On inelastic hydrogen atom collisions in stellar atmospheres

¹ Department of Physics and AstronomyUppsala University, Box 515, 75120 Uppsala, Sweden
e-mail: Paul.Barklem@physics.uu.se
² Department of Theoretical Physics, Herzen University, 191186 St. Petersburg, Russia
³ Université Paris-Est, Laboratoire Modélisation et Simulation Multi-Échelle, MSME UMR 8208 CNRS, 5 Bd Descartes, 77454 Marne-la-Vallée, France
⁴ LERMA and UMR 8112 of CNRS, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France
⁵ Laboratoire de Chimie et Physique Quantique, UMR 5626 du CNRS, IRSAMC, Université Paul Sabatier, 118 Rte de Narbonne, 31062 Toulouse, France

Received: 18 February 2011
Accepted: 10 April 2011

Abstract

The influence of inelastic hydrogen atom collisions on non-LTE spectral line formation has been, and remains to be, a significant source of uncertainty for stellar abundance analyses, due to the difficulty in obtaining accurate data for low-energy atomic collisions either experimentally or theoretically. For lack of a better alternative, the classical “Drawin formula” is often used. Over recent decades, our understanding of these collisions has improved markedly, predominantly through a number of detailed quantum mechanical calculations. In this paper, the Drawin formula is compared with the quantum mechanical calculations both in terms of the underlying physics and the resulting rate coefficients. It is shown that the Drawin formula does not contain the essential physics behind direct excitation by H atom collisions, the important physical mechanism being quantum mechanical in character. Quantitatively, the Drawin formula compares poorly with the results of the available quantum mechanical calculations, usually significantly overestimating the collision rates by amounts that vary markedly between transitions.