Using three-dimensional magnetohydrodynamics simulations, we investigate general properties of a blast wave shock interacting with interstellar clouds. The pre-shock cloudy medium is generated as a natural consequence of the thermal instability that simulates realistic clumpy interstellar clouds and their diffuse surrounding. The shock wave that sweeps the cloudy medium generates a turbulent shell through the vorticity generations that are induced by shock-cloud interactions. In the turbulent shell, the magnetic field is amplified as a result of turbulent dynamo action. The energy density of the amplified magnetic field can locally grow comparable to the thermal energy density, particularly at the transition layers between clouds and the diffuse surrounding. In the case of a young supernova remnant (SNR) with a shock velocity 10 3 km s −1 , the corresponding strength of the magnetic field is approximately 1 mG. The propagation speed of the shock wave is significantly stalled in the clouds because of the high density, while the shock maintains a high velocity in the diffuse surrounding. In addition, when the shock wave hits the clouds, reflection shock waves are generated that propagate back into the shocked shell. From these simulation results, many observational characteristics of a young SNR RX J1713.7−3946 that is suggested to be interacting with molecular clouds, can be explained as follows: The reflection shocks can accelerate particles in the turbulent downstream region where the magnetic field strength reaches 1mG, which causes short-time variability of synchrotron X-rays. Since the shock velocity is stalled locally in the clouds, the temperature in the shocked cloud is suppressed far below 1 keV. Thus, thermal Xray line emission would be faint even if the SNR is interacting with molecular clouds. We also find that the photon index of the π 0 -decay gamma rays generated by cosmic-ray protons can be 1.5 (corresponding energy flux is νF ν ∝ ν 0.5 ), because the penetration depth of high-energy particles into the clumpy clouds depends on their energy. This suggests that, if we rely only on the spectral study, the hadronic gamma-ray emission is indistinguishable from the leptonic inverse Compton emission. We propose that the spatial correlation of the gamma-ray, X-ray, and CO line emission regions can be conclusively used to understand the origin of gamma rays from RX J1713.7−3946.

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.

We describe an overall picture of galactic-scale star formation. Recent high-resolution magneto-hydrodynamical simulations of twofluid dynamics with cooling, heating, and thermal conduction have shown that the formation of molecular clouds requires multiple episodes of supersonic compression. This finding enables us to create a scenario in which molecular clouds form in interacting shells or bubbles on a galactic scale. First, we estimated the ensemble-averaged growth rate of molecular clouds on a timescale longer than a million years. Next, we performed radiation hydrodynamics simulations to evaluate the destruction rate of magnetized molecular clouds by the stellar far-ultraviolet radiation. We also investigated the resulting star formation efficiency within a cloud, which amounts to a low value (a few percent) if we adopt the power-law exponent ∼− 2.5 for the mass distribution of stars in the cloud. We finally describe the time evolution of the mass function of molecular clouds on a long timescale (>1 Myr) and discuss the steady state exponent of the power-law slope in various environments.

RX J1713.7−3946 is the most remarkable TeV γ-ray SNR which emits γ-rays in the highest energy range. We made a new combined analysis of CO and H I in the SNR and derived the total protons in the interstellar medium (ISM). We have found that the inclusion of the H I gas provides a significantly better spatial match between the TeV γ-rays and ISM protons than the H 2 gas alone. In particular, the southeastern rim of the γ-ray shell has a counterpart only in the H I. The finding shows that the ISM proton distribution is consistent with the hadronic scenario that comic ray (CR) protons react with ISM protons to produce the γ-rays. This provides another step forward for the hadronic origin of the γ-rays by offering one of the necessary conditions missing in the previous hadronic interpretations. We argue that the highly inhomogeneous distribution of the ISM protons is crucial in the origin of the γ-rays. Most of the neutral gas was likely swept up by the stellar wind of an OB star prior to the SNe to form a low-density cavity and a swept-up dense wall. The cavity explains the low-density site where the diffusive shock acceleration of charged particles takes place with suppressed thermal X-rays, whereas the CR protons can reach the target protons in the wall to produce the γ-rays. The present finding allows us to estimate the total CR proton energy to be ∼10 48 ergs, 0.1 % of the total energy of a SNe.

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We examine MHD simulations of the propagation of a strong shock wave through the interstellar two-phase medium composed of small-scale cloudlets and diffuse warm neutral medium in two-dimensional geometry. The pre-shock two-phase medium is provided as a natural consequence of the thermal instability that is expected to be ubiquitous in the interstellar medium. We show that the shock-compressed shell becomes turbulent owing to the preshock density inhomogeneity and magnetic field amplification takes place in the shell. The maximum field strength is determined by the condition that plasma β ∼ 1, which gives the field strength on the order of 1 mG in the case of shock velocity ∼ 10 3 km s −1 . The strongly magnetized region shows filamentary and knot-like structures in two-dimensional simulations. The spatial scale of the regions with magnetic field of ∼1 mG in our simulation is roughly 0.05 pc which is comparable to the spatial scale of the X-ray hot spots recently discovered in supernova remnants where the magnetic field strength is indicated to be amplified up to the order of 1 mG. This result may also suggest that the turbulent region with locally strong magnetic field is expected to be spread out in the region with frequent supernova explosions, such as in the Galactic center and starburst galaxies.

Recent observations suggest that intensive molecular cloud collision can trigger massive star/cluster formation. The most important physical process caused by the collision is a shock compression. In this paper, the influence of a shock wave on the evolution of a molecular cloud is studied numerically by using isothermal magnetohydrodynamics (MHD) simulations with the effect of self-gravity. Adaptive-mesh-refinement and sink particle techniques are used to follow long-time evolution of the shocked cloud. We find that the shock compression of turbulent inhomogeneous molecular cloud creates massive filaments, which lie perpendicularly to the background magnetic field as we have pointed out in a previous paper. The massive filament shows global collapse along the filament, which feeds a sink particle located at the collapse center. We observe high accretion rateṀ acc > 10 −4 M sun yr −1 that is high enough to allow the formation of even O-type stars. The most massive sink particle achieves M > 50 M sun in a few times 10 5 yr after the onset of the filament collapse.

We develop an unconditionally stable numerical method for solving the coupling between two fluids (frictional forces/heatings, ionization, and recombination), which can adequately solve the evolution of a partially ionized medium from weak-coupling to strong-coupling regimes. By using two-dimensional two-fluid magnetohydrodynamical simulations based on this method, we investigate the dynamical condensation process of thermally unstable gas that is provided by the shock waves in a weakly ionized and magnetized interstellar medium. If we neglect the effect of magnetic field, it is known that condensation driven by thermal instability can generate high-density clouds whose physical condition corresponds to molecular clouds (precursor of molecular clouds). In this paper we study the effect of magnetic field on the evolution of supersonic converging H i flows and focus on the case in which the orientation of magnetic field to converging flows is orthogonal. We show that the magnetic pressure gradient parallel to the flows prevents the formation of high-density and highYcolumn density clouds, but instead generates fragmented, filamentary H i clouds. With this restricted geometry, magnetic field drastically diminishes the opportunity of fast molecular cloud formation directly from the warm neutral medium, in contrast to the case without magnetic field.