Volume 438, Number 1, July IV 2005
|Page(s)||L9 - L12|
|Published online||06 July 2005|
Letter to the Editor
The projection factor of δ Cephei
A calibration of the Baade-Wesselink method using the CHARA Array
LESIA, UMR8109, Observatoire de Paris-Meudon, 5, place Jules Janssen, 92195 Meudon Cedex, France e-mail: firstname.lastname@example.org
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
Accepted: 7 June 2005
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).
Key words: techniques: interferometric / stars: variables: Cepheids / stars: individual: δ Cep / cosmology: distance scale
© ESO, 2005
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