6,722 research outputs found
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
Overcoming the boundary layer turbulence at Dome C: ground-layer adaptive optics versus tower
The unique atmospheric conditions present at sites such as Dome C on the Antarctic plateau are very favorable for high spatial resolution astronomy. At Dome C, the majority of the optical turbulence is confined to a 30 to 40 m thick stable boundary layer that results from the strong temperature inversion created by the heat exchange between the air and the ice-covered ground. To fully realize the potential of the exceptionally calm free atmosphere, this boundary layer must be overcome. In this article we compare the performance of two methods proposed to beat the boundary layer: mounting a telescope on a tower that physically puts it above the turbulent layer, and installing a telescope at ground level with a ground-layer adaptive optics system. A case is also made to combine these two methods to further improve the image quality
Job creation in small and medium sized enterprises: Federal Republic of Germany, France, Netherlands, Belgium, Luxembourg. Vol. II: Main report. Programme of research and actions on development of the labour market
Pulse Profiles, Accretion Column Dips and a Flare in GX 1+4 During a Faint State
The Rossi X-ray Timing Explorer (RXTE) spacecraft observed the X-ray pulsar
GX 1+4 for a period of 34 hours on July 19/20 1996. The source faded from an
intensity of ~20 mCrab to a minimum of <~0.7 mCrab and then partially recovered
towards the end of the observation. This extended minimum lasted ~40,000
seconds. Phase folded light curves at a barycentric rotation period of
124.36568 +/- 0.00020 seconds show that near the center of the extended minimum
the source stopped pulsing in the traditional sense but retained a weak dip
feature at the rotation period. Away from the extended minimum the dips are
progressively narrower at higher energies and may be interpreted as
obscurations or eclipses of the hot spot by the accretion column. The pulse
profile changed from leading-edge bright before the extended minimum to
trailing-edge bright after it. Data from the Burst and Transient Source
Experiment (BATSE) show that a torque reversal occurred <10 days after our
observation. Our data indicate that the observed rotation departs from a
constant period with a Pdot/P value of ~-1.5% per year at a 4.5 sigma
significance. We infer that we may have serendipitously obtained data, with
high sensitivity and temporal resolution about the time of an accretion disk
spin reversal. We also observed a rapid flare which had some precursor
activity, close to the center of the extended minimum.Comment: 19 pages, 6 figures, accepted for publication in Astrophysical
Journal (tentatively scheduled for vol. 529 #1, 20 Jan 2000
UNSWIRF: A Tunable Imaging Spectrometer for the Near-Infrared
We describe the specifications, characteristics, calibration, and analysis of
data from the University of New South Wales Infrared Fabry-Perot (UNSWIRF)
etalon. UNSWIRF is a near-infrared tunable imaging spectrometer, used primarily
in conjunction with IRIS on the AAT, but suitable for use as a visitor
instrument at other telescopes. The etalon delivers a resolving power in excess
of 4000 (corresponding to a velocity resolution ~75 km/s), and allows imaging
of fields up to 100" in diameter on the AAT at any wavelength between 1.5 and
2.4 microns for which suitable blocking filters are available.Comment: 16 pages, 10 figures, uses psfig.sty and html.sty (included). To
appear in Publications of the Astronomical Society of Australi
Momentum transfer for momentum transfer-free which-path experiments
We analyze the origin of interference disappearance in which-path double
aperture experiments. We show that we can unambiguously define an observable
momentum transfer between the quantum particle and the path detector and we
prove in particular that the so called ``momentum transfer free'' experiments
can be in fact logically interpreted in term of momentum transfer.Comment: to appear in Phys. Rev . A (2006). (7 pages, 2 figures
A New Look At Carbon Abundances In Planetary Nebulae. III. DDDM1, IC 3568, IC4593, NGC 6210, NGC 6720, NGC 6826, & NGC 7009
This paper is the third in a series reporting on a study of carbon abundances
in a carefully chosen sample of planetary nebulae representing a large range in
progenitor mass and metallicity. We make use of the IUE Final Archive database
containing consistently-reduced spectra to measure line strengths of C III]
1909 along with numerous other UV lines for the planetary nebulae DDDM1, IC
3568, IC 4593, NGC 6210, NGC 6720, NGC 6826, & NGC 7009. We combine the IUE
data with line strengths from optical spectra obtained specifically to match
the IUE slit positions as closely as possible, to determine values for the
abundance ratios He/H, O/H, C/O, N/O, and Ne/O. The ratio of C III] 1909/C II
4267 is found to be effective for merging UV and optical spectra when He II
1640/4686 is unavailable. Our abundance determination method includes a 5-level
program whose results are fine-tuned by corrections derived from detailed
photoionization models constrained by the same set of emission lines. All
objects appear to have subsolar levels of O/H, and all but one show N/O levels
above solar. In addition, the seven planetary nebulae span a broad range in C/O
values. We infer that many of our objects are matter bounded, and thus the
standard ionization correction factor for N/O may be inappropriate for these
PNe. Finally, we estimate C/O using both collisionally-excited and
recombination lines associated with C+2 and find the well established result
that abundances from recombination lines usually exceed those from
collisionally-excited lines by several times.Comment: 36 pages, 7 tables, 2 figures, latex. Tables and figures supplied as
two separate postscript files. Accepted for publication in Ap
Tectonic setting and geochemistry of Miocene alkalic basalts from the Jones Mountains, West Antarctica
Laminar flow of two miscible fluids in a simple network
When a fluid comprised of multiple phases or constituents flows through a
network, non-linear phenomena such as multiple stable equilibrium states and
spontaneous oscillations can occur. Such behavior has been observed or
predicted in a number of networks including the flow of blood through the
microcirculation, the flow of picoliter droplets through microfluidic devices,
the flow of magma through lava tubes, and two-phase flow in refrigeration
systems. While the existence of non-linear phenomena in a network with many
inter-connections containing fluids with complex rheology may seem
unsurprising, this paper demonstrates that even simple networks containing
Newtonian fluids in laminar flow can demonstrate multiple equilibria.
The paper describes a theoretical and experimental investigation of the
laminar flow of two miscible Newtonian fluids of different density and
viscosity through a simple network. The fluids stratify due to gravity and
remain as nearly distinct phases with some mixing occurring only by diffusion.
This fluid system has the advantage that it is easily controlled and modeled,
yet contains the key ingredients for network non-linearities. Experiments and
3D simulations are first used to explore how phases distribute at a single
T-junction. Once the phase separation at a single junction is known, a network
model is developed which predicts multiple equilibria in the simplest of
networks. The existence of multiple stable equilibria is confirmed
experimentally and a criteria for their existence is developed. The network
results are generic and could be applied to or found in different physical
systems
Where is the best site on Earth? Domes A, B, C and F, and Ridges A and B
The Antarctic plateau contains the best sites on earth for many forms of
astronomy, but none of the existing bases was selected with astronomy as the
primary motivation. In this article, we try to systematically compare the
merits of potential observatory sites.We include South Pole, Domes A, C, and F,
and also Ridge B (running northeast from Dome A), and what we call "Ridge A"
(running southwest from Dome A). Our analysis combines satellite data,
published results, and atmospheric models, to compare the boundary layer,
weather, aurorae, airglow, precipitable water vapor, thermal sky emission,
surface temperature, and the free atmosphere, at each site. We find that all
Antarctic sites are likely to be compromised for optical work by airglow and
aurorae. Of the sites with existing bases, Dome A is easily the best overall;
but we find that Ridge A offers an even better site. We also find that Dome F
is a remarkably good site. Dome C is less good as a thermal infrared or
terahertz site, but would be able to take advantage of a predicted "OH hole"
over Antarctica during spring.Comment: Revised version. 16 pages, 21 figures (22 in first version).
Submitted to PASP 16/05/09, accepted 13/07/09; published 20/08/0
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