NEI Modelling of the ISM - Turbulent Dissipation & Hausdorff Dimension

High-resolution non-ideal magnetohydrodynamical simulations of the turbulent magnetized ISM, powered by supernovae types Ia and II at Galactic rate, including self-gravity and non-equilibriuim ionization (NEI), taking into account the time evolution of the ionization structure of H, He, C, N, O, Ne, Mg, Si, S and Fe, were carried out. These runs cover a wide range (from kpc to sub-parsec) of scales, providing resolution independent information on the injection scale, extended self-similarity and the fractal dmension of the most dissipative structures.Comment: 2 pages, 1 figure. Invited talk to the Joint Discussion 15 "Magnetic Fields in Diffuse Media". To appear in Highlights of Astronomy, Vol. 15, Proc. of the XXVIIth IAU General Assembly, Rio de Janeiro, Brazil, August 2009, eds. E. G. Dal Pino & A. Lazarian, Cambridge University Pres

Superbubble evolution in disk galaxies, I. Study of blow-out by analytical models

Galactic winds are a common phenomenon in starburst galaxies in the local universe as well as at higher redshifts. Their sources are superbubbles driven by sequential supernova explosions in star forming regions, which carve out large holes in the interstellar medium and eject hot, metal enriched gas into the halo and to the galactic neighborhood. We investigate the evolution of superbubbles in exponentially stratified disks. We present advanced analytical models for the expansion of such bubbles and calculate their evolution in space and time. With these models one can derive the energy input that is needed for blow-out of superbubbles into the halo and derive the break-up of the shell, since Rayleigh-Taylor instabilities develop soon after a bubble starts to accelerate into the halo. The approximation of Kompaneets is modified in order to calculate velocity and acceleration of a bubble analytically. Our new model differs from earlier ones, because it presents for the first time an analytical calculation for the expansion of superbubbles in an exponential density distribution driven by a time-dependent energy input rate. The time-sequence of supernova explosions of OB-stars is modeled using their main sequence lifetime and an initial mass function. We calculate the morphology and kinematics of superbubbles powered by three different kinds of energy input and we derive the energy input required for blow-out as a function of the density and the scale height of the ambient interstellar medium. The Rayleigh-Taylor instability timescale in the shell is calculated in order to estimate when the shell starts to fragment and finally breaks up. Analytical models are a very efficient tool for comparison to observations, like e.g. the Local Bubble and the W4 bubble discussed in this paper, and also give insight into the dynamics of superbubble evolution.Comment: 18 pages, 11 figure

3D Hydrodynamic Simulations of the Galactic Supernova Remnant CTB 109

Using detailed 3D hydrodynamic simulations we study the nature of the Galactic supernova remnant (SNR) CTB 109 (G109.1-1.0), which is well-known for its semicircular shape and a bright diffuse X-ray emission feature inside the SNR. Our model has been designed to explain the observed morphology, with a special emphasis on the bright emission feature inside the SNR. Moreover, we determine the age of the remnant and compare our findings with X-ray observations. With CTB 109 we test a new method of detailed numerical simulations of diffuse young objects, using realistic initial conditions derived directly from observations. We performed numerical 3D simulations with the RAMSES code. The initial density structure has been directly taken from $^{12}$CO emission data, adding an additional dense cloud, which, when it is shocked, causes the bright emission feature. From parameter studies we obtained the position $(\ell , b)=(109.1545^\circ , -1.0078^\circ)$ for an elliptical cloud with $n_\text{cloud}=25~\text{cm}^{-3}$ based on the preshock density from Chandra data and a maximum diameter of 4.54 pc, whose encounter with the supernova (SN) shock wave generates the bright X-ray emission inside the SNR. The calculated age of the remnant is about 11,000 yr according to our simulations. In addition, we can also determine the most probable site of the SN explosion. Hydrodynamic simulations can reproduce the morphology and the observed size of the SNR CTB 109 remarkably well. Moreover, the simulations show that it is very plausible that the bright X-ray emission inside the SNR is the result of an elliptical dense cloud shocked by the SN explosion wave. We show that numerical simulations using observational data for an initial model can produce meaningful results.Comment: 9 pages, 6 figures, accepted for publication in A&

Non-relativistic free-free emission due to n−n-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 $0 \leq (x,y) \leq 1$ kpc size in the galactic plane and $-10 \leq z \leq 10$ 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$^{-3}$ 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 $10^{2}<$T$\leq 10^{3.9}$ K, while for T$\leq 100$ 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 $10^{2}<$T$\leq 10^{3.9}$ K with $\sim30$ of the disk mass enclosed in the T$\leq 10^{3}$ 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

Testing Global ISM Models: A Detailed Comparison of Ovi Column Densities with FUSE and Copernicus Data

We study the ovi distribution in space and time in a representative section of the Galactic disk by 3D adaptive mesh refinement HD and MHD simulations of the ISM, including the disk-halo-disk circulation. The simulations describe a supernova driven ISM on large (10 kpc) and small (1.25 pc) scales over a sufficiently large timescale (400 Myrs) in order to establish a global dynamical equilibrium. The Ovi column density, N(Ovi), is monitored through lines of sight measurements at different locations in the simulated disk. One has been deliberately chosen to be inside of a hot bubble, like our own Local Bubble, while the other locations are random. We obtain a correlation between N(Ovi) and distance, which is independent of the observer's vantage point in the disk. In particular, the location of the observer inside a hot bubble does not have any influence on the correlation, because the contribution of an individual bubble (with a typical extension of 100 pc) is negligibly small. We find a remarkable agreement between the Ovi column densities (as a function of distance) and the averaged Ovi density (~1.8x10^{-8}$cm$^{-3}$) in the disk from our simulations and the values observed with Copernicus, and FUSE. Our results strongly support the important r\^ole of turbulent mixing in the distribution of Ovi clumps in the ISM. Supernova induced turbulence is quite strong and unavoidable due to shearing motions in the ISM and operates on a large range of scales.Comment: 4 pages using emulateapj5 style, 5 figures including a simulation image. Accepted for publication in ApJ Letter