The Transition Zone in Balmer-Dominated Shocks

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${}^{-2}$ $n_0^{-1}$, 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, ${\Theta }_{\mathrm{shift}}$, between the centroids of ${\xi }_b$ and ${\xi }_n$ is unique for a given shock velocity and $f_{\mathrm{ion}}$, the pre-shock ion fraction. Measurements of ${\Theta }_{\mathrm{shift}}$ can be used to constrain $n_0$.

25 pages, 8 figures. Accepted by Astrophysical Journal

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