Radiative transfer of hydrogen lines from supernova remnant shock waves: contributions of 2s-state hydrogen atoms

Abstract

Radiative transfer in hydrogen lines in supernova remnant (SNR) shock waves is studied taking into account the population of the hydrogen atom 2s-state. Measurements of Balmer line emission, especially of H~$\alpha$, are often relied upon to derive physical conditions in the SNR shock. On the other hand, Lyman series photons, especially Ly~$\beta$, are mostly absorbed by upstream hydrogen atoms. As a result, atoms are excited to the 3p state, and then emit H~$\alpha$ by the spontaneous transition from 3p to 2s. Thus, the nature of H~$\alpha$ depends on how many Ly~$\beta$ photons are converted to H~$\alpha$ photons. Moreover, the Balmer lines can be scattered by the 2s-state hydrogen atoms, which are excited not only by collisional excitation but also by the Lyman-Balmer conversion. It is shown for example that the H~$\alpha$ photons are scattered if the shock propagates into an H~${}_{\mathrm{I}}$ cloud with a density of $\sim 30\mathrm{c}{\mathrm{m}}^{-3}$ and a size of $\sim 1$~pc. We find that the line profile of H~$\alpha$ becomes asymmetric resulting from the difference between line centre frequencies among the transitions from 3s to 2p, from 3p to 2s and from 3d to 2p. We also find that the broad-to-narrow ratio of H~$\alpha$, which is often used to estimate the ion-electron temperature equilibrium, varies at most $\simeq 10$ per cent depending on the ionization degree of the upstream medium because of incomplete conversion of Lyman lines to Balmer lines.

16 pages, accepted for publication in MNRAS

References in corpus (14)

• Measuring the cosmic ray acceleration efficiency of a supernova remnant
• Spatial Structure and Collisionless Electron Heating in Balmer-dominated Shocks
• Evidence for nonlinear diffusive shock acceleration of cosmic-rays in the 2006 outburst of the recurrent nova RS Ophiuchi
• Two-photon transitions in hydrogen and cosmological recombination
• Resolved shock structure of the Balmer-dominated filaments in Tycho's supernova remnant: Cosmic-ray precursor?
• Balmer-Dominated Shocks Revisited
• SUBARU HDS Observations of a Balmer-Dominated Shock in Tycho's Supernova Remnant
• Formation of HI Clouds in Shock-compressed Interstellar Medium: Physical Origin of Angular Correlation Between Filamentary Structure and Magnetic Field
• On cosmic-ray production efficiency at supernova remnant shocks propagating into realistic diffuse interstellar medium
• Non-Maxwellian Proton Velocity Distributions in Nonradiative Shocks
• Broad Lyman-Alpha Emission from Supernova Remnants in Young Galaxies
• Balmer filaments in Tycho's supernova remnant: an interplay between cosmic-ray and broad-neutral precursors
• A Systematic Study of the Thermal and Nonthermal Emission in the Supernova Remnant RCW 86 with Suzaku
• Polarized Balmer Line Emission from Supernova Remnant Shock Waves Efficiently Accelerating Cosmic Rays

Cited by in corpus (2)

• The warm outer layer of a Little Red Dot as the source of [Fe II] and collisional Balmer lines with scattering wings
• X-ray Line Diagnostics of Ion Temperature at Cosmic-Ray Accelerating Collisionless Shocks