Astron. Astrophys. 331, 1051-1058 (1998)

1. Introduction

Detailed stellar abundance analyses have generally targeted weak lines lying on the linear portion of the curve of growth, where , for W and the equivalent width and wavelength respectively. Weak lines have been favoured over stronger ones for a number of reasons including:

• their equivalent widths exhibit maximum sensitivity to abundance;
• they are less affected by the uncertain atmospheric parameters of microturbulence and gravity; and
• the broad "damping" wings of strong lines require that the collision broadening be known.

As the strengths of weak absorption lines reflect the opacity in the Doppler-dominated line core more-so than that in the Lorentzian wings, the sometimes-large uncertainties in collision broadening calculations have generally been regarded as unimportant in the analysis of weak lines. Some twenty-five years have passed since Blackwell, Calamai, & Willis (1972) discussed the effects of uncertainties in damping and microturbulence on abundance analyses, and it is over a decade and a half since Gurtovenko & Kondrashova (1980) reviewed available data on damping. In the meantime, high S/N spectra have become the norm and gf values accurate to within a few percent are more common, and greater accuracy is expected from stellar abundance analyses than might previously have been the case. In view of this, it is appropriate to revisit the issue of collision broadening of weak lines, and to be aware of lingering uncertainties in this aspect of an abundance analysis. It is the purpose of this Research Note to draw attention again to the importance of collision broadening even for weak lines, and to highlight the consequences of adopting various formalisms for computing it.

The paper is set out as follows. In Sect. 2, collision broadening is discussed in general terms, followed in Sect. 3 by a description of several collision broadening formalisms. In Sect. 4, damping widths are computed for a set of lines according to different prescriptions, and abundances are computed and discussed for several of these cases in Sect. 5, using solar data as the illustration.

© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998
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