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Astron. Astrophys. 317, 968-975 (1997)

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Radiative forces on lithium: effects of electron recoil and background opacities

J. Richer 1, 2, G. Michaud 1, 2 and G. Massacrier 3

1 CEntre de Recherche en Calcul Appliqué (CERCA), 5160 boul. Décarie, bureau 400, Montréal, Québec, H3X 2H9, Canada
2 Département de Physique, Université de Montréal, Case Postale 6128, Succursale A, Montréal, Québec, H3C 3J7, Canada
3 Centre de Recherche Astronomique de Lyon (UMR 142 du CNRS), École Normale Supérieure de Lyon, 46 allée d'Italie, F-69364 Lyon Cédex 07, France

Received 16 April 1996 / Accepted 11 June 1996

Abstract

Recent results on momentum transfer during photoionization processes determine how the ionizing photon momentum is split between the ejected electron and the recoiling ion. This leads to improved radiative force calculations. The consequences of this recoil for radiative forces are demonstrated by comparing for lithium the new accurate results with calculations based on simpler approximations commonly used in the past. We show that at some densities and temperatures typical of deep stellar envelopes, the total radiative force on lithium becomes negative (outgoing stellar radiation pushing the ions inwards); however this negative force is then negligible compared to gravity. In stellar envelope calculations, the exact results for hydrogenoid states are well approximated by the simpler formula that Sommerfeld obtained for the hydrogen 1s state. We also show that the force on Li depends significantly on the abundance of other elements, especially helium and oxygen, that block photon flux at critical wavelengths.

Key words: atomic processes – diffusion – radiative transfer – stars: abundances – stars: chemically peculiar

Send offprint requests to: G. Michaud, E-mail: michaudg@cerca.umontreal.ca

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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