Letter to the Editor

The projection factor of δ Cephei

A calibration of the Baade-Wesselink method using the CHARA Array

¹ LESIA, UMR8109, Observatoire de Paris-Meudon, 5, place Jules Janssen, 92195 Meudon Cedex, France e-mail: antoine.merand@obspm.fr
² National Optical Astronomical Observatory 950 North Cherry Avenue, Tucson, AZ 85719, USA
³ Center for High Angular Resolution Astronomy, Georgia State University, PO Box 3965, Atlanta, Georgia 30302-3965, USA

Received: 6 May 2005
Accepted: 7 June 2005

Abstract

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, δ Cep. The value we determined, ± 0.06, confirms the generally adopted value of within 1.5 sigmas. Our value is in line with recent theoretical predictions of Nardetto et al. (2004, A&A, 428, 131).