Understanding the dynamical structure of pulsating stars: The Baade-Wesselink projection factor of the δ Scuti stars AI Velorum and β Cassiopeiae⋆
1 Laboratoire Lagrange, UMR 7293, UNS/CNRS/OCA, BP 4229, 06304 Nice Cedex 4, France
2 Institut de Recherche en Astrophysique et Planétologie, UMR 5277, 57 avenue d’Azereix, 65000 Tarbes, France
3 INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
4 Institute of Astronomy of the Russian Academy of Sciences, 48 Pjatnitskaya Str., 109017 Moscow, Russia
5 Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, CL Concepción, Chile
Received: 23 November 2012
Accepted: 2 January 2013
Aims. The Baade-Wesselink method of distance determination is based on the oscillations of pulsating stars. The key parameter of this method is the projection factor used to convert the radial velocity into the pulsation velocity. Our analysis was aimed at deriving for the first time the projection factor of δ Scuti stars, using high-resolution spectra of the high-amplitude pulsator AI Vel and of the fast rotator β Cas.
Methods. The geometric component of the projection factor (i.e. p0) was calculated using a limb-darkening model of the intensity distribution for AI Vel, and a fast-rotator model for β Cas. Then, using SOPHIE/OHP data for β Cas and HARPS/ESO data for AI Vel, we compared the radial velocity curves of several spectral lines forming at different levels in the atmosphere and derived the velocity gradient associated to the spectral-line-forming regions in the atmosphere of the star. This velocity gradient was used to derive a dynamical projection factor p.
Results. We find a flat velocity gradient for both stars and finally p = p0 = 1.44 for AI Vel and p = p0 = 1.41 for β Cas. By comparing Cepheids and δ Scuti stars, these results bring valuable insights into the dynamical structure of pulsating star atmospheres. They suggest that the period-projection factor relation derived for Cepheids is also applicable to δ Scuti stars pulsating in a dominant radial mode.
Key words: stars: oscillations / stars: atmospheres / stars: variables:δScuti / techniques: spectroscopic
© ESO, 2013