The p and k−factors of the Classical Cepheid δ Cep

Nicolas Nardetto, Ennio Poretti, Richard I. Anderson, Andrei Fokin, Alexandre Gallenne, Wolfgang Gieren, Dariusz Graczyk, Pierre Kervella, Philippe Mathias, Antoine Mérand, Denis Mourard, Hilding Neilson, Grzegorz Pietrzyński, Bogumił Pilecki, Monica Rainer and Jesper Storm

Probing the dynamical structure of Cepheid atmospheres is of primary importance in order to better constrain the cosmic distance ladder. It is indeed now well-known that the projection factor (or p-factor), that is used in the Baade-Wesselink method of distance determination of Cepheids, is sensitive to the atmo- spheric velocity gradient and other subtle dynamical effects. Relying on HARPS-N high resolution spectroscopic observations, we have measured the atmospheric velocity gradient of δ Cep for the first time, and we found an agreement with the hydrodynamical model of the star. Besides, the mysterious k-term, a residual γ-velocity of the order of 1-2 km s^-1 , seen in HARPS-N data of δ Cep, is now physically understood thanks to recently published predicted spectroscopic properties of two-dimensional time-dependent Cepheid models.

Proceedings of the Polish Astronomical Society, vol. 6, 217-221 (2018)

Download full article as PDF file: