659 research outputs found
Multi-Wavelength Implications of the Companion Star in Eta Carinae
Eta Carinae is considered to be a massive colliding wind binary system with a
highly eccentric (e \sim 0.9), 5.54-yr orbit. However, the companion star
continues to evade direct detection as the primary dwarfs its emission at most
wavelengths. Using three-dimensional (3-D) SPH simulations of Eta Car's
colliding winds and radiative transfer codes, we are able to compute synthetic
observables across multiple wavebands for comparison to the observations. The
models show that the presence of a companion star has a profound influence on
the observed HST/STIS UV spectrum and H-alpha line profiles, as well as the
ground-based photometric monitoring. Here, we focus on the Bore Hole effect,
wherein the fast wind from the hot secondary star carves a cavity in the dense
primary wind, allowing increased escape of radiation from the hotter/deeper
layers of the primary's extended wind photosphere. The results have important
implications for interpretations of Eta Car's observables at multiple
wavelengths.Comment: 5 pages, 4 figures, To be published in the proceedings of the meeting
'Four Decades of Research on Massive Stars' in honor of Tony Moffat, 11-15
July 2011, Saint-Michel-des-Saints, Quebe
Particle dynamics of a cartoon dune
The spatio-temporal evolution of a downsized model for a desert dune is
observed experimentally in a narrow water flow channel. A particle tracking
method reveals that the migration speed of the model dune is one order of
magnitude smaller than that of individual grains. In particular, the erosion
rate consists of comparable contributions from creeping (low energy) and
saltating (high energy) particles. The saltation flow rate is slightly larger,
whereas the number of saltating particles is one order of magnitude lower than
that of the creeping ones. The velocity field of the saltating particles is
comparable to the velocity field of the driving fluid. It can be observed that
the spatial profile of the shear stress reaches its maximum value upstream of
the crest, while its minimum lies at the downstream foot of the dune. The
particle tracking method reveals that the deposition of entrained particles
occurs primarily in the region between these two extrema of the shear stress.
Moreover, it is demonstrated that the initial triangular heap evolves to a
steady state with constant mass, shape, velocity, and packing fraction after
one turnover time has elapsed. Within that time the mean distance between
particles initially in contact reaches a value of approximately one quarter of
the dune basis length
Constraints on decreases in Eta Carinae's mass loss from 3D hydrodynamic simulations of its binary colliding winds
Recent work suggests that the mass-loss rate of the primary star (Eta A) in
the massive colliding wind binary Eta Carinae dropped by a factor of 2-3
between 1999 and 2010. We present results from large- (r=1545au) and small-
(r=155au) domain, 3D smoothed particle hydrodynamic (SPH) simulations of Eta
Car's colliding winds for 3 Eta A mass-loss rates (2.4, 4.8, and 8.5 x 10^-4
M_sun/yr), investigating the effects on the dynamics of the binary wind-wind
collision (WWC). These simulations include orbital motion, optically thin
radiative cooling, and radiative forces. We find that Eta A's mass-loss rate
greatly affects the time-dependent hydrodynamics at all spatial scales
investigated. The simulations also show that the post-shock wind of the
companion star (Eta B) switches from the adiabatic to the radiative-cooling
regime during periastron passage. The SPH simulations together with 1D
radiative transfer models of Eta A's spectra reveal that a factor of 2 or more
drop in Eta A's mass-loss rate should lead to substantial changes in numerous
multiwavelength observables. Recent observations are not fully consistent with
the model predictions, indicating that any drop in Eta A's mass-loss rate was
likely by a factor < 2 and occurred after 2004. We speculate that most of the
recent observed changes in Eta Car are due to a small increase in the WWC
opening angle that produces significant effects because our line-of-sight to
the system lies close to the dense walls of the WWC zone. A modest decrease in
Eta A's mass-loss rate may be responsible, but changes in the wind/stellar
parameters of Eta B cannot yet be fully ruled out. We suggest observations
during Eta Car's next periastron in 2014 to further test for decreases in Eta
A's mass-loss rate. If Eta A's mass-loss rate is declining and continues to do
so, the 2014 X-ray minimum should be even shorter than that of 2009.Comment: 38 pages, 25 figures, 1 table. Accepted for publication in MNRA
Observation of Density Segregation inside Migrating Dunes
Spatiotemporal patterns in nature, such as ripples or dunes, formed by a
fluid streaming over a sandy surface show complex behavior despite their simple
forms. Below the surface, the granular structure of the sand particles is
subject to self-organization processes, exhibiting such phenomena as reverse
grading when larger particles are found on top of smaller ones. Here we report
results of an experimental investigation with downscaled model dunes revealing
that, if the particles differ not in size but in density, the heavier
particles, surprisingly, accumulate in the central core close to the top of the
dune. This finding contributes to the understanding of sedimentary structures
found in nature and might be helpful to improve existing dating methods for
desert dunes.Comment: 4 pages, 5 figures, submitted to Physical Review E Rapid
Communications, in prin
Systematic evidence on migrating and extractable food contact chemicals: Most chemicals detected in food contact materials are not listed for use
Food packaging is important for today’s globalized food system, but food contact materials (FCMs) can also be a source of hazardous chemicals migrating into foodstuffs. Assessing the impacts of FCMs on human health requires a comprehensive identification of the chemicals they contain, the food contact chemicals (FCCs). We systematically compiled the “database on migrating and extractable food contact chemicals” (FCCmigex) using information from 1210 studies. We found that to date 2881 FCCs have been detected, in a total of six FCM groups (Plastics, Paper & Board, Metal, Multi-materials, Glass & Ceramic, and Other FCMs). 65% of these detected FCCs were previously not known to be used in FCMs. Conversely, of the more than 12’000 FCCs known to be used, only 1013 are included in the FCCmigex database. Plastic is the most studied FCM with 1975 FCCs detected. Our findings expand the universe of known FCCs to 14,153 chemicals. This knowledge contributes to developing non-hazardous FCMs that lead to safer food and support a circular economy
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