502 research outputs found
RBF neural net based classifier for the AIRIX accelerator fault diagnosis
The AIRIX facility is a high current linear accelerator (2-3.5kA) used for
flash-radiography at the CEA of Moronvilliers France. The general background of
this study is the diagnosis and the predictive maintenance of AIRIX. We will
present a tool for fault diagnosis and monitoring based on pattern recognition
using artificial neural network. Parameters extracted from the signals recorded
on each shot are used to define a vector to be classified. The principal
component analysis permits us to select the most pertinent information and
reduce the redundancy. A three layer Radial Basis Function (RBF) neural network
is used to classify the states of the accelerator. We initialize the network by
applying an unsupervised fuzzy technique to the training base. This allows us
to determine the number of clusters and real classes, which define the number
of cells on the hidden and output layers of the network. The weights between
the hidden and the output layers, realising the non-convex union of the
clusters, are determined by a least square method. Membership and ambiguity
rejection enable the network to learn unknown failures, and to monitor
accelerator operations to predict future failures. We will present the first
results obtained on the injector.Comment: 3 pages, 4 figures, LINAC'2000 conferenc
Gravitational Stirring in Planetary Debris Disks
We describe gravitational stirring models of planetary debris disks using a
new multi-annulus planetesimal evolution code. The current code includes
gravitational stirring and dynamical friction; future studies will include
coagulation, fragmentation, Poynting-Robertson drag, and other physical
processes. We use the results of our calculations to investigate the physical
conditions required for small bodies in a planetesimal disk to reach the
shattering velocity and begin a collisional cascade. Our results demonstrate
that disks composed primarily of bodies with a single size will not undergo a
collisional cascade which produces small dust grains at 30-150 AU on timescales
of 1 Gyr or smaller. Disks with a size distribution of bodies reach conditions
necessary for a collisional cascade in 10 Myr to 1 Gyr if the disk is at least
as massive as a minimum mass solar nebula and if the disk contains objects with
radii of 500 km or larger. The estimated 500 Myr survival time for these disks
is close to the median age of roughly 400 Myr derived for nearby stars with
dusty disks.Comment: 23 pages of text + 16 Figures; to appear in the Astronomical Journal,
January 200
NICMOS Imaging of the HR 4796A Circumstellar Disk
We report the first near infrared (NIR) imaging of a circumstellar annular
disk around the young (~8 Myr), Vega-like star, HR 4796A. NICMOS coronagraph
observations at 1.1 and 1.6 microns reveal a ring-like symmetrical structure
peaking in reflected intensity 1.05 arcsec +/- 0.02 arcsec (~ 70 AU) from the
central A0V star. The ring geometry, with an inclination of 73.1 deg +/- 1.2
deg and a major axis PA of 26.8 deg +/- 0.6 deg, is in good agreement with
recent 12.5 and 20.8 micron observations of a truncated disk (Koerner, et al.
1998). The ring is resolved with a characteristic width of less than 0.26
arcsec (17 AU) and appears abruptly truncated at both the inner and outer
edges. The region of the disk-plane inward of ~60 AU appears to be relatively
free of scattering material. The integrated flux density of the part of the
disk that is visible (greater than 0.65 arcsec from the star) is found to be
7.5 +/- 0.5 mJy and 7.4 +/- 1.2 mJy at 1.1 and 1.6 microns, respectively.
Correcting for the unseen area of the ring yields total flux densities of 12.8
+/- 1.0 mJy and 12.5 +/- 2.0 mJy, respectively (Vega magnitudes = 12.92 /+-
0.08 and 12.35 +/-0.18). The NIR luminosity ratio is evaluated from these
results and ground-based photometry of the star. At these wavelengths
Ldisk(lambda)/L*(lambda) = 1.4 +/- 0.2E-3 and 2.4 +/- 0.5E-3, giving reasonable
agreement between the stellar flux scattered in the NIR and that which is
absorbed in the visible and re-radiated in the thermal infrared. The somewhat
red reflectance of the disk at these wavelengths implies mean particle sizes in
excess of several microns, larger than typical interstellar grains. The
confinement of material to a relatively narrow annular zone implies dynamical
constraints on the disk particles by one or more as yet unseen bodies.Comment: 14 pages, 1 figure for associated gif file see:
http://nicmosis.as.arizona.edu:8000/AAS99/FIGURE1_HR4796A_ApJL.gif . Accepted
13 January 1999, Astrophyical Journal Letter
Signatures of Planets in Spatially Unresolved Disks
Main sequence stars are commonly surrounded by debris disks, composed of cold
dust continuously replenished by a reservoir of undetected dust-producing
planetesimals. In a planetary system with a belt of planetesimals (like the
Solar System's Kuiper Belt) and one or more interior giant planets, the
trapping of dust particles in the mean motion resonances with the planets can
create structure in the dust disk, as the particles accumulate at certain
semimajor axes. Sufficiently massive planets may also scatter and eject dust
particles out of a planetary system, creating a dust depleted region inside the
orbit of the planet. In anticipation of future observations of spatially
unresolved debris disks with the Spitzer Space Telescope, we are interested in
studying how the structure carved by planets affects the shape of the disk's
spectral energy distribution (SED), and consequently if the SED can be used to
infer the presence of planets. We numerically calculate the equilibrium spatial
density distributions and SEDs of dust disks originated by a belt of
planetesimals in the presence of interior giant planets in different planetary
configurations, and for a representative sample of chemical compositions. The
dynamical models are necessary to estimate the enhancement of particles near
the mean motion resonances with the planets, and to determine how many
particles drift inside the planet's orbit. Based on the SEDs and predicted
colors we discuss what types of planetary systems can be
distinguishable from one another and the main parameter degeneracies in the
model SEDs.Comment: 40 pages (pre-print form), including 16 figures. Published in ApJ
200
The Inner Rings of Beta Pictoris
We present Keck images of the dust disk around Beta Pictoris at 17.9 microns
that reveal new structure in its morphology. Within 1" (19 AU) of the star, the
long axis of the dust emission is rotated by more than 10 degrees with respect
to that of the overall disk. This angular offset is more pronounced than the
warp detected at 3.5" by HST, and in the opposite direction. By contrast, the
long axis of the emission contours at ~ 1.5" from the star is aligned with the
HST warp. Emission peaks between 1.5" and 4" from the star hint at the presence
of rings similar to those observed in the outer disk at ~ 25" with HST/STIS. A
deconvolved image strongly suggests that the newly detected features arise from
a system of four non-coplanar rings. Bayesian estimates based on the primary
image lead to ring radii of 14+/-1 AU, 28+/-3 AU, 52+/-2 AU and 82+/-2 AU, with
orbital inclinations that alternate in orientation relative to the overall disk
and decrease in magnitude with increasing radius. We believe these new results
make a strong case for the existence of a nascent planetary system around Beta
Pic.Comment: 5 pages, 2 figures, PDF format. Published in ApJL, December 20,200
The Warped Circumstellar Disk of HD100546
We propose that the two armed spiral features seen in visible Hubble Space
Telescope images of scattered light in HD100546's circumstellar disk are caused
by the illumination of a warped outer disk. A tilt of 6-15 degrees from the
symmetry plane can cause the observed surface brightness variations providing
the disk is very twisted (highly warped) at radii greater than 200 AU where the
spiral features are seen. Dust lanes are due in part to shadowing in the
equatorial plane from the inner disk within a radius of 100 AU. HD100546's
outer disk, if viewed edge-on, would appear similar to that of Beta Pictorus. A
disk initially misaligned with a planetary system, becomes warped due to
precession induced by planetesimal bodies and planets. However, the twistedness
of HD100546's disk cannot be explained by precession during the lifetime of the
system induced by a single Jovian mass planet within the clearing at ~13 AU.
One possible explanation for the corrugated disk is that precession was induced
by massive of bodies embedded in the disk at larger radius. This would require
approximately a Jupiter mass of bodies well outside the central clearing at 13
AU and within the location of the spiral features or at radii approximately
between 50-200 AU.Comment: submitted to Ap
A probable giant planet imaged in the Beta Pictoris disk
Since the discovery of its dusty disk in 1984, Beta Pictoris has become the
prototype of young early-type planetary systems, and there are now various
indications that a massive Jovian planet is orbiting the star at ~ 10 AU.
However, no planets have been detected around this star so far. Our goal was to
investigate the close environment of Beta Pic, searching for planetary
companion(s). Deep adaptive-optics L'-band images of Beta Pic were recorded
using the NaCo instrument at the Very Large Telescope. A faint point-like
signal is detected at a projected distance of ~ 8 AU from the star, within the
North-East side of the dust disk. Various tests were made to rule out with a
good confidence level possible instrumental or atmospheric artifacts. The
probability of a foreground or background contaminant is extremely low, based
in addition on the analysis of previous deep Hubble Space Telescope images. The
object L'=11.2 apparent magnitude would indicate a typical temperature of ~1500
K and a mass of ~ 8 Jovian masses. If confirmed, it could explain the main
morphological and dynamical peculiarities of the Beta Pic system. The present
detection is unique among A-stars by the proximity of the resolved planet to
its parent star. Its closeness and location inside the Beta Pic disk suggest a
formation process by core accretion or disk instabilities rather than a
binary-like formation process.Comment: 5 pages, 3 figures, 1 table. A&A Letters, in pres
Adaptive Optics Imaging of the AU Microscopii Circumstellar Disk: Evidence for Dynamical Evolution
We present an H-band image of the light scattered from circumstellar dust
around the nearby (10 pc) young M star AU Microscopii (AU Mic, GJ 803, HD
197481), obtained with the Keck adaptive optics system. We resolve the disk
both vertically and radially, tracing it over 17-60 AU from the star. Our AU
Mic observations thus offer the possibility to probe at high spatial resolution
(0.04" or 0.4 AU per resolution element) for morphological signatures of the
debris disk on Solar-System scales. Various sub-structures (dust clumps and
gaps) in the AU Mic disk may point to the existence of orbiting planets. No
planets are seen in our H-band image down to a limiting mass of 1 M_Jup at >20
AU, although the existence of smaller planets can not be excluded from the
current data. Modeling of the disk surface brightness distribution at H-band
and R-band, in conjunction with the optical to sub-millimeter spectral energy
distribution, allows us to constrain the disk geometry and the dust grain
properties. We confirm the nearly edge-on orientation of the disk inferred from
previous observations, and deduce an inner clearing radius <=10 AU. We find
evidence for a lack of small grains in the inner (<60 AU) disk, either as a
result of primordial disk evolution, or because of destruction by
Poynting-Robertson and/or corpuscular drag. A change in the power-law index of
the surface brightness profile is observed near 33 AU, similar to a feature
known in the profile of the beta Pic circumstellar debris disk. By comparing
the time scales for inter-particle collisions and Poynting-Robertson drag
between the two systems, we argue that the breaks are linked to one of these
two processes.Comment: 17 pages, 7 figures, 1 table; accepted by Ap
VLT/NACO coronagraphic observations of fine structures in the disk of beta Pictoris
We present ground-based observations of the disk around the A-type star
Pictoris to perform a close inspection of the inner disk morphology.
Images were collected with NACO, the AO-assisted near-IR instrument on the VLT
(ESO) which includes two types of coronagraphs: classical Lyot masks and phase
masks. In this program we took advantage of both types of coronagraphs in two
spectral bands, H-band for the Lyot mask and Ks-band for the phase mask. In
addition, we simulated an extended object to understand the limitations in
deconvolution of coronagraphic images. The reduced coronagraphic images allow
us to carefully measure the structures of the debris disk and reveal a number
of asymmetries of which some were not reported before (position, elevation and
thickness of the warp). In this program, the circumstellar material is visible
as close as 0.7" (AU) owing to the phase mask while the Lyot mask
generates artifacts which hamper the detection of the dust at separations
closer than 1.2" (AU). The point source detection limit is compared to
recently published observations of a planet candidate. Finally, the simulations
show that deconvolution of coronagraphic data may indeed produce artificial
patterns within the image of a disk.Comment: 13 pages, to appear in Astronomy and Astrophysic
Dissecting the Moth: Discovery of an off-centered ring in the HD 61005 debris disk with high-resolution imaging
The debris disk known as "The Moth" is named after its unusually asymmetric
surface brightness distribution. It is located around the ~90 Myr old G8V star
HD 61005 at 34.5 pc and has previously been imaged by the HST at 1.1 and 0.6
microns. Polarimetric observations suggested that the circumstellar material
consists of two distinct components, a nearly edge-on disk or ring, and a
swept-back feature, the result of interaction with the interstellar medium. We
resolve both components at unprecedented resolution with VLT/NACO H-band
imaging. Using optimized angular differential imaging techniques to remove the
light of the star, we reveal the disk component as a distinct narrow ring at
inclination i=84.3 \pm 1.0{\deg}. We determine a semi-major axis of a=61.25 \pm
0.85 AU and an eccentricity of e=0.045 \pm 0.015, assuming that periastron is
located along the apparent disk major axis. Therefore, the ring center is
offset from the star by at least 2.75 \pm 0.85 AU. The offset, together with a
relatively steep inner rim, could indicate a planetary companion that perturbs
the remnant planetesimal belt. From our imaging data we set upper mass limits
for companions that exclude any object above the deuterium-burning limit for
separations down to 0.3". The ring shows a strong brightness asymmetry along
both the major and minor axis. A brighter front side could indicate
forward-scattering grains, while the brightness difference between the NE and
SW components can be only partly explained by the ring center offset,
suggesting additional density enhancements on one side of the ring. The
swept-back component appears as two streamers originating near the NE and SW
edges of the debris ring.Comment: 6 pages, 6 figures. Accepted to Astronomy and Astrophysics letter
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