7,398 research outputs found
Ion-neutral decoupling in the nonlinear KelvinâHelmholtz instability: Case of field-aligned flow
This is the author accepted manuscript. The final version is available from AIP Publishing via the DOI in this recordThe nonlinear magnetic Kelvin-Helmholtz instability (KHi), and the turbulence it creates, appears in many astrophysical
systems. This includes those systems where the local plasma conditions are such that the plasma is not fully ionised,
for example in the lower solar atmosphere and molecular clouds. In a partially ionised system, the fluids couple
via collisions which occur at characteristic frequencies, therefore neutral and plasma species become decoupled for
sufficiently high-frequency dynamics. Here we present high-resolution 2D two-fluid simulations of the nonlinear KHi
for a system that traverses the dynamic scales between decoupled fluids and coupled dynamics. We discover some
interesting phenomena, including the presence of a density coupling that is independent of the velocity coupling. Using
these simulations we analyse the heating rate, and two regimes appear. The first is a regime where the neutral flow
is decoupled from the magnetic field that is characterised with a constant heating rate, then at larger scales the strong
coupling approximation holds and the heating rate. At large scales with the KHi layer width to the 2 power. There is
an energy cascade in the simulation, but the nature of the frictional heating means the heating rate is determined by the
largest scale of the turbulent motions, a fact that has consequences for understanding turbulent dissipation in multi-fluid
systems.Science and Technology Facilities Council (STFC
Augmented reality meeting table: a novel multi-user interface for architectural design
Immersive virtual environments have received widespread attention as providing possible replacements for the media and systems that designers traditionally use, as well as, more generally, in providing support for collaborative work. Relatively little attention has been given to date however to the problem of how to merge immersive virtual environments into real world work settings, and so to add to the media at the disposal of the designer and the design team, rather than to replace it. In this paper we report on a research project in which optical see-through augmented reality displays have been developed together with prototype decision support software for architectural and urban design. We suggest that a critical characteristic of multi user augmented reality is its ability to generate visualisations from a first person perspective in which the scale of rendition of the design model follows many of the conventions that designers are used to. Different scales of model appear to allow designers to focus on different aspects of the design under consideration. Augmenting the scene with simulations of pedestrian movement appears to assist both in scale recognition, and in moving from a first person to a third person understanding of the design. This research project is funded by the European Commission IST program (IST-2000-28559)
The UV Scattering Halo of the Central Source Associated with Eta Carinae
We have made an extensive study of the UV spectrum of Eta Carinae, and find
that we do not directly observe the star and its wind in the UV. Because of
dust along our line of sight, the UV light that we observe arises from
bound-bound scattering at large impact parameters. We obtain a reasonable fit
to the UV spectrum by using only the flux that originates outside 0.033". This
explains why we can still observe the primary star in the UV despite the large
optical extinction -- it is due to the presence of an intrinsic coronagraph in
the Eta Carinae system, and to the extension of the UV emitting region. It is
not due to peculiar dust properties alone. We have computed the spectrum of the
purported companion star, and show that it could only be directly detected in
the UV spectrum preferentially in the Far Ultraviolet Spectroscopic Explorer
(FUSE) spectral region (912-1175 Ang.). However, we find no direct evidence for
a companion star, with the properties indicated by X-ray studies and studies of
the Weigelt blobs, in UV spectra. This might be due to reprocessing of the
companion's light by the dense stellar wind of the primary. Broad FeII and
[FeII] emission lines, which form in the stellar wind, are detected in spectra
taken in the SE lobe, 0.2" from the central star. The wind spectrum shows some
similarities to the spectra of the B & D Weigelt blobs, but also shows some
marked differences in that high excitation lines, and lines pumped by Ly-alpha,
are not seen. The detection of the broad lines lends support to our
interpretation of the UV spectrum, and to our model for Eta Carinae.Comment: To appear in ApJ. 57 pages with 18 figure
The Atomic Physics Underlying the Spectroscopic Analysis of Massive Stars and Supernovae
We have developed a radiative transfer code, CMFGEN, which allows us to model
the spectra of massive stars and supernovae. Using CMFGEN we can derive
fundamental parameters such as effective temperatures and surface gravities,
derive abundances, and place constraints on stellar wind properties. The last
of these is important since all massive stars are losing mass via a stellar
wind that is driven from the star by radiation pressure, and this mass loss can
substantially influence the spectral appearance and evolution of the star.
Recently we have extended CMFGEN to allow us to undertake time-dependent
radiative transfer calculations of supernovae. Such calculations will be used
to place constraints on the supernova progenitor, to place constraints on the
supernova explosion and nucleosynthesis, and to derive distances using a
physical approach called the "Expanding Photosphere Method". We describe the
assumptions underlying the code and the atomic processes involved. A crucial
ingredient in the code is the atomic data. For the modeling we require accurate
transition wavelengths, oscillator strengths, photoionization cross-sections,
collision strengths, autoionization rates, and charge exchange rates for
virtually all species up to, and including, cobalt. Presently, the available
atomic data varies substantially in both quantity and quality.Comment: 8 pages, 2 figures, Accepted for publication in Astrophysics & Space
Scienc
Properties of WNh stars in the Small Magellanic Cloud: evidence for homogeneous evolution
We derive the physical properties of three WNh stars in the SMC to constrain
stellar evolution beyond the main sequence at low metallicity and to
investigate the metallicity dependence of the clumping properties of massive
stars. We compute atmosphere models to derive the stellar and wind properties
of the three WNh targets. A FUV/UV/optical/near-infrared analysis gives access
to temperatures, luminosities, mass loss rates, terminal velocities and stellar
abundances. All stars still have a large hydrogen mass fraction in their
atmosphere, and show clear signs of CNO processing in their surface abundances.
One of the targets can be accounted for by normal stellar evolution. It is a
star with initial mass around 40-50 Msun in, or close to, the core He burning
phase. The other two objects must follow a peculiar evolution, governed by fast
rotation. In particular, one object is likely evolving homogeneously due to its
position blue-ward of the main sequence and its high H mass fraction. The
clumping factor of one star is found to be 0.15+/-0.05. This is comparable to
values found for Galactic Wolf-Rayet stars, indicating that within the
uncertainties, the clumping factor does not seem to depend on metallicity.Comment: 16 pages. A&A accepte
The qWR star HD 45166. II. Fundamental stellar parameters and evidence of a latitude-dependent wind
The enigmatic object HD 45166 is a qWR star in a binary system with an
orbital period of 1.596 day, and presents a rich emission-line spectrum in
addition to absorption lines from the companion star (B7 V). As the system
inclination is very small (i=0.77 +- 0.09 deg), HD 45166 is an ideal laboratory
for wind-structure studies. The goal of the present paper is to determine the
fundamental stellar and wind parameters of the qWR star. A radiative transfer
model for the wind and photosphere of the qWR star was calculated using the
non-LTE code CMFGEN. The wind asymmetry was also analyzed using a
recently-developed version of CMFGEN to compute the emerging spectrum in
two-dimensional geometry. The temporal-variance spectrum (TVS) was calculated
for studying the line-profile variations. Abundances, stellar and wind
parameters of the qWR star were obtained. The qWR star has an effective
temperature of Teff=50000 +- 2000 K, a luminosity of log(L/Lsun)=3.75 +- 0.08,
and a corresponding photospheric radius of Rphot=1.00 Rsun. The star is
helium-rich (N(H)/N(He) = 2.0), while the CNO abundances are anomalous when
compared either to solar values, to planetary nebulae, or to WR stars. The
mass-loss rate is Mdot = 2.2 . 10^{-7} Msun/yr, and the wind terminal velocity
is vinf=425 km/s. The comparison between the observed line profiles and models
computed under different latitude-dependent wind densities strongly suggests
the presence of an oblate wind density enhancement, with a density contrast of
at least 8:1 from equator to pole. If a high velocity polar wind is present
(~1200 km/s), the minimum density contrast is reduced to 4:1. The wind
parameters determined are unusual when compared to O-type stars or to typical
WR stars. (abridged)Comment: 16 pages, 13 figures, accepted for publication in A&
Centrality Measures in Spatial Networks of Urban Streets
We study centrality in urban street patterns of different world cities
represented as networks in geographical space. The results indicate that a
spatial analysis based on a set of four centrality indices allows an extended
visualization and characterization of the city structure. Planned and
self-organized cities clearly belong to two different universality classes. In
particular, self-organized cities exhibit scale-free properties similar to
those found in the degree distributions of non-spatial networks.Comment: 4 pages, 3 figure
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