Spatial Structure and Collisionless Electron Heating in Balmer-dominated Shocks

TL;DR

This paper models the kinetic processes in Balmer-dominated shocks of supernova remnants, revealing how broad neutral kinetics influence shock structure and emission, and providing refined shock parameter estimates from observations.

Contribution

It introduces a comprehensive treatment of broad neutral kinetics in shock models, improving the accuracy of shock velocity and electron temperature estimates from Balmer line observations.

Findings

Inferred shock velocities are lower than previous estimates by up to 30%.

Electron temperatures are found to be lower by less than 10-30%.

The study establishes a relation between temperature ratio and shock velocity.

Abstract

Balmer-dominated shocks in supernova remnants (SNRs) produce strong hydrogen lines with a two-component profile composed of a narrow contribution from cold upstream hydrogen atoms, and a broad contribution from hydrogen atoms that have undergone charge transfer reactions with hot protons. Observations of emission lines from edge-wise shocks in SNRs can constrain the gas velocity and collisionless electron heating at the shock front. Downstream hydrogen atoms engage in charge transfer, excitation and ionization reactions, defining an interaction region called the shock transition zone. The properties of hot hydrogen atoms produced by charge transfers (called broad neutrals) are critical for accurately calculating the structure and radiation from the shock transition zone. This paper is the third in a series describing the kinetic, fluid and emission properties of Balmer-dominated shocks, and is the first to properly treat the effect of broad neutral kinetics on shock transition zone structure. We use our models to extract shock parameters from observations of Balmer-dominated SNRs. We find that inferred shock velocities and electron temperatures are lower than those of previous calculations by <10% for v_s<1500 km/s, and by 10-30% for v_s>1500 km/s. This effect is primarily due to the fact that excitation by proton collisions and charge transfer to excited levels favor the high speed part of the neutral hydrogen velocity distribution. Our results have a strong dependence on the ratio of electron to proton temperatures, \beta=T_e/T_p, which allows us to construct a relation \beta(v_s) between the temperature ratio and shock velocity. We compare our calculations to previous results by Ghavamian et al. (2007).