Abstract:
Linear stability of a current sheet that is subject to an impulsive acceleration due to a shock passage is studied with the effect of guide magnetic field. We find that the current sheet embedded in relativistically magnetized plasma always shows a Richtmyer-Meshkov type instability, while it depends on the density structure in the Newtonian limit. The growth of the instability is expected to generate turbulence around the current sheet that can induce so-called turbulent reconnection whose rate is essentially free from plasma resistivity. Thus, the instability can be applied as a triggering mechanism of rapid magnetic energy release in variety of high-energy astrophysical phenomena such as pulsar wind nebulae, gamma-ray bursts, and active galactic nuclei, where the shock wave is supposed to play a crucial role.

Abstract:
Recent observations of molecular clouds around rich massive star clusters including NGC3603, Westerlund 2, and M20 revealed that the formation of massive stars could be triggered by a cloud-cloud collision. By using three-dimensional, isothermal, magnetohydrodynamics simulations with the effect of self-gravity, we demonstrate that massive, gravitationally unstable, molecular cloud cores are formed behind the strong shock waves induced by the cloud-cloud collision. We find that the massive molecular cloud cores have large effective Jeans mass owing to the enhancement of the magnetic field strength by shock compression and turbulence in the compressed layer. Our results predict that massive molecular cloud cores formed by the cloud-cloud collision are filamentary and threaded by magnetic fields perpendicular to the filament.

Abstract:
We examine the linear stability and nonlinear growth of the thermal instability in isobarically contracting gas with various metallicities and FUV field strengths. When the H2 cooling is suppressed by FUV fields (G_0>10^-3) or the metallicity is high enough (Z/Zs>10^-3), the interstellar medium is thermally unstable in the temperature range 100-7000 K owing to the cooling by CII and OI fine-structure lines. In this case, a bi-phasic medium with a bimodal density probability distribution is formed as a consequence of the thermal instability. The characteristic scales of the thermal instability become smaller with increasing metallicity. Comparisons of the nonlinear simulations with different resolution indicates that the maximum scale of the thermal instability should be resolved with more than 60 cells to follow runaway cooling driven by the thermal instability. Under sufficiently weak FUV fields and with low metallcity, the density range of the thermal instability shrinks owing to dominance of H2 cooling. As the FUV intensity is reduced, bi-phasic structure becomes less remarkable and disappears eventually. Our basic results suggest that, in early galaxies, i) the thermal instability has little effect for the medium with Z/Zs<10^-4, ii) fragmentation by the thermal instability could determine mass spectrum of star clusters for 10^-40.04, although the threshold metallicity depends on the conditions such as thermal pressure, FUV strength and redshift.

Abstract:
We present a new numerical method of special relativistic resistive magnetohydrodynamics with scalar resistivity that can treat a range of phenomena, from nonrelativistic to relativistic (shock, contact discontinuity, and Alfv\'en wave). The present scheme calculates the numerical flux of fluid by using an approximate Riemann solver, and electromagnetic field by using the method of characteristics. Since this scheme uses appropriate characteristic velocities, it is capable of accurately solving problems that cannot be approximated as ideal magnetohydrodynamics and whose characteristic velocity is much lower than light velocity. The numerical results show that our scheme can solve the above problems as well as nearly ideal MHD problems. Our new scheme is particularly well suited to systems with initially weak magnetic field, and mixed phenomena of relativistic and non-relativistic velocity; for example, MRI in accretion disk, and super Alfv\'enic turbulence.

Abstract:
Formation of interstellar clouds as a consequence of thermal instability is studied using two-dimensional two-fluid magnetohydrodynamic simulations. We consider the situation of converging, supersonic flows of warm neutral medium in the interstellar medium that generate a shocked slab of thermally unstable gas in which clouds form. We found, as speculated in paper I, that in the shocked slab magnetic pressure dominates thermal pressure and the thermal instability grows in the isochorically cooling, thermally unstable slab that leads formation of HI clouds whose number density is typically n < 100 cm^-3, even if the angle between magnetic field and converging flows is small. We also found that even if there is a large dispersion of magnetic field, evolution of the shocked slab is essentially determined by the angle between the mean magnetic field and converging flows. Thus, the direct formation of molecular clouds by piling up warm neutral medium does not seem a typical molecular cloud formation process, unless the direction of supersonic converging flows is biased to the orientation of mean magnetic field by some mechanism. However, when the angle is small, the HI shell generated as a result of converging flows is massive and possibly evolves into molecular clouds, provided gas in the massive HI shell is piled up again along the magnetic field line. We expect that another subsequent shock wave can pile up again the gas of the massive shell and produce a larger cloud. We thus emphasize the importance of multiple episodes of converging flows, as a typical formation process of molecular clouds.

Abstract:
Using 3D MHD simulation with the effects of radiative cooling/heating, chemical reactions, and thermal conduction, we investigate the formation of molecular cloud in the ISM. We consider the formation of molecular cloud by accretion of the HI clouds as suggested in recent observations. The simulation shows that the initial HI medium is compressed and piled up behind the shock waves induced by the accretion flows. Since the initial medium is highly inhomogeneous as a consequence of the thermal instability, the formed molecular cloud becomes very turbulent owing to the development of the Richtmyer-Meshkov instability. The structure of the post shock region is composed of dense cold gas (T<100 K) and diffuse warm gas (T>1,000 K), which are spatially well mixed owing to the turbulence. Because the energy source of the turbulence is the accretion flows, the turbulence is highly anisotropic biased toward the direction of accretion flows. The kinetic energy of the turbulence dominates the thermal, magnetic, and gravitational energies in the total 10 Myr evolution. However, the kinetic energy measured by using the CO-fraction-weighted density is comparable to the other energies. This suggests that the true kinetic energy of turbulence in molecular cloud as a hole can be much larger than the kinetic energy of turbulence estimated by line-width of molecular emissions. The clumps in the molecular cloud show statistically homogeneous evolution as follows: The typical plasma beta of the clumps is roughly constant ~ 0.4. The size-velocity dispersion relation show dv ~ 1.5 km s^{-1} (l/1 pc)^{0.5}, irrespective of the density. The clumps evolve toward magnetically supercritical, gravitationally unstable cores. The clumps seem to evolve into cores that satisfy the condition for fragmentation into binary. These statistical properties may provide the initial condition of star formation.

Abstract:
The critical strength of a magnetic field required for the suppression of the Richtmyer-Meshkov instability (RMI) is investigated numerically by using a two-dimensional single-mode analysis. For the cases of MHD parallel shocks, the RMI can be stabilized as a result of the extraction of vorticity from the interface. A useful formula describing a critical condition for MHD RMI has been introduced, and which is successfully confirmed by the direct numerical simulations. The critical field strength is found to be largely depending on the Mach number of the incident shock. If the shock is strong enough, even low-$\beta$ plasmas can be subject to the growth of the RMI.

Abstract:
Recent ALMA (Atacama Large Millimeter/submillimeter Array) observations of young protostellar objects detected warm SO emission, which could be associated with a forming protostellar disk. In order to investigate if such warm gas can be produced by accretion shock onto the forming disk, we calculate the sputtering and thermal desorption of various grain surface species in one dimensional shock waves. We find that thermal desorption is much more efficient than the sputtering in the post-shock region. While H$_{2}$O can be thermally desorbed, if the accretion velocity is larger than 8 km s$^{-1}$ with the pre-shock gas number density of 10$^{9}$ cm$^{-3}$, SO is desorbed, if the accretion velocity $\gtrsim$ 2 km s$^{-1}$ and $\gtrsim$ 4km s$^{-1}$, with the pre-shock density of 10$^{9}$ cm$^{-3}$ and 10$^{8}$ cm$^{-3}$, respectively. We also find that the column density of hydrogen nuclei in warm post-shock gas is $N_{{\rm warm}} \sim 10^{21}$ cm$^{-2}$.

Abstract:
Relativistic astrophysical phenomena such as gamma-ray bursts (GRBs) and active galactic nuclei often require long-lived strong magnetic field. Here, we report on three-dimensional special-relativistic magnetohydrodynamic (MHD) simulations to explore the amplification and decay of macroscopic turbulence dynamo excited by the so-called Richtmyer-Meshkov instability (RMI; a Rayleigh-Taylor type instability). This instability is an inevitable outcome of interactions between shock and ambient density fluctuations. We find that the magnetic energy grows exponentially in a few eddy turnover times, and then, following the decay of kinetic turbulence, decays with a temporal power-law exponent of -0.7. The magnetic-energy fraction can reach $epsilon_B \sim$ 0.1 but depends on the initial magnetic field strength. We find that the magnetic energy grows by at least two orders of magnitude compared to the magnetic energy immediately behind the shock. This minimum degree of the amplification does not depend on the amplitude of the initial density fluctuations, while the growth timescale and the maximum magnetic energy depend on the degree of inhomogeneity in the density. The transition from Kolmogorov cascade to MHD critical balance cascade occurs at $\sim$ 1/10th the initial inhomogeneity scale, which limits the maximum synchrotron polarization to less than 2%. New results include the avoidance of electron cooling with RMI turbulence, the turbulent photosphere model via RMI, and the shallow decay of the early afterglow from RMI. We also performed a simulation of freely decaying turbulence with relativistic velocity dispersion. We find that relativistic turbulence begins to decay much faster than one eddy-turnover time because of fast shock dissipation, which does not support the relativistic turbulence model by Narayan & Kumar.

Abstract:
We report turbulence effects on magnetic reconnection in relativistic plasmas using 3-dimensional relativistic resistive magnetohydrodynamics simulations. We found reconnection rate became independent of the plasma resistivity due to turbulence effects similarly to non-relativistic cases. We also found compressible turbulence effects modified the turbulent reconnection rate predicted in non-relativistic incompressible plasmas; The reconnection rate saturates and even decays as the injected velocity approaches to the Alfv\'en velocity. Our results indicate the compressibility cannot be neglected when compressible component becomes about half of incompressible mode occurring when the Alfv\'en Mach number reaches about $0.3$. The obtained maximum reconnection rate is around $0.05$ to $0.1$, which will be able to reach around $0.1$ to $0.2$ if injection scales are comparable to the sheet length.