The Transition Zone in Balmer-Dominated Shocks

TL;DR

This paper investigates the structure of the post-shock region in supernova remnants, focusing on the shock transition zone, line emissivities, and how measurements can constrain pre-shock densities.

Contribution

It provides a detailed analysis of the shock transition zone, models the spatial emissivities of Hα lines, and introduces a method to constrain pre-shock densities using centroid shifts.

Findings

The shock transition zone width is approximately 10^{15} cm$^{-2}$ $n_0^{-1}$.

The FWHM of the broad Hα line relates directly to shock velocity for high-velocity shocks.

Spatial shift measurements of Hα line centroids can constrain pre-shock ionization fractions and densities.

Abstract

We examine the structure of the post-shock region in supernova remnants (SNRs). The ``shock transition zone'' is set up by charge transfer and ionization events between atoms and ions, and has a width $\sim 10^{15}$ cm$^{-2}$ $n^{-1}_0$, where $n_0$ is the total pre-shock density (including both atoms and ions). For Balmer-dominated SNRs with shock velocity $v_s \gtrsim 1000$ km s$^{-1}$, the Rankine-Hugoniot conditions for ion velocity and temperature are obeyed instantly, leaving the full width at half-maximum (FWHM) of the broad H$\alpha$ line versus $v_s$ relation intact. However, the spatial variation in the post-shock densities is relevant to the problem of Ly$\alpha$ resonant scattering in young, core-collapse SNRs. Both two- (pre-shock atoms and ions) and three-component (pre-shock atoms, broad neutrals and ions) models are considered. We compute the spatial emissivities of the broad ($\xi_b$) and narrow ($\xi_n$) H$\alpha$ lines; a calculation of these emissivities in SN 1006 is in general agreement with the computed ones of Raymond et al. (2007). The (dimensionless) spatial shift, $\Theta_{\rm{shift}}$, between the centroids of $\xi_b$ and $\xi_n$ is unique for a given shock velocity and $f_{\rm{ion}}$, the pre-shock ion fraction. Measurements of $\Theta_{\rm{shift}}$ can be used to constrain $n_0$.