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Issue A&A
Volume 438, Number 1, July IV 2005
Page(s) L9 - L12
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200500139



A&A 438, L9-L12 (2005)
DOI: 10.1051/0004-6361:200500139

Letter

The projection factor of $\mathsf{\delta}$ Cephei

A calibration of the Baade-Wesselink method using the CHARA Array
A. Mérand1, P. Kervella1, V. Coudé du Foresto1, S. T. Ridgway1, 2, 3, J. P. Aufdenberg2, T. A. ten Brummelaar3, D. H. Berger3, J. Sturmann3, L. Sturmann3, N. H. Turner3 and H. A. McAlister3

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, $\delta$ 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, A&A, 428, 131).


Key words: techniques: interferometric -- stars: variables: Cepheids -- stars: individual: $\delta$ Cep -- cosmology: distance scale

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© ESO 2005


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