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

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          Abstract

          We investigate the formation of molecular clouds from atomic gas by using three-dimensional magnetohydrodynamical simulations including chemical reactions and heating/cooling processes. We consider super-Alfv\'enic head-on colliding flows of atomic gas possessing the two-phase structure. We examine how the molecular cloud formation changes depending on the angle \(\theta\) between the upstream flow and mean magnetic field. If the atomic gas is compressed almost along the mean magnetic field, super-Alfv\'enic anisotropic turbulence is maintained by accretion of the highly inhomogeneous upstream atomic gas, and the post-shock layer rapidly expands. Even a small obliqueness of the magnetic field drastically changes the physical properties of the post-shock layers. The shock compression amplifies the tangential component of the magnetic field which weakens the post-shock turbulence, making the post-shock layer denser. If the magnetic field is further inclined to the upstream flow, the shock-amplified magnetic pressure suppresses gas compression, leading to an extended post-shock layer. Our results, therefore, show that there is a critical angle \(\theta_\mathrm{cr}\). Compression with \(\theta<\theta_\mathrm{cr}\) generates largely-extended turbulence-dominated cold clouds. Around \(\theta\sim \theta_\mathrm{cr}\), dense cold clouds form. For \(\theta\gg \theta_\mathrm{cr}\), the strong magnetic pressure suppresses the formation of cold clouds. Efficient MC formation is expected if \(\theta\) is less than a few times \(\theta_\mathrm{cr}\).

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          An Unsplit Godunov Method for Ideal MHD via Constrained Transport in Three Dimensions

          We present a single step, second-order accurate Godunov scheme for ideal MHD which is an extension of the method described by Gardiner & Stone (2005) to three dimensions. This algorithm combines the corner transport upwind (CTU) method of Colella for multidimensional integration, and the constrained transport (CT) algorithm for preserving the divergence-free constraint on the magnetic field. We describe the calculation of the PPM interface states for 3D ideal MHD which must include multidimensional ``MHD source terms'' and naturally respect the balance implicit in these terms by the \({\bf\nabla\cdot B}=0\) condition. We compare two different forms for the CTU integration algorithm which require either 6- or 12-solutions of the Riemann problem per cell per time-step, and present a detailed description of the 6-solve algorithm. Finally, we present solutions for test problems to demonstrate the accuracy and robustness of the algorithm.
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            Author and article information

            Journal
            11 June 2018
            Article
            1806.03824
            f8678afb-1af9-4d5e-902b-521afe998be5

            http://arxiv.org/licenses/nonexclusive-distrib/1.0/

            History
            Custom metadata
            21 pages, 19 figures, submitted to ApJ,Comments are welcome
            astro-ph.GA

            Galaxy astrophysics
            Galaxy astrophysics

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