The structure of radiative shock waves. II. The multilevel hydrogen atom

Abstract

Models of steady-state plane-parallel shock waves propagating through the unperturbed hydrogen gas of temperature T=6000K and density rho = 1e-10 gm/cm^3 are computed for upstream velocities from 15 km/s to 70 km/s. The shock wave structure is considered in terms of the self-consistent solution of the radiation transfer, fluid dynamics and rate equations for 2 <= L <= 4 atomic bound levels with a continuum. The radiative flux Frad emergent from the shock wave was found to be independent of the lower limit nu_L of the frequency range provided that nu_L < nu_2, where nu_2 is the Balmer continuum head frequency, At the same time the decrease of nu_L is accompanied by decrease of the Lyman continuum flux and leads to smaller heating and weaker ionization of the hydrogen gas in the radiative precursor. For all models the size of the radiative precursor is of 10^4 cm and corresponds to several mean free paths of photons at the frequency of the Lyman continuum edge nu_1. The compression ratio at the discontinuous jump gradually increases with increasing upstream velocity U_1, reaches the maximum of ρ^+/ρ^- = 3.62 at U_1 = 55 km/s and slowly decreases for larger U_1 due to the strong rize of the preshock gas temperature. The radiative flux from the shock wave was determined as a function of the upstream velocity U_1 and its ratio to the total energy flux in the shock wave C_2 was found to range within 0.18 < Frad/C_2 < 0.92 for 15 km/s <= U_1 <= 65 km/s. Thus, at upstream velocities U_1 > 60 km/s the shock wave losses more than 90% of its total energy due to radiation. For all shock wave models the role of collisional processes in both bound-bound and bound-free atomic transitions was found to be negligible in comparison with corresponding radiative processes.