2007ApJ...668..275H

We examine the structure of the postshock 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 ~(10¹⁵/cm2)n^–1₀, where n₀ is the total preshock density (including both atoms and ions). For Balmer-dominated SNRs with shock velocity v_s≳1000 km/s, the Rankine-Hugoniot conditions for ion velocity and temperature are obeyed instantly, leaving the full width at half-maximum (FWHM) of the broad Hα line versus v_s relation intact. However, the spatial variation in the postshock densities is relevant to the problem of Lyα resonant scattering in young, core-collapse SNRs. Both two- (preshock atoms and ions) and three-component (preshock atoms, broad neutrals, and ions) models are considered. We compute the spatial emissivities of the broad (ξ_b) and narrow (ξ_n) Hα lines; a calculation of these emissivities in SN 1006 is in general agreement with the computed ones of Raymond and coworkers. The (dimensionless) spatial shift, Θ_shift, between the centroids of ξ_b and ξ_nis unique for a given shock velocity and f_ion, the preshock ion fraction. Measurements of Θ_shift can be used to constrain n₀.