Universal Properties of Dense Clumps in Magnetized Molecular Clouds Formed through Shock Compression of Two-phase Atomic Gases

Abstract

We investigate the formation of molecular clouds from atomic gas by using three-dimensional magnetohydrodynamical simulations, including nonequilibrium chemical reactions, heating/cooling processes, and self-gravity by changing the collision speed V 0 and the angle θ between the magnetic field and colliding flow. We found that the efficiency of the dense-gas formation depends on θ. For small θ, anisotropic super-Alfvénic turbulence delays the formation of gravitationally unstable clumps. An increase in θ develops shock-amplified magnetic fields along which the gas accumulates, creating prominent filamentary structures. We further investigate the statistical properties of dense clumps identified with different density thresholds. The statistical properties of the dense clumps with lower densities depend on V 0 and θ because their properties are inherited from the global turbulence structure of molecular clouds. By contrast, denser clumps appear to have asymptotic universal statistical properties, which do not significantly depend on the properties of the colliding flow. The internal velocity dispersions approach subsonic and plasma β becomes order of unity. We develop an analytic formula of the virial parameter that reproduces the simulation results reasonably well. This property may be one of the reasons for the universality of the initial mass function of stars.

CO enhancement by magnetohydrodynamic waves

“…When the magnetic field is perpendicular to the filament axis, the carbon-based species abundances have shallower gradients toward the center of the filament. Furthermore, amplified magnetic fields from colliding flows form a critical angle with the upstream velocity, which determines whether molecule formation is suppressed or enhanced (Iwasaki et al 2019;Iwasaki & Tomida 2022). Overall, all the aforementioned numerical studies suggest that the ISM magnetic field affects the formation of molecular gas mainly by setting a preferred orientation along which gas accumulation, and hence molecule formation, can take place.…”

Simulation of Head-on Collisions between Filamentary Molecular Clouds Threaded by a Lateral Magnetic Field and Subsequent Evolution

“…Magnetic fields are also essential for understanding the early stages of star formation in various aspects, such as changing the character of interstellar turbulence (e.g., Cho et al 2002;Federrath & Klessen 2012), modifying the gas compression by shocks (e.g., Inoue & Fukui 2013;Iwasaki & Tomida 2022), achieving stability against gravitational instability (e.g., Nakano & Nakamura 1978), and transferring the angular momentum from a star-forming gas (e.g., Mouschovias & Paleologou 1980;Tomisaka 2000). Orientation of the magnetic field is observed using near-infrared, far-infrared, and millimeter-wave polarizations.…”

Formation Mechanism of Odd- and Even-Numbered Hydrogen Cluster Cations Using the Direct Ab Initio Molecular Dynamics Approach

“…Space chemical reactions can occur in interstellar molecular clouds (IMCs), dust surfaces, and planetary atmospheres. − IMCs are composed of molecular hydrogen (H 2 ); therefore, the molecules in IMCs are surrounded by H 2 . When H 2 molecules are exposed to cosmic rays, they are ionized and radical cations of hydrogen molecule (H 2 + ) are formed.…”