When Shock Waves Collide

TL;DR

This paper investigates the behavior of Mach stems formed by intersecting shock waves in astrophysical plasmas through numerical simulations and laboratory experiments, revealing their formation conditions, stability, and relevance to observed stellar jet features.

Contribution

It provides the first combined experimental and numerical analysis of Mach stem dynamics in astrophysical-like supersonic plasmas, confirming analytical predictions and exploring their stability.

Findings

Mach stems form at critical intersection angles in supersonic plasmas.

Large Mach stems are more stable and can regrow after destruction.

Mach stems can explain variable bright knots in stellar jet observations.

Abstract

Supersonic outflows from objects as varied as stellar jets, massive stars and novae often exhibit multiple shock waves that overlap one another. When the intersection angle between two shock waves exceeds a critical value, the system reconfigures its geometry to create a normal shock known as a Mach stem where the shocks meet. Mach stems are important for interpreting emission-line images of shocked gas because a normal shock produces higher postshock temperatures and therefore a higher-excitation spectrum than an oblique one does. In this paper we summarize the results of a series of numerical simulations and laboratory experiments designed to quantify how Mach stems behave in supersonic plasmas that are the norm in astrophysical flows. The experiments test analytical predictions for critical angles where Mach stems should form, and quantify how Mach stems grow and decay as intersection angles between the incident shock and a surface change. While small Mach stems are destroyed by surface irregularities and subcritical angles, larger ones persist in these situations, and can regrow if the intersection angle changes to become more favorable. The experimental and numerical results show that although Mach stems occur only over a limited range of intersection angles and size scales, within these ranges they are relatively robust, and hence are a viable explanation for variable bright knots observed in HST images at the intersections of some bow shocks in stellar jets.