Introduction

In cool stars such as the sun, the photospheric temperature is sufficiently high to dissociate molecular hydrogen into atomic hydrogen but is insufficient to ionise hydrogen. The electrons present are produced by ionisation of metals leading to an electron number density which is about four orders of magnitude lower than the hydrogen atom density. As hydrogen atoms are the dominant particle species, the collisional broadening of most metallic lines is dominated by collisions with hydrogen atoms. To synthesise spectral lines in cool stars, particularly those with well developed damping wings, it is essential to have satisfactory line broadening data. For the last fifty years astrophysicists have used the broadening theory of Lindholm (1942) and Foley (1946) in a form presented by Unsöld (1955) which assumes a van der Waals interaction between the absorbing atom and the perturbing hydrogen atom. This theory has long been recognised as totally inadequate (see for example Holweger and Müller 1974). Enhancement factors over the van der Waals broadening, which in general range anywhere from 1 to 4, have been used by a number of workers to fit solar spectral lines. One of the main reasons that the use of this theory has persisted is the lack of a suitable alternative that is easily used and is applicable to many spectral lines.

Anstee and O'Mara (1995), Anstee, O'Mara and Ross (1997), Barklem and O'Mara (1997) and Barklem, O'Mara and Ross (1998) have shown that the general theory of broadening developed by Anstee and O'Mara (1991) leads to solar abundances of certain elements which are consistent with their meteoritic values to within the uncertainties involved, making this theory a natural successor to van der Waals theory. Our aim is to make the determination of line broadening cross-sections from this theory even more accessible to users by means of fortran code available over the World Wide Web. More detailed and possibly more accurate calculations of broadening parameters have been undertaken by others, however usually on a line by line basis.

© Copyright Astronomical Society of Australia 1997