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

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 Added by Jiro Shimoda
 Publication date 2019
  fields Physics
and research's language is English




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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~$_{rm I}$ cloud with a density of $sim30~{rm cm^{-3}}$ 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.



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Ab initio study of the density-dependent population and lifetime of the long-lived $(mu p)_{2s}$ and the yield of $(mu p)_{1s}$ atoms with kinetic energy 0.9 keV have been performed for the first time. The direct Coulomb $2sto 1s$ deexcitation is proved to be the dominant quenching mechanism of the $2s$ state at kinetic energy below $2p$ threshold and explain the lifetime of the metastable $2s$ state and high-energy 0.9 keV component of $(mu p)_{1S}$ observed at low densities. The cross sections of the elastic, Stark and Coulomb deexcitation processes have been calculated in the close-coupling approach taking into account for the first time both the closed channels and the threshold effects due to vacuum polarization shifts of the $ns$ states. The cross sections are used as the input data in the detailed study of the atomic cascade kinetics. The theoretical predictions are compared with the known experimental data at low densities. The 40% yield of the 0.9 keV$(mu p)_{1s}$ atoms is predicted for liquid hydrogen density.
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