Turbulence variation in the atmosphere of delta Cephei

Using 288 high resolution spectra obtained over 3 consecutive years (1993-1 995), the microturbulence velocity curve of delta Cephei is determined with a high temporal resolution. From the observed FWHM variation of the unblen ded Fe I 5576.0883 Angstrom absorption line, and from nonlinear nonadiabati c pulsating models, it is shown that the global compression/expansion of th e atmosphere provokes the variation of the turbulence. The turbulent veloci ty reaches its maximum, of about 7 km s(-1), at minimum radius, and decreas es up to 2 km s(-1) during the expansion of the star. It is also shown that the compression waves and shocks certainly provoke some rapid turbulence v ariations during their propagation in the atmosphere. The turbulence amplif ication due to these waves, being in general smaller than the global compre ssion effect, increases with the wave amplitude.