7,679 research outputs found
The potential for astrometry in the infrared
Infrared interferometry promises to be a useful astrometric technique. Preliminary measurements of the star alpha Orionis made with a heterodyne interferometer exhibit phase coherence over a period of at least 1000 seconds. The measurements were equivalent to a positional determination of 60 milliarcsecond accuracy every 5 seconds of integration
PILOT: design and capabilities
The proposed design for PILOT is a general-purpose, wide-field 1 degree 2.4m,
f/10 Ritchey-Chretien telescope, with fast tip-tilt guiding, for use 0.5-25
microns. The design allows both wide-field and diffraction-limited use at these
wavelengths. The expected overall image quality, including median seeing, is
0.28-0.3" FWHM from 0.8-2.4 microns. Point source sensitivities are estimated.Comment: 4 pages, Proceedings of 2nd ARENA conference 'The Astrophysical
Science Cases at Dome C', Potsdam, 17-21 September 200
Mocassin: A fully three-dimensional Monte Carlo photoionization code
The study of photoionized environments is fundamental to many astrophysical
problems. Up to the present most photoionization codes have numerically solved
the equations of radiative transfer by making the extreme simplifying
assumption of spherical symmetry. Unfortunately very few real astronomical
nebulae satisfy this requirement. To remedy these shortcomings, a
self-consistent, three-dimensional radiative transfer code has been developed
using Monte Carlo techniques. The code, Mocassin, is designed to build
realistic models of photoionized nebulae having arbitraries geometry and
density distributions with both the stellar and diffuse radiation fields
treated self-consistently. In addition, the code is capable of tretating on or
more exciting stars located at non-central locations. The gaseous region is
approximated by a cuboidal Cartesian grid composed of numerous cells. The
physical conditions within each grid cell are determined by solving the thermal
equilibrium and ionization balance equations This requires a knowledge of the
local primary and secondary radiation fields, which are calculated
self-consistently by locally simulating the individual processes of ionization
and recombination. The main structure and computational methods used in the
Mocassin code are described in this paper. Mocassin has been benchmarked
against established one-dimensional spherically symmetric codes for a number of
standard cases, as defined by the Lexington/Meudon photoionization workshops
(Pequignot et al., 1986; Ferland et al., 1995; Pequignot et al.,
2001)\citep{pequignot86,ferland95, pequignot01}. The results obtained for the
benchmark cases are satisfactory and are presented in this paper. A performance
analysis has also been carried out and is discussed here.Comment: 17 pages, 4 figures, 1 appendix Changes: appendix adde
Chemical abundances for Hf 2-2, a planetary nebula with the strongest known heavy element recombination lines
We present high quality optical spectroscopic observations of the planetary
nebula (PN) Hf 2-2. The spectrum exhibits many prominent optical recombination
lines (ORLs) from heavy element ions. Analysis of the H {\sc i} and He {\sc i}
recombination spectrum yields an electron temperature of K, a factor
of ten lower than given by the collisionally excited [O {\sc iii}] forbidden
lines. The ionic abundances of heavy elements relative to hydrogen derived from
ORLs are about a factor of 70 higher than those deduced from collisionally
excited lines (CELs) from the same ions, the largest abundance discrepancy
factor (adf) ever measured for a PN. By comparing the observed O {\sc ii}
4089/4649 ORL ratio to theoretical value as a function of
electron temperature, we show that the O {\sc ii} ORLs arise from ionized
regions with an electron temperature of only K. The current
observations thus provide the strongest evidence that the nebula contains
another previously unknown component of cold, high metallicity gas, which is
too cool to excite any significant optical or UV CELs and is thus invisible via
such lines. The existence of such a plasma component in PNe provides a natural
solution to the long-standing dichotomy between nebular plasma diagnostics and
abundance determinations using CELs on the one hand and ORLs on the other.Comment: 12 pages, 5 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
Characterization of soil and postlaunch pad debris from Cape Canaveral launch complex and analysis of soil interaction with aqueous HCl
Soil samples were fractionated and analyzed in order to assess the physical and chemical interactions of entrained soil with solid-rocket exhaust clouds. The sandy soil consisted primarily of quartz (silica) particles, 30 to 500 microns in diameter, and also contained seashell fragments. Differential and cumulative soil-mass size distributions are presented along with mineralogy, elemental compositions, and solution pH histories. About 90 percent of the soil mass consisted of particles 165 microns in diameter. Characteristic reaction times in aqueous HC1 slurries varied from a few minutes to several days, and capacities for reaction under acidic conditions varied from 10 to 40 g HCl/kg soil, depending on particle size. Airborne lifetimes of particles 165 microns are conservatively 30 min, and this major grouping is predicted to represent a small short-term chemical sink for up to 5% of the total HC1. The smaller and more minor fractions, below a 165 micron diameter, may act as giant cloud condensation nuclei over much longer airborne lifetimes. Finally, the demonstrated time dependency of neutralization is a complicating factor; it can influence the ability to deduce in-cloud HCl scavenging with reaction and can affect the accuracy of measured chemical compositions of near-field wet deposition
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