The Early Stage of Molecular Cloud Formation by Compression of Two-phase Atomic Gases

Abstract

We investigate the formation of molecular clouds from atomic gas by using three-dimensional magnetohydrodynamic simulations, including non-equilibrium chemical reactions and heating/cooling processes. We consider super-Alfvénic head-on colliding flows of atomic gas possessing the two-phase structure that consists of HI clouds and surrounding warm diffuse gas. We examine how the formation of molecular clouds depends on the angle θ between the upstream flow and the mean magnetic field. We find that there is a critical angle θ cr above which the shock-amplified magnetic field controls the post-shock gas dynamics. If the atomic gas is compressed almost along the mean magnetic field (θ ≪ θ cr ), super-Alfvénic anisotropic turbulence is maintained by the accretion of the highly inhomogeneous upstream atomic gas. As a result, a greatly extended turbulence-dominated post-shock layer is generated. Around θ ∼ θ cr , the shock-amplified magnetic field weakens the post-shock turbulence, leading to a dense post-shock layer. For θ ≫ θ cr , the strong magnetic pressure suppresses the formation of cold dense clouds. Efficient molecular cloud formation is expected if θ is less than a few times θ cr . Developing an analytic model and performing a parameter survey, we obtain an analytic formula for the critical angle as a function of the mean density, collision speed, and field strength of the upstream atomic gas. The critical angle is found to be less than ∼ 15 • as long as the field strength is larger than 1 µG, indicating that the probability of occurrence of compression with θ < θ cr is limited if shock waves come from various directions.

Bimodal Behavior and Convergence Requirement in Macroscopic Properties of the Multiphase Interstellar Medium Formed by Atomic Converging Flows

“…The turbulence dominantly supports the shock-compressed layer at 3 Myr and turb reaches 12.4 % at maximum. Such a high conversion rate is consistent with the one driven by the interaction between shocks and density inhomogeneity (e.g., Inoue et al 2012Inoue et al , 2013Iwasaki et al 2019). Inoue et al 2013 showed that the growth velocity of the Richtmyer-Meshkov instability (Richtmyer 1960;Nishihara et al 2010) is able to account for the velocity dispersion of the turbulence.…”

“…They essentially form the multiphase ISM with supersonic turbulence consistent with observations (e.g., Larson 1981;Heyer & Brunt 2004). There have been also converging-flow studies that extensively investigate the effect of magnetic fields (Hennebelle & Pérault 2000;Inoue & Inutsuka 2008Hennebelle et al 2008;Heitsch et al 2009;Vázquez-Semadeni et al 2011;Inoue & Inutsuka 2012;Valdivia et al 2016;Inoue & Inutsuka 2016;Iwasaki et al 2019;c.f., van Loo et al 2007c.f., van Loo et al , 2010 and gas-phase metallicity (Inoue & Omukai 2015), reporting that the flow direction is redirected by magnetic fields and the critical metallicity is ∼ 0.04Z 6 , above which the ISM becomes biphasic.…”

“…We found that the x-component of δv 2 dw amounts to > 70 % of the total δv 2 dw in all density range in any ∆ρ 0 cases. Such anisotropic turbulence is reported even in magnetized converging flow simulations when the magnetic fields are mostly parallel to the gas flow (e.g., Vázquez-Semadeni et al 2007;Inoue & Inutsuka 2012;Iwasaki et al 2019).…”

“…Large-scale simulations overestimate dense gas mass available for star formation if their sub-grid models immediately convert 100 percent of the diffuse WNM into CNM and molecular gas within 3 Myr. It is left for future studies to numerically simulate another shock passage through the multiphase ISM whose CNM mass fraction is already ∼ 50 % (c.f., Inoue & Inutsuka 2012;Iwasaki et al 2019).…”

Formation of the Musca filament: evidence for asymmetries in the accretion flow due to a cloud–cloud collision

“…In the case of a (H I ) cloudcloud collision with some primordial density substructure, numerical simulations show that such a collision is prone to concentrate the pre-existent structures (e.g. Inoue & Inutsuka 2012;Inoue & Fukui 2013;Inoue et al 2018;Iwasaki et al 2019). This concentration of matter can originate from a locally curved magnetic field leading to dense structures perpendicular to the magnetic field.…”