Self-Similar Polytropic Champagne Flows in H II Regions

TL;DR

This paper develops self-similar hydrodynamic models of champagne flows in H II regions, incorporating polytropic gas behavior, shocks, and expanding cavities, and compares these models with numerical simulations.

Contribution

It introduces a new class of self-similar solutions for champagne flows in polytropic gases, including effects of shocks and central cavities, extending previous isothermal models.

Findings

Polytropic models show significant differences from isothermal ones, especially for n<1.

Self-similar solutions match well with numerical simulations of H II region expansion.

Expansion of central cavities can be modeled with self-similar shock solutions.

Abstract

We explore large-scale hydrodynamics of H II regions for various self-similar shock flows of a polytropic gas cloud under self-gravity and with quasi-spherical symmetry. We formulate cloud dynamics by invoking specific entropy conservation along streamlines and obtain global self-similar "champagne flows" for a conventional polytropic gas with shocks as a subclass. Molecular cloud cores are ionized and heated to high temperatures after the onset of nuclear burning of a central protostar. We model subsequent evolutionary processes in several ways and construct possible self-similar shock flow solutions. We may neglect the mass and gravity of the central protostar. The ionization and heating of the surrounding medium drive outflows in the inner cloud core and a shock travels outwards, leading to the so-called "champagne phase" with an expanding outer cloud envelope. Complementarily, we also consider the expansion of a central cavity around the centre. As the inner cloud expands plausibly due to powerful stellar winds, a cavity (i.e., `void' or `bubble') can be created around the centre, and when the cavity becomes sufficiently large, one may neglect the gravity of the central protostar. We thus present self-similar shock solutions for "champagne flows" with an expanding central void. We compare our solutions with isothermal solutions and find that the generalization to the polytropic regime brings about significant differences of the gas dynamics, especially for cases of n<1, where n is a key scaling index in the self-similar transformation. We also compare our global polytropic self-similar solutions with numerical simulations on the expansion of H II regions.