Introduction

Recently Fokin, Gillet and Breitfellner (1996, hereafter FGB) showed that the velocity field in the atmosphere of a pulsating star ($\delta$Cephei) is an important line broadening source. This means that it is imperative to take into account the large amplitude compression waves and shock waves propagating in this type of atmosphere to evaluate line profiles and physical quantities which can be deduced from them such as the turbulence velocity.

In their determination of the turbulence curve, Breitfellner and Gillet (1993) do not consider the effect of waves. Consequently, they obtain an overestimation of the turbulence velocity with a maximum around 13km/s and a minimum value of 7.5km/s. Their turbulence curve pre- sents three obvious peaks. The main one occurs around the maximum compression. It appears broadened on its left side (phase interval 0.75-0.80). A moderate peak is visible around phase $\varphi=0.6$ and a smaller one is present at phase 0.1 after the maximum compression ($\varphi=0.9$).

Contrary to Breitfellner and Gillet (1993), FGB do not limit the determination of the turbulence velocity to one line parameter (the Full Width Half Maximum or FWHM) but they use the complete line profile. Nevertheless, as already explained by FGB, this technique is difficult to apply because presently opacity tables are not sufficient accurate to correctly calculate line profiles, especially at low temperatures (<5000K). Indeed, the relative flux of theoretical profiles are often weaker than the observed ones (see Fig.7 of FBG). Consequently, now it is difficult to achieve a relevant solution by the comparison of calculated and observed line profiles. The FWHM of an absorption line is a less sensitive line parameter. In particular, its value is weakly affected by non-LTE effects. It is the reason why it seems preferable to limit the determination of the turbulence velocity to this single parameter. The use of the residual flux, the equivalent width or the whole line profile does not seem actually appropriate.

Section2 presents new spectroscopic high resolution observations of the variable star $\delta$Cephei which allow us to determine, from an unblended FeI absorption line, the shape of the FWHM-curve. Section3 is devoted to the description of the main line parameter variations together with a qualitative interpretation in the framework of the nonlinear nonadiabatic models used to describe the pulsation of the atmosphere. The obtained turbulence velocity curve is discussed in Sect.4 and some comments and concluding remarks are given in Sect.5.