2014A&A...561A.151N

High-resolution spectroscopy is a powerful tool to study the dynamical structure of a pulsating star's atmosphere. We aim at comparing the line asymmetry and velocity of the two δ Sct stars ρ Pup and DX Cet with previous spectroscopic data obtained on classical Cepheids and β Cep stars. We obtained, analysed and discuss HARPS high-resolution spectra of ρ Pup and DX Cet. We derived the same physical quantities as used in previous studies, which are the first-moment radial velocities and the bi-Gaussian spectral line asymmetries. The identification of f=7.098/d as a fundamental radial mode and the very accurate Hipparcos parallax promote ρ Pup as the best standard candle to test the period-luminosity relations of δ Sct stars. The action of small-amplitude nonradial modes can be seen as well-defined cycle-to-cycle variations in the radial velocity measurements of ρ Pup. Using the spectral-line asymmetry method, we also found the centre-of-mass velocities of ρ Pup and DX Cet, V_γ=47.49±0.07 km/s and V_γ=25.75±0.06 km/s, respectively. By comparing our results with previous HARPS observations of classical Cepheids and β Cep stars, we confirm the linear relation between the atmospheric velocity gradient and the amplitude of the radial velocity curve, but only for amplitudes larger than 22.5 km/s. For lower values of the velocity amplitude (i.e., <22.5 km/s), our data on ρ Pup seem to indicate that the velocity gradient is null, but this result needs to be confirmed with additional data. We derived the Baade-Wesselink projection factor p=1.36±0.02 for ρ Pup and p=1.39±0.02 for DX Cet. We successfully extended the period-projection factor relation from classical Cepheids to δ Scuti stars.