Introduction

The strength of a line is characterized by its equivalent width (see figure 4). The continuous background – or continuous spectrum – being taken as unity, the equivalent width is the surface enclosed by the line profile. The abscissa can be expressed either in terms of wavelength or on a frequency (or wavenumber) scale. The equivalent width is given in the same units as the abscissa and can thus be expressed in angstrom units (1 Å = 10−8 cm), or in reciprocal centimeters. Habitually one speaks of a strong line as a line having an equivalent width (W> 1 Å and of a weak line as one having W<0.1 Å.

The difficulties in measuring equivalent widths usually come from the tracing of the continuum. It can happen that there are too many lines in the region that one is studying, so that the region is so crowded that the background becomes invisible. This happens in all late type stars because of the number of lines present in their spectra. The larger the plate factor is (in Å mm−1), the more the lines are compressed and one cannot any longer see a stretch of unperturbed continuum. A partial solution is to use lower plate factors, which may allow one to find (narrow) windows of the true continuum.

In early type stars fewer lines are present and therefore one might think that the true continuum is easy to trace; but among these stars one finds a large number of rapid rotators. If rotation is very fast it draws out the line profiles, increasing superposition of neighboring lines and again the continuous spectrum is perturbed.