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Supernovae are known to be the dominant energy source for driving turbulence in the interstellar medium. Yet, their effect on magnetic field amplification in spiral galaxies is still poorly understood. Analytical models based on the uncorrelated-ensemble approach predicted that any created field will be expelled from the disk before a significant amplification can occur. By means of direct simulations of supernova-driven turbulence, we demonstrate that this is not the case. Accounting for vertical stratification and galactic differential rotation, we find an exponential amplification of the mean field on timescales of 100Myr. The self-consistent numerical verification of such a “fast dynamo” is highly beneficial in explaining the observed strong magnetic fields in young galaxies. We, furthermore, highlight the importance of rotation in the generation of helicity by showing that a similar mechanism based on Cartesian shear does not lead to a sustained amplification of the mean magnetic field. This finding impressively confirms the classical picture of a dynamo based on cyclonic turbulence.
Theoretical studies have revealed that dust grains are usually moving fast through the turbulent interstellar gas, which could have significant effects upon interstellar chemistry by modifying grain accretion. This effect is investigated in this work on the basis of numerical gas-grain chemical modelling. Major features of the grain motion effect in the typical environment of dark clouds (DC) can be summarized as follows: (1) decrease of gas-phase (both neutral and ionic) abundances and increase of surface abundances by up to 2-3 orders of magnitude; (2) shifts of the existing chemical jumps to earlier evolution ages for gas-phase species and to later ages for surface species by factors of about 10; (3) a few exceptional cases in which some species turn out to be insensitive to this effect and some other species can show opposite behaviours too. These effects usually begin to emerge from a typical DC model age of about 10(5) yr. The grain motion in a typical cold neutral medium (CNM) can help overcome the Coulomb repulsive barrier to enable effective accretion of cations on to positively charged grains. As a result, the grain motion greatly enhances the abundances of some gas-phase and surface species by factors up to 2-6 or more orders of magnitude in the CNM model. The grain motion effect in a typical molecular cloud (MC) is intermediate between that of the DC and CNM models, but with weaker strength. The grain motion is found to be important to consider in chemical simulations of typical interstellar medium.
Origin and symmetry of the observed global magnetic fields in galaxies are not fully understood. We intend to clarify the question of the magnetic field origin and investigate the global action of the magneto-rotational instability (MRI) in galactic disks with the help of 3D global magneto-hydrodynamical (MHD) simulations. The calculations were done with the time-stepping ZEUS 3D code using massive parallelization. The alpha-Omega dynamo is known to be one of the most efficient mechanisms to reproduce the observed global galactic fields. The presence of strong turbulence is a pre-requisite for the alpha-Omega dynamo generation of the regular magnetic fields. The observed magnitude and spatial distribution of turbulence in galaxies present unsolved problems to theoreticians. The MRI is known to be a fast and powerful mechanism to generate MHD turbulence and to amplify magnetic fields. We find that the critical wavelength increases with the increasing of magnetic fields during the simulation, transporting the energy from critical to larger scales. The final structure, if not disrupted by supernovae explosions, is the structure of `thin layers' of thickness of about 100 pcs. An important outcome of all simulations is the magnitude of the horizontal components of the Reynolds and Maxwell stresses. The result is that the MRI-driven turbulence is magnetic-dominated: its magnetic energy exceeds the kinetic energy by a factor of 4. The Reynolds stress is small and less than 1% of the Maxwell stress. The angular momentum transport is thus completely dominated by the magnetic field fluctuations. The volume-averaged pitch angle is always negative with a magnitude of about -30. The non-saturated MRI regime is lasting sufficiently long to fill the time between the galactic encounters, independently of strength and geometry of the initial field. Therefore, we may claim the observed pitch angles can be due to MRI action in the gaseous galactic disks. The MRI is also shown to be a very fast instability with e-folding time proportional to the time of one rotation. Steep rotation curves imply a stronger growth for the magnetic energy due to MRI. The global e-folding time is from 44 Myr to 100 Myr depending on the rotation profile. Therefore, MRI can explain the existence of rather large magnetic field in very young galaxies. We also have reproduced the observed rms values of velocities in the interstellar turbulence as it was observed in NGC 1058. We have shown with the simulations that the averaged velocity dispersion of about 5 km/s is a typical number for the MRI-driven turbulence in galaxies, which agrees with observations. The dispersion increases outside of the disk plane, whereas supernovae-driven turbulence is found to be concentrated within the disk. In our simulations the velocity dispersion increases a few times with the heights. An additional support to the dynamo alpha-effect in the galaxies is the ability of the MRI to produce a mix of quadrupole and dipole symmetries from the purely vertical seed fields, so it also solves the seed-fields problem of the galactic dynamo theory. The interaction of magneto-rotational instability and random supernovae explosions remains an open question. It would be desirable to run the simulation with the supernovae explosions included. They would disrupt the calm ring structure produced by global MRI, may be even to the level when we can no longer blame MRI to be responsible for the turbulence.
Context. Supernova remnants are known as sources of Galactic cosmic rays for their nonthermal emission of radio waves, X-rays, and gamma rays. However, the observed soft broken power-law spectra are hard to reproduce within standard acceleration theory based on the assumption of Bohm diffusion and steady-state calculations. Aims. We point out that a time-dependent treatment of the acceleration process together with a self-consistent treatment of the scattering turbulence amplification is necessary. Methods. We numerically solve the coupled system of transport equations for cosmic rays and isotropic Alfvenic turbulence. The equations are coupled through the growth rate of turbulence determined by the cosmic-ray gradient and the spatial diffusion coefficient of cosmic rays determined by the energy density of the turbulence. The system is solved on a comoving expanding grid extending upstream for dozens of shock radii, allowing for the self-consistent study of cosmic-ray diffusion in the vicinity of their acceleration site. The transport equation for cosmic rays is solved in a test-particle approach. Results. We demonstrate that the system is typically not in a steady state. In fact, even after several thousand years of evolution, no equilibrium situation is reached. The resulting time-dependent particle spectra strongly differ from those derived assuming a steady state and Bohm diffusion. Our results indicate that proper accounting for the evolution of the scattering turbulence and hence the particle diffusion coefficient is crucial for the formation of the observed soft spectra. In any case, the need to continuously develop magnetic turbulence upstream of the shock introduces nonlinearity in addition to that imposed by cosmic-ray feedback.
Nonthermal emission from the reverse shock of the youngest galactic supernova remnant G1.9+0.3
(2019)
Context. The youngest Galactic supernova remnant G1.9+0.3 is an interesting target for next-generation gamma-ray observatories. So far, the remnant is only detected in the radio and the X-ray bands, but its young age of approximate to 100 yr and inferred shock speed of approximate to 14 000 km s(-1) could make it an efficient particle accelerator. Aims. We aim to model the observed radio and X-ray spectra together with the morphology of the remnant. At the same time, we aim to estimate the gamma-ray flux from the source and evaluate the prospects of its detection with future gamma-ray experiments. Methods. We performed spherical symmetric 1D simulations with the RATPaC code, in which we simultaneously solved the transport equation for cosmic rays, the transport equation for magnetic turbulence, and the hydro-dynamical equations for the gas flow. Separately computed distributions of the particles accelerated at the forward and the reverse shock were then used to calculate the spectra of synchrotron, inverse Compton, and pion-decay radiation from the source. Results. The emission from G1.9+0.3 can be self-consistently explained within the test-particle limit. We find that the X-ray flux is dominated by emission from the forward shock while most of the radio emission originates near the reverse shock, which makes G1.9+0.3 the first remnant with nonthermal radiation detected from the reverse shock. The flux of very-high-energy gamma-ray emission from G1.9+0.3 is expected to be close to the sensitivity threshold of the Cherenkov Telescope Array. The limited time available to grow large-scale turbulence limits the maximum energy of particles to values below 100 TeV, hence G1.9+0.3 is not a PeVatron.
We report spatiotemporal chaos in the Oregonator model of the Belousov-Zhabotinsky reaction. Spatiotemporal chaos spontaneously develops in a regime, where the underlying local dynamics show stable limit cycle oscillations (diffusion-induced turbulence). We show that spatiotemporal chaos can be suppressed by a unidirectional flow in the system. With increasing flow velocity, we observe a transition scenario from spatiotemporal chaos via a regime of travelling waves to a stationary steady state. At large flow velocities, we recover the known regime of flow distributed oscillations.