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High-resolution spectroscopy of pulsating stars is a powerful tool to studythe dynamical structure of their atmosphere. Lines asymmetry is used to derivethe center-of-mass velocity of the star, while a direct measurement of theatmospheric velocity gradient helps determine the projection factor used in theBaade-Wesselink method of distance determination. We aim at deriving thecenter-of-mass velocity and the projection factor of the beta-Cephei staralpha-Lup. We present HARPS high spectral resolution observations of alpha-Lup.We calculate the first-moment radial velocities and fit the spectral lineprofiles by a bi-Gaussian to derive line asymmetries. Correlations between thegamma-velocity and the gamma-asymmetry (defined as the average values of theradial velocity and line asymmetry curves respectively) are used to derive thecenter-of-mass velocity of the star. By combining our spectroscopicdetermination of the atmospheric velocity gradient with a hydrodynamicalmodelof the photosphere of the star, we derive a semi-theoretical projectionfactor for alpha Lup. We find a center-of-mass velocity of Vgamma = 7.9 +/- 0.6km/s and that the velocity gradient in the atmosphere of alpha Lup isnull. Weapply to alpha Lup the usual decomposition of the projection factor into threeparts, p = p0 fgrad fog (originally developed for Cepheids), and derive aprojection factor of p = 1.43 +/-0.01. By comparing our results with previousHARPS observations of classical Cepheids, we also point out a linear relationbetween the atmospheric velocity gradient and the amplitude of the radialvelocity curve. Moreover, we observe a phase shift (Van Hoof effect), whereasalpha Lup has no velocity gradient.

astronomy and astrophysics

Understanding the dynamical structure of pulsating stars: The  center-of-mass velocity and the Baade-Wesselink projection factor of the  beta-Cephei star alpha-Lupi