The projection factor of delta Cephei A calibration of the Baade-Wesselink method using the CHARA Array

Mérand Antoine (1) , Pierre Kervella (1) , Vincent Coude Du Foresto (1) , Stephen T. Ridgway (1, 2, 3) , Jason P. Aufdenberg (2) , Theo A. ten Brummelaar (3) , David H. Berger (3) , Judit Sturmann (3) , Laszlo Sturmann (3) , Nils H. Turner (3) , Harold A. Mcalister (3)

1 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
2 NOAO - National Optical Astronomy Observatory
3 CHARA - Center for High Angular Resolution Astronomy

Cepheids play a key role in astronomy as standard candles for measuring intergalactic distances. Their distance is usually inferred from the Period-Luminosity relationship, calibrated using the semi-empirical Baade-Wesselink method. Using this method, the distance is known to a multiplicative factor, called the projection factor. Presently, this factor is computed using numerical models - it has hitherto never been measured directly. Based on our new interferometric measurements obtained with the CHARA Array and the already published parallax, we present a geometrical measurement of the projection factor of a Cepheid, delta Cep. The value we determined, p = 1.27$\pm$0.06, confirms the generally adopted value of p = 1.36 within 1.5 sigmas. Our value is in line with recent theoretical predictions of Nardetto et al. (2004).

Mots clés: Techniques: interferometric; Stars: variables: Cepheids; Stars: individual: $\delta$ Cep; Cosmology: distance scale

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]

Mérand Antoine, Pierre Kervella, Vincent Coude Du Foresto, Stephen T. Ridgway, Jason P. Aufdenberg, et al.. The projection factor of delta Cephei A calibration of the Baade-Wesselink method using the CHARA Array. 2005. ⟨hal-00005680⟩