852 research outputs found
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
The Role of p38 MAPK in Rhinovirus-Induced Monocyte Chemoattractant Protein-1 Production by Monocytic-Lineage Cells
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
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
Supersonic turbulence and structure of interstellar molecular clouds
The interstellar medium (ISM) provides a unique laboratory for highly
supersonic, driven hydrodynamics turbulence. We present a theory of such
turbulence, confirm it by numerical simulations, and use the results to explain
observational properties of interstellar molecular clouds, the regions where
stars are born.Comment: 5 pages, 3 figures include
Baseband and RF hardware impairments in full-duplex wireless systems: experimental characterisation and suppression
TCO evaluation in physical asset management : benefits and limitations for industrial adoption
Part 1: Knowledge-Based Performance ImprovementInternational audienceNowadays, the evaluation of the total cost of ownership (TCO) of an asset for supporting informed decision-making both for investments and managerial issues within the asset management framework is gaining increasing attention in industry. Nevertheless its application in practice is still limited. The aim of this paper is to analyze the benefits and limitations of the adoption of TCO evaluation in asset management. Based on a literature review, the paper defines a framework that categorizes the benefits and potential applications that a TCO model can have for different stakeholders. Together with that, industry related issues that influence its implementation are also considered. Finally, empirical evidences are analyzed through a multiple case study to understand if those benefits are recognized in practice and which are the limitations for the practical adoption of a TCO model that should allow exploiting such benefits
Simulations of galactic dynamos
We review our current understanding of galactic dynamo theory, paying
particular attention to numerical simulations both of the mean-field equations
and the original three-dimensional equations relevant to describing the
magnetic field evolution for a turbulent flow. We emphasize the theoretical
difficulties in explaining non-axisymmetric magnetic fields in galaxies and
discuss the observational basis for such results in terms of rotation measure
analysis. Next, we discuss nonlinear theory, the role of magnetic helicity
conservation and magnetic helicity fluxes. This leads to the possibility that
galactic magnetic fields may be bi-helical, with opposite signs of helicity and
large and small length scales. We discuss their observational signatures and
close by discussing the possibilities of explaining the origin of primordial
magnetic fields.Comment: 28 pages, 15 figure, to appear in Lecture Notes in Physics "Magnetic
fields in diffuse media", Eds. E. de Gouveia Dal Pino and A. Lazaria
Pleiotropic functions of the tumor- and metastasis-suppressing Matrix Metalloproteinase-8 in mammary cancer in MMTV-PyMT transgenic mice
Matrix metalloproteinase-8 (MMP-8; neutrophil collagenase) is an important regulator of innate immunity which has onco-suppressive actions in numerous tumor types
Global Dynamical Evolution of the ISM in Star Forming Galaxies - I. High Resolution 3D HD and MHD Simulations: Effect of the Magnetic Field
In star forming disk galaxies, matter circulation between stars and the
interstellar gas, and, in particular the energy input by random and clustered
supernova explosions, determine the dynamical and chemical evolution of the
ISM, and hence of the galaxy as a whole. Using a 3D MHD code with adaptive mesh
refinement developed for this purpose, we have investigated the r\^ole of
magnetized matter circulation between the gaseous disk and the surrounding
galactic halo. Special emphasis has been put on the effect of the magnetic
field with respect to the volume and mass fractions of the different ISM
``phases'', the relative importance of ram, thermal and magnetic pressures, and
whether the field can prevent matter transport from the disk into the halo. The
simulations were performed on a grid with an area of 1 kpc, centered on
the solar circle, extending kpc perpendicular to the galactic disk
with a resolution as high as 1.25 pc. The simulations were run for a time scale
of 400 Myr, sufficiently long to avoid memory effects of the initial setup, and
to allow for a global dynamical equilibrium to be reached in case of a constant
energy input rate. (...) We find that in general gas transport into the halo in
3D is not prevented by an initial disk parallel magnetic field, but only
delayed initially, for as long as it is needed to punch holes into the thick
magnetized gas disk. The mean volume filling factor of the hot phase in the
disk is similar in HD and MHD (the latter with a total field strength of 4.4
G) runs, amounting to for the Galactic supernova rate.Comment: Paper accepted for publication in A&A (31 Jan); 17 pages with 10
Figures including 6 images and 16 plots (all in low quality jpeg files). For
better resolution images contact the author
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