288 research outputs found
A Spitzer Study of Comets 2P/Encke, 67P/Churyumov-Gerasimenko, and C/2001 HT50 (LINEAR-NEAT)
We present infrared images and spectra of comets 2P/Encke,
67P/Churyumov-Gerasimenko, and C/2001 HT50 (LINEAR-NEAT) as part of a larger
program to observe comets inside of 5 AU from the sun with the Spitzer Space
Telescope. The nucleus of comet 2P/Encke was observed at two vastly different
phase angles (20 degrees and 63 degrees). Model fits to the spectral energy
distributions of the nucleus suggest comet Encke's infrared beaming parameter
derived from the near-Earth asteroid thermal model may have a phase angle
dependence. The observed emission from comet Encke's dust coma is best-modeled
using predominately amorphous carbon grains with a grain size distribution that
peaks near 0.4 microns, and the silicate contribution by mass to the sub-micron
dust coma is constrained to 31%. Comet 67P/Churyumov-Gerasimenko was observed
with distinct coma emission in excess of a model nucleus at a heliocentric
distance of 5.0 AU. The coma detection suggests that sublimation processes are
still active or grains from recent activity remain near the nucleus. Comet
C/2001 HT50 (LINEAR-NEAT) showed evidence for crystalline silicates in the
spectrum obtained at 3.2 AU and we derive a silicate-to-carbon dust ratio of
0.6. The ratio is an order of magnitude lower than that derived for comets
9P/Tempel 1 during the Deep Impact encounter and C/1995 O1 (Hale-Bopp).Comment: Accepted for publication in the Astrophysical Journal 48 pages, 15
figures, 10 table
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Investigation of impulsively loaded pressure vessels
Explosion containment vessels for containing from 2,000 to 3,000 five ton nuclear explosions are considered. Analysis methods appear adequate and lowest weights using the most advanced materials available in the next five years are projected.None of these materials can be fabricated today and all require extensive development. Present material technology limits the choice of materials and defines the weight. The addition of safety factors and fixtures (nozzles, etc.) will add to this weight considerably, and may well radically alter the vessel response. Improvements in the strength weight ratios of metals and glasses over those considered in this report do not appear reasonable at this time. Winding schemes to utilize the high strength of steel wires and somehow maintain a reasonable thickness appear to offer the most promise. A `ductile` beryllium would of course offer vast improvement, but no indications that this is being developed have appeared and all presently known beryllium is much too brittle
On Ultrasmall Silicate Grains in the Diffuse Interstellar Medium
The abundance of both amorphous and crystalline silicates in very small
grains is limited by the fact that the 10 micron silicate emission feature is
not detected in the diffuse ISM. On the basis of the observed IR emission
spectrum for the diffuse ISM, the observed ultraviolet extinction curve, and
the 10 micron silicate absorption profile, we obtain upper limits on the
abundances of ultrasmall (a < 15 Angstrom) amorphous and crystalline silicate
grains.
Contrary to previous work, as much as ~20% of interstellar Si could be in a <
15 Angstrom silicate grains without violating observational constraints. Not
more than ~5% of the Si can be in crystalline silicates (of any size).Comment: Submitted to ApJ Letters, 11 pages, 4 figures, Late
Broadband infrared photometry of comet Hale-Bopp with ISOPHOT
Comet Hale-Bopp was observed five times with ISOPHOT, the photometer on board ESA's Infrared Space Observatory (ISO) between 4.6 and 2.8 AU. Each time, broadband photometry was performed using 4 different detectors, 5 apertures and 10 filters covering the range between 3.6 and 170 μm. Background observations were performed with identical instrument settings at the same positions on the sky several days after the comet observations. The observation strategy and the data reduction steps are described in some detail, including the techniques to correct for variable detector responsivity. The resulting inband power values of the Hale-Bopp observations and their uncertainties are given. The mean uncertainty is 25%. The final fluxes were computed, taking into account the zodiacal background, possible offset of the comet's position from the center of the aperture, the brightness distribution within the coma, and the spectral energy distribution of the comet's emission. Strong thermal emission from a broad size distribution of dust particles was detected in all of the data sets, even at r = 4.6-4.9 AU pre-perihelion and 3.9 AU post-perihelion; the total thermal energy varied as r-3. The 7.3-12.8 μm color temperature was ~1.5 times the blackbody temperature, higher than that observed in any other comet. Silicate features at 10 and 25 μm were prominent in all 5 data sets, the largest heliocentric distances that silicate emission has been detected in a comet. The presence of crystalline water ice grains is suggested from the 60 μm excess emission at 4.6-4.9 AU, consistent with the observed QOH if the icy grains were slightly warmer than an equilibrium blackbody. The average albedo of the dust is higher than that of comet P/Halley, but lower than other albedo measurements for Hale-Bopp nearer perihelion. There is no evidence for a component of cold, bright icy grains enhancing the scattered light at 4.6 AU. Simple models for a mixture of silicate and absorbing grains were fit to the ISO spectra and photometry at 2.8 AU. The observed flux at λ >100 μm requires a size distribution in which most of the mass is concentrated in large particles. Dust production rates of order 1.5 x 105 kg s-1 at 2.8 AU and 3 x 104 kg s-1 at 4.6 AU have been found. They correspond to dust to gas mass ratios of 6 to 10
The Embedded Super Star Cluster of SBS0335-052
We analyze the infrared (6-100 micron) spectral energy distribution of the
blue compact dwarf and metal-poor (Z=Z_solar/41) galaxy SBS0335-052. With the
help of DUSTY (Ivezic et al. 1999), a program that solves the radiation
transfer equations in a spherical environment, we evaluate that the infrared
(IR) emission of SBS0335-052 is produced by an embedded super-star cluster
(SSC) hidden under 10^5 M_solar of dust, causing 30 mag of visual extinction.
This implies that one cannot detect any stellar emission from the 2x10^6
M_solar stellar cluster even at near-infrared (NIR) wavelengths. The derived
grain size distribution departs markedly from the widely accepted size
distribution inferred for dust in our galaxy (the so-called MRN distribution,
Mathis et al. 1977), but resembles what is seen around AGNs, namely an absence
of PAH and smaller grains, and grains that grow to larger sizes (around 1
micron). The fact that a significant amount of dust is present in such a
low-metallicity galaxy, hiding from UV and optical view most of the star
formation activity in the galaxy, and that the dust size distribution cannot be
reproduced by a standard galactic law, should be borne in mind when
interpreting the spectrum of primeval galaxies.Comment: 32 pages, 3 figures,accepted for publication in A
Annealing of Silicate Dust by Nebular Shocks at 10 AU
Silicate dust grains in the interstellar medium are known to be mostly
amorphous, yet crystalline silicate grains have been observed in many
long-period comets and in protoplanetary disks. Annealing of amorphous silicate
grains into crystalline grains requires temperatures > 1000 K, but exposure of
dust grains in comets to such high temperatures is incompatible with the
generally low temperatures experienced by comets. This has led to the proposal
of models in which dust grains were thermally processed near the protoSun, then
underwent considerable radial transport until they reached the gas giant planet
region where the long-period comets originated. We hypothesize instead that
silicate dust grains were annealed in situ, by shock waves triggered by
gravitational instabilities. We assume a shock speed of 5 km/s, a plausible
value for shocks driven by gravitational instabilities. We calculate the peak
temperatures of micron and submicron amorphous pyroxene grains of chondritic
composition under conditions typical in protoplanetary disks at 5 - 10 AU. Our
results also apply to chondritic amorphous olivine grains. We show that {\it in
situ} thermal annealing of submicron and micron-sized silicate dust grains can
occur, obviating the need for large-scale radial transport.Comment: 12 pages; includes 1 figure, 1 table; accepted by ApJ Letter
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