Ionization-Gasdynamic Simulations of Wind-Blown Nebulae around Massive Stars

Vikram V Dwarkadas

doi:10.3390/galaxies10010037

Abstract

Using a code that employs a self-consistent method for computing the effects of photo-ionization on circumstellar gas dynamics, we model the formation of wind-driven nebulae around massive stars. We take into account changes in stellar properties and mass-loss over the star’s evolution. Our simulations show how various properties, such as the density and ionization fraction, change throughout the evolution of the star. The multi-dimensional simulations reveal the presence of strong ionization front instabilities in the main-sequence phase, similar to those seen in galactic ionization fronts. Hydrodynamic instabilities at the interfaces lead to the formation of filaments and clumps that are continually being stripped off and mixed with the low density interior. Even though the winds start out as completely radial, the spherical symmetry is quickly destroyed, and the shocked wind region is manifestly asymmetrical. The simulations demonstrate that it is important to include the effects of the photoionizing photons from the star, and simulations that do not include this may fail to reproduce the observed density profile and ionization structure of wind-blown bubbles around massive stars.

Highlights

Massive stars (≥10M ) lose mass throughout their lifetime, via winds and eruptions.Many will subsequently end their lives in a cataclysmic supernova (SN) explosion, while others may collapse directly to a black hole [1]
If the star ends its life in a SN explosion, the resulting SN shock wave will expand within the bubble, and the dynamics and kinematics of the shock wave, dependent on the bubble parameters, will differ from evolution within the interstellar medium (ISM) [2]
Similar to the findings of Freyer et al [17], that the MS phase leads to substantial instabilities, the effects of which can persist throughout the evolution, and lead to increased turbulence within the wind blown region

Summary

Introduction

Massive stars (≥10M ) lose mass throughout their lifetime, via winds and eruptions. Many will subsequently end their lives in a cataclysmic supernova (SN) explosion, while others may collapse directly to a black hole [1]. This suggests the existence of some mechanism that tends to lower the interior temperature, or equivalently that the energy that would go into raising the temperature is being expended elsewhere The goal of this project is to carry out ionization-gasdynamic simulations of wind-blown bubbles followed by subsequent X-ray modelling. We will assess accurately the interior structure of Galaxies 2022, 10, 37 wind bubbles by studying the evolution in multi-dimensions, and use the results to compute the X-ray temperatures and examine the energy budget. In this paper we elaborate significantly on the work first outlined in Dwarkadas and Rosenberg [24], describing in detail the gasdynamic simulations, the density structure, as well as the onset and growth of instabilities in models of ionized wind blown bubbles around massive stars.

AVATAR Code

Spherically Symmetric Simulations

Ionization Fraction

Findings

Conclusions