Probing the dynamical structure of δ Cephei atmosphere

¹ Observatoire de la Côte d'Azur, Dpt. Gemini, UMR 6203, 06130 Grasse, France e-mail: Nicolas.Nardetto@obs-azur.fr
² Institute of Astronomy of the Russian Academy of Sciences, 48 Pjatnitskaya Str., Moscow 109017, Russia

Received: 29 September 2005
Accepted: 11 March 2006

Abstract

Context.Limb darkening and the projection factor are currently two limiting aspects of the interferometric Baade-Wesselink method of Cepheid distance determination.

Aims.We first quantify the impact of the phase dependence of limb darkening on the derived distance. We then study a new way to probe the dynamical structure of Cepheid's atmosphere through spectro-interferometric observations.

Methods.A hydrodynamical model of δ Cep is used to derive stellar disk intensity distribution in the continuum and in different spectral lines, together with the corresponding wavelength- and phase-dependent visibility curves.

Results.We find that considering a constant limb darkening in the visible leads to a systematic shift of about 0.02 in phase on the angular diameter curve. The derived distance is, however, not affected by this effect. Otherwise, for a spectroscopic resolution of in the visible, we find in the most favourable case (maximum contraction velocity) a signature on the visibility curve of about 7% that is clearly detectable by current spectro-interferometers. Nevertheless, the projection factor has only a 1% (or less) effect on the visibility curve.

Conclusions.The spectro-interferometry provides a new geometric view of Cepheid's atmosphere. However, the combination of different techniques (high resolution spectroscopy, spectro- and differential-interferometry) are now needed to efficiently constrain the physical parameters of Cepheids' atmosphere and, in particular the projection factor.