The structure of radiative shock waves. V. Hydrogen emission lines

TL;DR

This paper investigates the structure and emission line profiles of steady-state radiative shock waves in partially ionized hydrogen gas, revealing Doppler shifts, line doubling, and the effects of optical thickness and geometry.

Contribution

It provides detailed modeling of hydrogen emission lines in radiative shocks, including Doppler shifts and line profile structures, under various densities and shock velocities.

Findings

Doppler shifts in Balmer lines are about one-third of shock velocity.

Hydrogen emission regions are located in the hydrogen recombination zone behind the shock.

Double line profiles are observed in H-alpha at certain densities and velocities.

Abstract

We considered the structure of steady-state plane-parallel radiative shock waves propagating through the partially ionized hydrogen gas of temperature T_1 = 3000K and density 1e-12 gm/cm^3 <= \rho_1 <= 1e-9 gm/cm^3. The upstream Mach numbers range within 6 <= M_1 <= 14. In frequency intervals of hydrogen lines the radiation field was treated using the transfer equation in the frame of the observer for the moving medium, whereas the continuum radiation was calculated for the static medium. Doppler shifts in Balmer emission lines of the radiation flux emerging from the upstream boundary of the shock wave model were found to be roughly one-third of the shock wave velocity. The gas emitting the Balmer line radiation is located at the rear of the shock wave in the hydrogen recombination zone where the gas flow velocity in the frame of the observer is approximately one-half of the shock wave velocity. The ratio of the Doppler shift to the gas flow velocity of 0.7 results both from the small optical thickness of the shock wave in line frequencies and the anisotropy of the radiation field typical for the slab geometry. In the ambient gas with density of \rho_1 >= 1e-11 gm/cm^3 the flux in the H-alpha frequency interval reveals the double structure of the profile. A weaker H-beta profile doubling was found for \rho_1 >= 1e-10 gm/cm^3 and U_1 <= 50 km/s. The unshifted redward component of the double profile is due to photodeexcitation accompanying the rapid growth of collisional ionization in the narrow layer in front of the discontinuous jump.