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Aims. The Baade-Wesselink method of distance determination is based on theoscillations of pulsating stars. The key parameter of this method is theprojection factor used to convert the radial velocity into the pulsationvelocity. Our analysis was aimed at deriving for the first time the projectionfactor of delta Scuti stars, using high-resolution spectra of thehigh-amplitude pulsator AI Vel and of the fast rotator beta Cas. Methods. Thegeometric component of the projection factor (i.e. p0) was calculated using alimb-darkening model of the intensity distribution for AI Vel, and afast-rotator model for beta Cas. Then, using SOPHIE/OHP data for beta Cas andHARPS/ESO data for AI Vel, we compared the radial velocity curves of severalspectral lines forming at different levels in the atmosphere and derived thevelocity gradient associated to the spectral-line-forming regions in theatmosphere of the star. This velocity gradient was used to derive a dynamicalprojection factor p. Results. We find a flat velocity gradient for both starsand finally p = p0 = 1.44 for AI Vel and p = p0 = 1.41 for beta Cas. Bycomparing Cepheids and delta Scuti stars, these results bring valuable insightsinto the dynamical structure of pulsating star atmospheres. They suggest thatthe period-projection factor relation derived for Cepheids is also applicableto delta Scuti stars pulsating in a dominant radial mode.

Understanding the dynamical structure of pulsating stars: The Baade-Wesselink projection factor of the delta Scuti stars AI Vel and beta Cas