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High-resolution spectroscopy is a powerful tool to study the dynamicalstructure of pulsating stars atmosphere. We aim at comparing the line asymmetryand velocity of the two delta Sct stars rho Pup and DX Cet with previousspectroscopic data obtained on classical Cepheids and beta Cep stars. Weobtained, analysed and discuss HARPS high-resolution spectra of rho Pup and DXCet. We derived the same physical quantities as used in previous studies, whichare the first-moment radial velocities and the bi-Gaussian spectral lineasymmetries. The identification of f=7.098 (1/d) as a fundamental radial modeand the very accurate Hipparcos parallax promote rho Pup as the best standardcandle to test the period-luminosity relations of delta Sct stars. The actionof small-amplitude nonradial modes can be seen as well-defined cycle-to-cyclevariations in the radial velocity measurements of rho Pup. Using thespectral-line asymmetry method, we also found the centre-of-mass velocities ofrho Pup and DX Cet, V_gamma = 47.49 +/- 0.07 km/s and V_gamma = 25.75 +/- 0.06km/s, respectively. By comparing our results with previous HARPS observationsof classical Cepheids and beta Cep stars, we confirm the linear relationbetween the atmospheric velocity gradient and the amplitude of the radialvelocity curve, but only for amplitudes larger than 22.5 km/s. For lower valuesof the velocity amplitude (i.e., < 22.5 km/s), our data on rho Pup seem toindicate that the velocity gradient is null, but this result needs to beconfirmed with additional data. We derived the Baade-Wesselink projectionfactor p = 1.36 +/- 0.02 for rho Pup and p = 1.39 +/- 0.02 for DX Cet. Wesuccessfully extended the period-projection factor relation from classicalCepheids to delta Scuti stars.

Understanding the dynamical structure of pulsating stars. HARPS spectroscopy of the delta Scuti stars rho Pup and DX Cet