 375. The ability of the perturbative
method to separate the inner and outer contributions of the potential in
gravitational lenses is tested using SL2S02176-0513. In this lens, the
gravitational field of the central galaxy is dominated by a nearby group of
galaxies located at a distance of a few critical radius. The perturbative
functionals are re-constructed using local polynomials. The polynomial
interpolation is smoothed using Fourier series, and numerically fitted to HST
data using a non-linear minimization procedure. The potential inside and
outside the critical circle is derived from the reconstruction of the
perturbative fields. The inner and outer potential contours are very
different.The inner contours are consistent with the central galaxy, while the
outer contours are fully consistent with the perturbation introduced by the
group of galaxies. The ability of the perturbative method to separate the inner
and outer contribution is confirmed, and indicates that in the perturbative
approach the field of the central deflector can be separated from outer
perturbations. The separation of the inner and outer contribution is especially
important for the study of the shape of dark matter halo's as well as for the
statistical analysis of the effect of dark matter substructures
Hubble Space Telescope Imaging of Bright Lyman-break Galaxy Candidates from the Sloan Digital Sky Survey: Not LBGs After All
We present deep Hubble Space Telescope ACS and NICMOS images of six bright
Lyman-break galaxy candidates that were previously discovered in the Sloan
Digital Sky Survey. We find that five of the objects are consistent with
unresolved point sources. Although somewhat atypical of the class, they are
most likely LoBAL quasars, perhaps FeLoBALs. The sixth object, J1147, has a
faint companion galaxy located ~0.8 arcsec to the southwest. The companion
contributes ~8% of the flux in the observed-frame optical and infrared. It is
unknown whether this companion is located at the same redshift as J1147.Comment: 16 pages, 8 figures, 3 tables; accepted for publication in A
Turbulence Model Implementation and Verification in the SENSEI CFD Code
This paper outlines the implementation and verification of the negative Spalart-Allmaras turbulence model into the SENSEI CFD code. The SA-neg turbulence model is implemented in a flexible, object-oriented framework where additional turbulence models can be easily added. In addition to outlining the new turbulence modeling framework in SENSEI, an overview of the other general improvements to SENSEI is provided. The results for four 2D test cases are compared to results from CFL3D and FUN3D to verify that the turbulence models are implemented properly. Several differences in the results from SENSEI, CFL3D, and FUN3D are identified and are attributed to differences in the implementation and discretization order of the boundary conditions as well as the order of discretization of the turbulence model. When a solid surface is located near or intersects an inflow or outflow boundary, higher order boundary conditions should be used to limit their effect on the forces on the surface. When the turbulence equations are discretized using second order spatial accuracy, the edge of the eddy viscosity profile seems to be sharper than when a first order discretization is used. However, the discretization order of the turbulence equation does not have a significant impact on output quantities of interest, such as pressure and viscous drag, for the cases studied
HST/ACS Images of the GG Tauri Circumbinary Disk
Hubble Space Telescope Advanced Camera for Surveys images of the young binary
GG Tauri and its circumbinary disk in V and I bandpasses were obtained in 2002
and are the most detailed of this system to date. The confirm features
previously seen in the disk including: a "gap" apparently caused by shadowing
from circumstellar material; an asymmetrical distribution of light about the
line of sight on the near edge of the disk; enhanced brightness along the near
edge of the disk due to forward scattering; and a compact reflection nebula
near the secondary star. New features are seen in the ACS images: two short
filaments along the disk; localized but strong variations in disk intensity
("gaplets"); and a "spur" or filament extending from the reflection nebulosity
near the secondary. The back side of the disk is detected in the V band for the
first time. The disk appears redder than the combined light from the stars,
which may be explained by a varied distribution of grain sizes. The brightness
asymmetries along the disk suggest that it is asymmetrically illuminated by the
stars due to extinction by nonuniform circumstellar material or the illuminated
surface of the disk is warped by tidal effects (or perhaps both). Localized,
time-dependent brightness variations in the disk are also seen.Comment: 28 pages, 7 figures, accepted for publication in the Astronomical
Journa
HST and Spitzer Observations of the HD 207129 Debris Ring
A debris ring around the star HD 207129 (G0V; d = 16.0 pc) has been imaged in
scattered visible light with the ACS coronagraph on the Hubble Space Telescope
and in thermal emission using MIPS on the Spitzer Space Telescope at 70 microns
(resolved) and 160 microns (unresolved). Spitzer IRS (7-35 microns) and MIPS
(55-90 microns) spectrographs measured disk emission at >28 microns. In the HST
image the disk appears as a ~30 AU wide ring with a mean radius of ~163 AU and
is inclined by 60 degrees from pole-on. At 70 microns it appears partially
resolved and is elongated in the same direction and with nearly the same size
as seen with HST in scattered light. At 0.6 microns the ring shows no
significant brightness asymmetry, implying little or no forward scattering by
its constituent dust. With a mean surface brightness of V=23.7 mag per square
arcsec, it is the faintest disk imaged to date in scattered light.Comment: 28 pages, 8 figure
Quaoar: A Rock in the Kuiper belt
Here we report WFPC2 observations of the Quaoar-Weywot Kuiper belt binary.
From these observations we find that Weywot is on an elliptical orbit with
eccentricity of 0.14 {\pm} 0.04, period of 12.438 {\pm} 0.005 days, and a
semi-major axis of 1.45 {\pm} 0.08 {\times} 104 km. The orbit reveals a
surpsingly high Quaoar-Weywot system mass of 1.6{\pm}0.3{\times}10^21 kg. Using
the surface properties of the Uranian and Neptunian satellites as a proxy for
Quaoar's surface, we reanalyze the size estimate from Brown and Trujillo
(2004). We find, from a mean of available published size estimates, a diameter
for Quaoar of 890 {\pm} 70 km. We find Quaoar's density to be \rho = 4.2 {\pm}
1.3 g cm^-3, possibly the highest density in the Kuiper belt.Comment: 11 pages, 2 figures, accepted for publication in ApJ letters
Perturbative reconstruction of a gravitational lens: when mass does not follow light
The structure and potential of a complex gravitational lens is reconstructed
using the perturbative method presented in Alard 2007, MNRAS, 382L, 58; Alard
2008, MNRAS, 388, 375. This lens is composed of 6 galaxies belonging to a small
group. The lens inversion is reduced to the problem of reconstructing
non-degenerate quantities: the 2 fields of the perturbative theory of strong
gravitational lenses. Since in the perturbative theory the circular source
solution is analytical, the general properties of the perturbative solution can
be inferred directly from the data. As a consequence, the reconstruction of the
perturbative fields is not affected by degeneracy, and finding the best
solution is only a matter of numerical refinement. The local shape of the
potential and density of the lens are inferred from the perturbative solution,
revealing the existence of an independent dark component that does not follow
light. The most likely explanation is that the particular shape of the dark
halo is due to the merging of cold dark matter halos. This is a new result
illustrating the structure of dark halos at the scale of galaxies.Comment: Final version (Astronomy and Astrophysics in press
The Transit Light Curve of an Exozodiacal Dust Cloud
Planets embedded within debris disks gravitationally perturb nearby dust and
can create clumpy, azimuthally asymmetric circumstellar ring structures that
rotate in lock with the planet. The Earth creates one such structure in the
solar zodiacal dust cloud. In an edge-on system, the dust "clumps" periodically
pass in front of the star as the planet orbits, occulting and
forward-scattering starlight. In this paper, we predict the shape and magnitude
of the corresponding transit signal. To do so, we model the dust distributions
of collisional, steady-state exozodiacal clouds perturbed by planetary
companions. We examine disks with dusty ring structures formed by the planet's
resonant trapping of in-spiraling dust for a range of planet masses and
semi-major axes, dust properties, and disk masses. We synthesize edge-on images
of these models and calculate the transit signatures of the resonant ring
structures. The transit light curves created by dusty resonant ring structures
typically exhibit two broad transit minima that lead and trail the planetary
transit. We find that Jupiter-mass planets embedded within disks hundreds of
times denser than our zodiacal cloud can create resonant ring structures with
transit depths up to , possibly detectable with \emph{Kepler}.
Resonant rings produced by planets more or less massive than Jupiter produce
smaller transit depths. Observations of these transit signals may provide upper
limits on the degree of asymmetry in exozodiacal clouds.Comment: 31 pages, 7 figure
The Orbit, Mass, and Albedo of Transneptunian Binary 1999 RZ253
We have observed 1999 RZ253 with the Hubble Space Telescope at seven separate
epochs and have fit an orbit to the observed relative positions of this binary.
Two orbital solutions have been identified that differ primarily in the
inclination of the orbit plane. The best fit corresponds to an orbital period,
P=46.263 +0.006/-0.074 days, semimajor axis a=4,660 +/-170 km and orbital
eccentricity e=0.460 +/-0.013 corresponding to a system mass m=3.7 +/-0.4
x10^18 kg. For a density of rho = 1000 kg m^-3 the albedo at 477 nm is p = 0.12
+/-0.01, significantly higher than has been commonly assumed for objects in the
Kuiper Belt. Multicolor, multiepoch photometry shows this pair to have colors
typical for the Kuiper belt with a spectral gradient of 0.35 per 100 nm in the
range between 475 and 775 nm. Photometric variations at the four epochs we
observed were as large as 12 +/-3% but the sampling is insufficient to confirm
the existence of a lightcurve
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