Radiation-mediated Shock and Planar Shock Breakout in the Presence of Atomic Transition Lines

Abstract We numerically study fast radiation-mediated shocks (RMS; v / c ≲ 0.2) in two simplified problems within the context of a supernova shock breakout: (1) an RMS traveling in a uniform medium, and (2) an RMS escaping a planar power-law density profile ( ρ ∝ x n ). For a fully ionized plasma, it has been shown in the literature that insufficient photon production can yield a many-keV Comptonized spectrum at high shock velocities. Here, we incorporate, for the first time, opacity from bound species of heavy elements (solar composition) into these two problems, thereby increasing photon production via bound-free and bound–bound processes. We use a hydrodynamic coupled multigroup radiative diffusion code, including inelastic Compton scattering and frequency-dependent opacity. We show that a more realistic opacity promotes local thermal equilibrium (LTE) in cases previously expected to be Comptonized. In planar shock breakout (and correspondingly in the spherical case), this leads to typically a factor of 2—and at times orders of magnitude—lower peak emission wavelength. As a result, explosions in red supergiant stars (and part of blue supergiants) without stellar wind or extended envelopes are unlikely to produce observable X-ray emission. We provide a semi-analytic spectral energy distribution description of when LTE is maintained. We note that a correct calculation of shock-breakout radiation requires opacity tables that include bound yet highly ionized species, ruling out the use of certain line tables (such as the commonly used Kurucz table) for this problem.