On the circumstellar medium of massive stars and how it may appear in GRB observations

TL;DR

This paper uses hydrodynamical simulations to explore the circumstellar environments of massive stars, particularly those that could be GRB progenitors, and examines how these features might influence GRB observations.

Contribution

It provides detailed models of the circumstellar medium around a GRB progenitor candidate, highlighting the formation and evolution of transient structures and their potential observational signatures.

Findings

Massive stars create large, turbulent bubbles with temporary shells and clumps.

High-density environments lead to simpler, more confined circumstellar structures.

Most transient features dissipate by the end of the star's life.

Abstract

Massive stars lose a large fraction of their original mass over the course of their evolution. These stellar winds shape the surrounding medium according to parameters that are the result of the characteristics of the stars, varying over time as the stars evolve, leading to both permanent and temporary features that can be used to constrain the evolution of the progenitor star. Because long Gamma-Ray Bursts (GRBs) are thought to originate from massive stars, the characteristics of the circumstellar medium (CSM) should be observable in the signal of GRBs. This can occur directly, as the characteristics of the GRB-jet are changed by the medium it collides with, and indirectly because the GRB can only be observed through the extended circumstellar bubble that surrounds each massive star. We use computer simulations to describe the circumstellar features that can be found in the vicinity of massive stars and discuss if, and how, they may appear in GRB observations. Specifically, we make hydrodynamical models of the circumstellar environment of a rapidly rotating, chemically near-homogeneous star, which is represents a GRB progenitor candidate model. The simulations show that the star creates a large scale bubble of shocked wind material, which sweeps up the interstellar medium in an expanding shell. Within this bubble, temporary circumstellar shells, clumps and voids are created as a result of changes in the stellar wind. Most of these temporary features have disappeared by the time the star reaches the end of its life, leaving a highly turbulent circumstellar bubble behind. Placing the same star in a high density environment simplifies the evolution of the CSM as the more confined bubble prohibits the formation of some of the temporary structures.