1 Introduction

Hydrogen lines are of major interest in astrophysical problems. They are used, for example, to determine fundamental stellar parameters like effective temperature and surface gravity or to derive the electron density in solar flares. They also play a role in the macroscopic composition of stellar envelopes through the opacity and radiative diffusion processes, and also in the structure of stellar atmospheres.

These purposes require reliable tabulations extending from the line centres to the line wings. The first tables of Underhill & Waddel (1959) were followed by those of Vidal et al. (1973), which neglected the effects of the ion motions in the ionic broadening ("static ion approximation") and used a simplified description of electronic broadening by neglecting the time ordering effects. These assumptions are known to underestimate the line centre broadening, especially at low densities. Nethertheless these tabulations give a correct description of the line wings and are extensively used for the spectral synthesis of stellar atmospheres. In this context, they allow satisfactory reproduction of the variation of the stellar flux with the wavelength. These tabulations have been recently extended by Lemke (1997) to Paschen and Brackett lines. They extend possibilities of the diagnostics of stellar atmospheres towards the infrared.

The reason why the VCS tables were successful for stellar atmospheres lies in the great importance of radiative transfer in the line centre, which smears out most of the line structure. This situation is different in low density media like solar flares where using these tables can overestimate the density. Moreover, developments in high resolution spectroscopy, may require also more accurate astrophysical modelling and therefore improved input physics. One of us tried to work in this direction by publishing tables of Lyman, Balmer and Paschen series for the plasma conditions of stellar envelopes (Stehlé 1994a; 1996a); these tables have also been parametrized for the two first series (Clausset et al. 1994). Unfortunately, these tables were restricted to a limited range of density and temperatures, and were not suitable for stellar atmosphere applications. This is why we present here new extended calculations for a density range between 10¹⁰ and 10¹⁹ cm^-3and temperatures between 2500 and 1.3 10⁶ K. The extremes of these ranges of temperatures and densities are calculated essentially to allow more accurate interpolation in the tables.