General Polytropic Magnetofluid under Self-Gravity: Voids and Shocks

TL;DR

This paper investigates self-similar magnetohydrodynamic solutions for expanding voids in self-gravitating gases with a polytropic equation of state, revealing new zero-density boundary solutions and their astrophysical implications.

Contribution

It introduces novel void solutions with zero density boundary conditions in a general polytropic MHD context, expanding understanding of void dynamics in astrophysical environments.

Findings

Discovery of zero-density boundary void solutions

Existence of shell-type density profiles in voids

Void solutions crossing magnetosonic critical curves with shocks

Abstract

We study the self-similar magnetohydrodynamics (MHD) of a quasi-spherical expanding void (viz. cavity or bubble) in 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.