1,174 research outputs found
Dynamically dominant magnetic fields in the diffuse interstellar medium
Observations show that magnetic fields in the interstellar medium (ISM) often
do not respond to increases in gas density as would be naively expected for a
frozen-in field. This may suggest that the magnetic field in the diffuse gas
becomes detached from dense clouds as they form. We have investigated this
possibility using theoretical estimates, a simple magneto-hydrodynamic model of
a flow without mass conservation and numerical simulations of a thermally
unstable flow. Our results show that significant magnetic flux can be shed from
dense clouds as they form in the diffuse ISM, leaving behind a magnetically
dominated diffuse gas.Comment: 2 pages, 1 figure. In proceedings of IAU Symposium 259: "Cosmic
magnetic fields: from planets to stars and galaxies", eds. K.G. Strassmeier,
A.G. Kosovichev & J.E. Beckman in pres
Mixing Time Scales in a Supernova-Driven Interstellar Medium
We study the mixing of chemical species in the interstellar medium (ISM).
Recent observations suggest that the distribution of species such as deuterium
in the ISM may be far from homogeneous. This raises the question of how long it
takes for inhomogeneities to be erased in the ISM, and how this depends on the
length scale of the inhomogeneities. We added a tracer field to the
three-dimensional, supernova-driven ISM model of Avillez (2000) to study mixing
and dispersal in kiloparsec-scale simulations of the ISM with different
supernova (SN) rates and different inhomogeneity length scales. We find several
surprising results. Classical mixing length theory fails to predict the very
weak dependence of mixing time on length scale that we find on scales of
25--500 pc. Derived diffusion coefficients increase exponentially with time,
rather than remaining constant. The variance of composition declines
exponentially, with a time constant of tens of Myr, so that large differences
fade faster than small ones. The time constant depends on the inverse square
root of the supernova rate. One major reason for these results is that even
with numerical diffusion exceeding physical values, gas does not mix quickly
between hot and cold regions.Comment: 23 pages, 14 figures that include 7 simulation images and 19 plots,
accepted for publication at Ap
Slight respiratory irritation but not inflammation in mice exposed to (1-->3)-beta-D-glucan aerosols.
Airway irritation effects after single and repeated inhalation exposures to aerosols of beta-glucan (grifolan) were investigated in mice. In addition, the effects on serum total immunoglobulin E (IgE) production and histopathological inflammation in the respiratory tract were studied. The beta-glucan aerosols provoked slight sensory irritation in the airways, but the response was not concentration dependent at the levels studied. Slight pulmonary irritation was observed after repeated exposures. No effect was found on the serum total IgE levels, and no signs of inflammation were seen in the airways 6 h after the final exposure. The results suggest that, irrespective of previous fungal sensitization of the animals, inhaled beta-glucan may cause symptoms of respiratory tract irritation but without apparent inflammation. Respiratory tract irritation reported after inhalation of fungi may not be entirely attributed to beta-glucan
Astrophysical significance of the anisotropic kinetic alpha effect
The generation of large scale flows by the anisotropic kinetic alpha (AKA)
effect is investigated in simulations with a suitable time-dependent space- and
time-periodic anisotropic forcing lacking parity invariance. The forcing
pattern moves relative to the fluid, which leads to a breaking of the Galilean
invariance as required for the AKA effect to exist. The AKA effect is found to
produce a clear large scale flow pattern when the Reynolds number, R, is small
as only a few modes are excited in linear theory. In this case the
non-vanishing components of the AKA tensor are dynamically independent of the
Reynolds number. For larger values of R, many more modes are excited and the
components of the AKA tensor are found to decrease rapidly with increasing
value of R. However, once there is a magnetic field (imposed and of sufficient
strength, or dynamo-generated and saturated) the field begins to suppress the
AKA effect, regardless of the value of R. It is argued that the AKA effect is
unlikely to be astrophysically significant unless the magnetic field is weak
and R is small.Comment: 8 pages, 10 figures, submitted to A&
Galaxies in box: A simulated view of the interstellar medium
We review progress in the development of physically realistic three
dimensional simulated models of the galaxy.We consider the scales from star
forming molecular clouds to the full spiral disc. Models are computed using
hydrodynamic (HD) or magnetohydrodynamic (MHD) equations and may include cosmic
ray or tracer particles. The range of dynamical scales between the full galaxy
structure and the turbulent scales of supernova (SN) explosions and even cloud
collapse to form stars, make it impossible with current computing tools and
resources to resolve all of these in one model. We therefore consider a
hierarchy of models and how they can be related to enhance our understanding of
the complete galaxy.Comment: Chapter in Large Scale Magnetic Fields in the Univers
The small-scale dynamo in a multiphase supernova-driven medium
Magnetic fields grow quickly, even at early cosmological times, suggesting
the action of a small-scale dynamo (SSD) in the interstellar medium (ISM) of
galaxies. Many studies have focused on idealized, isotropic, homogeneous,
turbulent driving of the SSD. Here we analyze more realistic simulations of
supernova-driven turbulence to understand how it drives an SSD. We find that
SSD growth rates are intermittently variable as a result of the evolving
multiphase ISM structure. Rapid growth in the magnetic field typically occurs
in hot gas, with the highest overall growth rates occurring when the fractional
volume of hot gas is large. SSD growth rates correlate most strongly with
vorticity, which also correlates well with gas temperature. Rotational energy
exceeds irrotational energy in all phases, but particularly in the hot phase
while SSD growth is most rapid. Supernova (SN) rate does not significantly
affect the ISM average kinetic energy density. Rather, higher temperatures
associated with high SN rates tend to increase SSD growth rates. SSD saturates
with total magnetic energy density around 5% of equipartition to kinetic energy
density, increasing slightly with magnetic Prandtl number. While magnetic
energy density in the hot gas can exceed that of the other phases when SSD
grows most rapidly, it saturates below 5% of equipartition with kinetic energy
in the hot gas, while in the cold gas it attains 100%. Fast, intermittent
growth of the magnetic field appears to be a characteristic behavior of
SN-driven, multiphase turbulence.Comment: 27 pages, 12 figures, 4 table
Influence of the oxygen partial pressure on the growth and optical properties of RF-sputtered anatase TiO2 thin films
Titanium dioxide (TiO2) films with thicknesses around 300 nm were deposited on glass substrates by
reactive radio frequency (RF) magnetron sputtering at constant RF sputtering power (200 W), high sputtering pressure and room temperature. The effects of the oxygen presence on the growth and properties
of the films were investigated using mixtures of Ar and O2 with different O2/(Ar + O2) ratios (from 0.0 to
0.3) during the sample deposition. The crystalline properties and surface morphology were characterized
using X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. The optical properties
were studied by ultraviolet–visible–near infrared (UV–Vis–NIR) spectroscopy, and the refractive index
and the thickness of the samples were obtained using the Swanepoel method. The obtained results indicate that all the TiO2 films grew with an anatase phase and with an improved crystallinity at O2/(Ar + O2)
= 0.2. However, AFM studies show that the grain size and surface roughness decrease as the O2/(Ar + O2)
ratio increases from 0.0 to 0.3. Moreover, a maximum refractive index was obtained for the sample prepared at O2/(Ar + O2) = 0.2
Magnetic Flux Expulsion in the Powerful Superbubble Explosions and the Alpha-Omega Dynamo
The possibility of the magnetic flux expulsion from the Galaxy in the
superbubble (SB) explosions, important for the Alpha-Omega dynamo, is
considered. Special emphasis is put on the investigation of the downsliding of
the matter from the top of the shell formed by the SB explosion which is able
to influence the kinematics of the shell. It is shown that either Galactic
gravity or the development of the Rayleigh-Taylor instabilities in the shell
limit the SB expansion, thus, making impossible magnetic flux expulsion. The
effect of the cosmic rays in the shell on the sliding is considered and it is
shown that it is negligible compared to Galactic gravity. Thus, the question of
possible mechanism of flux expulsion in the Alpha-Omega dynamo remains open.Comment: MNRAS, in press, 11 pages, 9 figure
Vorticity production through rotation, shear and baroclinicity
In the absence of rotation and shear, and under the assumption of constant
temperature or specific entropy, purely potential forcing by localized
expansion waves is known to produce irrotational flows that have no vorticity.
Here we study the production of vorticity under idealized conditions when there
is rotation, shear, or baroclinicity, to address the problem of vorticity
generation in the interstellar medium in a systematic fashion. We use
three-dimensional periodic box numerical simulations to investigate the various
effects in isolation. We find that for slow rotation, vorticity production in
an isothermal gas is small in the sense that the ratio of the root-mean-square
values of vorticity and velocity is small compared with the wavenumber of the
energy-carrying motions. For Coriolis numbers above a certain level, vorticity
production saturates at a value where the aforementioned ratio becomes
comparable with the wavenumber of the energy-carrying motions. Shear also
raises the vorticity production, but no saturation is found. When the
assumption of isothermality is dropped, there is significant vorticity
production by the baroclinic term once the turbulence becomes supersonic. In
galaxies, shear and rotation are estimated to be insufficient to produce
significant amounts of vorticity, leaving therefore only the baroclinic term as
the most favorable candidate. We also demonstrate vorticity production visually
as a result of colliding shock fronts.Comment: 9 pages, 10 figures, Accepted for publication in A&
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