General polytropic magnetofluid under self-gravity: Voids and shocks |
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| Authors: | Yu-Qing Lou Ren-Yu Hu |
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| Institution: | aPhysics Department and Tsinghua Center for Astrophysics (THCA), Tsinghua University, Beijing 100084, China;bDepartment of Astronomy and Astrophysics, The University of Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637, USA;cNational Astronomical Observatories, Chinese Academy of Sciences, A20, Datun Road, Beijing 100012, China |
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| Abstract: | We study the self-similar magnetohydrodynamics (MHD) of a quasi-spherical expanding void (viz. cavity or bubble) surrounding the centre of a self-gravitating gas sphere with a general polytropic equation of state. We show various analytic asymptotic solutions near the void boundary in different parameter regimes and obtain the corresponding void solutions by extensive numerical explorations. We find novel void solutions of zero density on the void boundary. These new void solutions exist only in a general polytropic gas and feature shell-type density profiles. These void solutions, if not encountering the magnetosonic critical curve (MCC), generally approach the asymptotic expansion solution far from the central void with a velocity proportional to radial distance. We identify and examine free-expansion solutions, Einstein–de Sitter expansion solutions, and thermal-expansion solutions in three different parameter regimes. Under certain conditions, void solutions may cross the MCC either smoothly or by MHD shocks, and then merge into asymptotic solutions with finite velocity and density far from the centre. Our general polytropic MHD void solutions provide physical insight for void evolution, and may have astrophysical applications such as massive star collapses and explosions, shell-type supernova remnants and hot bubbles in the interstellar and intergalactic media, and planetary nebulae. |
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| Keywords: | ISM: bubbles MHD Planetary nebulae: general Shock waves Star: winds outflows Supernova remnants |
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