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Astron. Astrophys. 324, 799-804 (1997)


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Semiclassical collisional functions in a non ideal plasma

N. Ben Nessib 1, S. Sahal-Bréchot 2 and Z. Ben Lakhdar 1

1 Laboratoire de Physique Atomique et Moléculaire, Département de Physique, Faculté des Sciences de Tunis, 1060 Campus Universitaire, Tunis, Tunisia
2 Laboratoire " Atomes et Molécules en Astrophysique", Unité associée au C.N.R.S. No 812, Département Atomes et Molécules en Astrophysique, Observatoire de Paris-Meudon, F-92195 Meudon Cedex, France

Received 1 October 1996 / Accepted 2 January 1997

Abstract

Collisions between atoms (or ions) and electrons play an important role in Astrophysics for spectroscopic diagnostics and modellisation of stellar interiors and atmospheres. Plasma shielding effects due to electron and ion correlations are not negligible in the physical conditions of white dwarfs, owing to their high density. They can also play a role in the case of rather cool stars and for hydrogen lines, because their excited levels are quasi-degenerated.

The model which usually describes collective effects is the Debye-Hückel potential: the two-particle Coulomb field is shielded by the ensemble of surrounding electrons. However, for low plasma temperatures, when the electron density increases, the plasma becomes non ideal. Consequently this potential is no longer valid and the Coulomb cutoff potential is more appropriate. The Coulomb cutoff potential is in fact especially suitable for the case of a strongly non ideal plasma.

In the present paper, we calculate inelastic cross-sections by describing the electron atom interaction with a Coulomb cutoff potential and using a semiclassical perturbation approach. These cross-sections enter the expressions of the collisional linewidths in the impact approximation, and the statistical equilibrium equations leading to atomic populations for non-LTE studies. The case of cross-sections which are needed for impact polarization studies are also treated.

Key words: atomic processes – line: formation plasmas – stars: interiors – stars: dwarfs – stars: atmospheres

Send offprint requests to: S. Sahal-Bréchot (sahal@obspm.fr)

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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