Volume 551, March 2013
|Number of page(s)||13|
|Published online||27 February 2013|
Isotropic inelastic and superelastic collisional rates in a multiterm atom
Instituto de Astrofísica de Canarias, C. vía Láctea s/n, 38205 La Laguna,
2 Departamento de Astrofísica, Facultad de Física, Universidad de La Laguna, 38206, La Laguna, Tenerife, Spain
3 Dipartimento di Fisica e Astronomia, Università di Firenze, Largo E. Fermi 2, 50125 Firenze, Italy
4 Consejo Superior de Investigaciones Científicas, Spain
Accepted: 4 November 2012
The spectral line polarization of the radiation emerging from a magnetized astrophysical plasma depends on the state of the atoms within the medium, whose determination requires considering the interactions between the atoms and the magnetic field, between the atoms and photons (radiative transitions), and between the atoms and other material particles (collisional transitions). In applications within the framework of the multiterm model atom (which accounts for quantum interference between magnetic sublevels pertaining either to the same J-level or to different J-levels within the same term) collisional processes are generally neglected when solving the master equation for the atomic density matrix. This is partly due to the lack of experimental data and/or of approximate theoretical expressions for calculating the collisional transfer and relaxation rates (in particular the rates for interference between sublevels pertaining to different J-levels, and the depolarizing rates due to elastic collisions). In this paper we formally define and investigate the transfer and relaxation rates due to isotropic inelastic and superelastic collisions that enter the statistical equilibrium equations for the atomic density matrix of a multiterm atom. Under the hypothesis that the interaction between the collider and the atom can be described by a dipolar operator, we provide expressions that relate the collisional rates for interference between different J-levels to the usual collisional rates for J-level populations, for which experimental data or approximate theoretical expressions are generally available. We show that the rates for populations and interference within the same J-level reduce to those previously obtained for the multilevel model atom (where quantum interference is assumed to be present only between magnetic sublevels pertaining to any given J-level). Finally, we apply the general equations to the case of a two-term atom with unpolarized lower term, illustrating the impact of inelastic and superelastic collisions on the scattering line polarization through radiative transfer calculations in a slab of stellar atmospheric plasma anisotropically illuminated by the photospheric radiation field.
Key words: atomic processes / line: formation / polarization / radiative transfer / scattering / stars: atmospheres
© ESO, 2013
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