Models of the circumstellar medium of evolving, massive runaway stars moving through the Galactic plane

TL;DR

This study uses hydrodynamical simulations to analyze how mass loss and velocity influence the morphology and emission properties of bow shocks around massive runaway stars in the Galactic plane.

Contribution

It provides new insights into the effects of thermal conduction and stellar parameters on bow shock structure and emission, highlighting infrared as the best observational waveband.

Findings

Thermal conduction significantly influences bow shock shape and size.

Infrared emission is dominant and best for detecting bow shocks.

Brightest bow shocks are from high-mass stars with low velocities.

Abstract

At least 5 per cent of the massive stars are moving supersonically through the interstellar medium (ISM) and are expected to produce a stellar wind bow shock. We explore how the mass loss and space velocity of massive runaway stars affect the morphology of their bow shocks. We run two-dimensional axisymmetric hydrodynamical simulations following the evolution of the circumstellar medium of these stars in the Galactic plane from the main sequence to the red supergiant phase. We find that thermal conduction is an important process governing the shape, size and structure of the bow shocks around hot stars, and that they have an optical luminosity mainly produced by forbidden lines, e.g. [OIII]. The Ha emission of the bow shocks around hot stars originates from near their contact discontinuity. The H$\alpha$ emission of bow shocks around cool stars originates from their forward shock, and is too faint to be observed for the bow shocks that we simulate. The emission of optically-thin radiation mainly comes from the shocked ISM material. All bow shock models are brighter in the infrared, i.e. the infrared is the most appropriate waveband to search for bow shocks. Our study suggests that the infrared emission comes from near the contact discontinuity for bow shocks of hot stars and from the inner region of shocked wind for bow shocks around cool stars. We predict that, in the Galactic plane, the brightest, i.e. the most easily detectable bow shocks are produced by high-mass stars moving with small space velocities.