506 research outputs found

    From Large to Small Scales: Global Models of the ISM

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    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

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    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

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    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

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    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

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    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 n‚ąín-distribution of electrons - Radiative cooling and thermally averaged and total Gaunt factors

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    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"

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    We have carried out 1.25 pc resolution MHD simulations of the ISM, on a Cartesian grid of 0‚ȧ(x,y)‚ȧ10 \leq (x,y) \leq 1 kpc size in the galactic plane and ‚ąí10‚ȧz‚ȧ10-10 \leq z \leq 10 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‚ąí3^{-3} 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 102<10^{2}<T‚ȧ103.9\leq 10^{3.9} K, while for T‚ȧ100\leq 100 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 102<10^{2}<T‚ȧ103.9\leq 10^{3.9} K with ‚ąľ30\sim30% of the disk mass enclosed in the T‚ȧ103\leq 10^{3} 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 őļ\kappa and Maxwellian distributions of electrons

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    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 őļ\kappa-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 10‚ąí1010^{-10} in units of z2Ryz^2Ry. 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 őļ\kappa parameter, which shows increasing deviations (with respect to the results obtained with the use of the Maxwellian distribution) with decreasing őļ\kappa. 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

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    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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