506 research outputs found
From Large to Small Scales: Global Models of the ISM
We review large scale modelling of the ISM with emphasis on the importance to
include the disk-halo-disk duty cycle and to use a dynamical refinement of the
grid (in regions where steep variations of density and pressure occur) for a
realistic modelling of the ISM. We also discuss the necessity of convergence of
the simulation results by comparing 0.625, 1.25 and 2.5 pc resolution
simulations and show that a minimum grid resolution of 1.25 pc is required for
quantitatively reliable results, as there is a rapid convergence for Delta x
\leq 1.1 pc.Comment: 10 pages, 8 figures that include 2 simulation images (in jpeg format)
and 6 plots (4 in ps and 2 in jpeg formats), to appear in the proceedings of
``From Observations to Self-Consistent Modelling of the ISM in Galaxies'',
Kluwe
The History and Future of the Local and Loop I Bubbles
The Local and Loop I superbubbles are the closest and best investigated
supernova (SN) generated bubbles and serve as test laboratories for
observations and theories of the interstellar medium. Since the morphology and
dynamical evolution of bubbles depend on the ambient density and pressure
distributions, a realistic modelling of the galactic environment is crucial for
a detailed comparison with observations. We have performed 3D high resolution
(down to 1.25 pc on a kpc-scale grid) hydrodynamic simulations of the Local
Bubble (LB) and the neighbouring Loop I (L1) superbubble in a realistically
evolving inhomogeneous background ISM, disturbed already by SN explosions at
the Galactic rate for 200 Myr before the LB and L1 are generated. The LB is the
result of 19 SNe occurring in a moving group, which passed through the present
day local HI cavity. We can reproduce (i) the OVI column density in absorption
within the LB in agreement with COPERNICUS and recent FUSE observations, giving
N(OVI) <2 10^{13} cm^-2 and N(OVI)<7 10^{12} cm^-2, respectively, (ii) the
observed sizes of the Local and Loop I superbubbles, (iii) the interaction
shell between LB and L1, discovered with ROSAT, (iv) constrain the age of the
LB to be 14.5+0.7/-0.4 Myr, (v) predict the merging of the two bubbles in about
3 Myr, when the interaction shell starts to fragment, (vi) the generation of
blobs like the Local Cloud as a consequence of a dynamical instability. We find
that evolving superbubbles strongly deviate from idealised self-similar
solutions due to ambient pressure and density gradients, as well as due to
turbulent mixing and mass loading. Hence, at later times the hot interior can
break through the surrounding shell, which may also help to explain the
puzzling energy "deficit" observed in LMC bubbles.Comment: Accepted for publication in Astronomy and Astrophysics Letters. The
paper contains 5 pages and 11 figures. Fig. 1a replaced by correct figur
Disk-Halo Interaction - I. Three-Dimensional Evolution of the Galactic Disk
The results of a three-dimensional model for disk-halo interaction are
presented here. The model considers explicitly the input of energy and mass by
isolated a nd correlated supernovae in the disk. Once disrupted by the
explosions, the disk never returns to its initial state. Instead it approaches
a state where a thin HI disk is formed in the Galactic plane overlayed by thick
HI and HII gas disk w ith scale heights of 500 pc and of 1 to 1.5 kpc,
respectively. The upper parts o f the thick HII disk (the diffuse ionized
medium) act as a disk-halo interface a nd its formation and stability are
directly correlated to the supernova rate per unit area in the simulated disk.Comment: 19 pages, 18 figures; to appear in Monthly Notices of Royal
Astronomical Societ
A Half-Century of Productivity Growth and Structural Change in Canadian Agriculture: An Overview
The primary agriculture sector in Canada experienced impressive productivity growth over the 1961-2007 period, outperforming by far productivity growth seen in the Canadian business sector as a whole. In the period in question, the agriculture sector also experienced profound structural changes, from the massive decline in the use of labour input due to mechanization, to the increased use of intermediate inputs (such as fertilizers and pesticides) in the production process. The objective of this article is to highlight some of the most important structural changes observed by Canadian agriculture over the past 50 years, and link them to the robust productivity growth in the sector.
3D HD and MHD Adaptive Mesh Refinement Simulations of the Global and Local ISM
We have performed high resolution 3D simulations with adaptive mesh
refinement, following the ISM evolution in a star forming galaxy both on small
(10 kpc) scales, enabling us to track structures in cooling
shock compressed regions as well as the entire Galactic fountain flow. It is
shown in an MHD run that the latter one is not inhibited by a large scale disk
parallel magnetic field. The fountain plays a vital role in limiting the volume
filling factor of the hot gas. Contrary to classical models most of the gas
between 100K and 8000 K is found to be thermally unstable. On scales of
superbubbles we find that the internal temperature structure is rather
inhomogeneous for an old object like our Local Bubble, leading to low OVI
column densities, consistent with observations.Comment: 8 pages, 4 figures that include 1 simulation image in jpeg format and
3 plots, to be published in How Does the Galaxy Works?, E.J. Alfaro, E.Perez,
J. Franco (eds.), Kluwer, Dordrech
Non-relativistic free-free emission due to distribution of electrons - Radiative cooling and thermally averaged and total Gaunt factors
Tracking the thermal evolution of plasmas, characterized by an
n-distribution, using numerical simulations, requires the determination of the
emission spectra and of the radiative losses due to free-free emission from the
correspond- ing temperature averaged and total Gaunt factors. Detailed
calculations of the latter are presented, associated to n-distributed electrons
with the parameter n ranging from 1 (corresponding to the Maxwell-Boltzmann
distribu- tion) to 100. The temperature averaged and total Gaunt factors, with
decreasing n tend to those obtained with the Maxwell-Boltzmann distribution.
Radiative losses due to free-free emission in a plasma evolving under
collisional ionization equilibrium conditions and composed by H, He, C, N, O,
Ne, Mg, Si, S, and Fe ions, are presented. These losses decrease with the
decrease in the parameter n reaching a minimum when n = 1, and, thus converging
to the losses of a thermal plasma.
Tables of the thermal averaged and total Gaunt factors calculated for n
distributions and a wide range electron and photon energies are presented.Comment: Accepted for publication in ApJS. 70 pages, 7 figures and 11 table
MHD Simulations of the ISM: The Importance of the Galactic Magnetic Field on the ISM "Phases"
We have carried out 1.25 pc resolution MHD simulations of the ISM, on a
Cartesian grid of kpc size in the galactic plane and kpc into the halo, thus being able to fully trace the
time-dependent evolution of the galactic fountain. The simulations show that
large scale gas streams emerge, driven by SN explosions, which are responsible
for the formation and destruction of shocked compressed layers. The shocked gas
can have densities as high as 800 cm and lifetimes up to 15 Myr. The
cold gas is distributed into filaments which tend to show a preferred
orientation due to the anisotropy of the flow induced by the galactic magnetic
field. Ram pressure dominates the flow in the unstable branch T K, while for T K (stable branch) magnetic pressure takes
over. Near supernovae thermal and ram pressures determine the dynamics of the
flow. Up to 80% of the mass in the disk is concentrated in the thermally
unstable regime T K with of the disk mass
enclosed in the T K gas. The hot gas in contrast is controlled by
the thermal pressure, since magnetic field lines are swept towards the dense
compressed walls.Comment: 8 pages, 8 figures (in jpeg format) that include 2 simulations images
and 6 plots. Paper accepted by the referee for publication in the proceedings
of ``Magnetic fields and star formation: theory versus observations'', kluwe
Temperature-averaged and total free-free Gaunt factors for and Maxwellian distributions of electrons
Aims. Optically thin plasmas may deviate from thermal equilibrium and thus,
electrons (and ions) are no longer described by the Maxwellian distribution.
Instead they can be described by -distributions. The free-free spectrum
and radiative losses depend on the temperature-averaged (over the electrons
distribution) and total Gaunt factors, respectively. Thus, there is a need to
calculate and make available these factors to be used by any software that
deals with plasma emission.
Methods. We recalculated the free-free Gaunt factor for a wide range of
energies and frequencies using hypergeometric functions of complex arguments
and the Clenshaw recurrence formula technique combined with approximations
whenever the difference between the initial and final electron energies is
smaller than in units of . We used double and quadruple
precisions. The temperature- averaged and total Gaunt factors calculations make
use of the Gauss-Laguerre integration with 128 nodes.
Results. The temperature-averaged and total Gaunt factors depend on the
parameter, which shows increasing deviations (with respect to the
results obtained with the use of the Maxwellian distribution) with decreasing
. Tables of these Gaunt factors are provided.Comment: Paper accepted for publication in Astronomy & Astrophysics. Contains
5 pages, 4 figures, and 9 table
The Evolution of the Large-scale ISM: Bubbles, Superbubbles and Non-Equilibrium Ionization
The ISM, powered by SNe, is turbulent and permeated by a magnetic field (with
a mean and a turbulent component). It constitutes a frothy medium that is
mostly out of equilibrium and is ram pressure dominated on most of the
temperature ranges, except for T 1E6 K, where magnetic and
thermal pressures dominate, respectively. Such lack of equilibrium is also
imposed by the feedback of the radiative processes into the ISM flow. Many
models of the ISM or isolated phenomena, such as bubbles, superbubbles, clouds
evolution, etc., take for granted that the flow is in the so-called collisional
ionization equilibrium (CIE). However, recombination time scales of most of the
ions below 1E6 K are longer than the cooling time scale. This implies that the
recombination lags behind and the plasma is overionized while it cools. As a
consequence cooling deviates from CIE. This has severe implications on the
evolution of the ISM flow and its ionization structure. Here, besides reviewing
several models of the ISM, including bubbles and superbubbles, the validity of
the CIE approximation is discussed, and a presentation of recent developments
in modeling the ISM by taking into account the time-dependent ionization
structure of the flow in a full-blown numerical 3D high resolution simulation
is presented.Comment: 15 pages, 6 figures with 15 panels. Invited review for "The Dynamic
ISM: A celebration of the Canadian Galactic Plane Survey" conference;
Naramata BC, Canada June 6-10, 2010. To be published in the ASP Conference
Serie
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