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

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, p = 1.27 ± 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).

[1]  Wesley A. Traub,et al.  New Frontiers in Stellar Interferometry , 2007 .

[2]  Pascal Bordé,et al.  A catalog of bright calibrator stars for 200-m baseline near-infrared stellar interferometry , 2004, astro-ph/0412251.

[3]  D. James,et al.  The dependence of differential rotation on temperature and rotation , 2004, astro-ph/0410575.

[4]  D. Bersier,et al.  Self consistent modelling of the projection factor for interferometric distance determination , 2004 .

[5]  D. Bersier,et al.  Cepheid distances from infrared long-baseline interferometry - I. VINCI/VLTI observations of seven Galactic Cepheids , 2003, astro-ph/0311525.

[6]  J. Armstrong,et al.  Theoretical Limb Darkening for Classical Cepheids. II. Corrections for the Geometric Baade-Wesselink Method , 2003, astro-ph/0304084.

[7]  D. Ségransan,et al.  The diameters of $\alpha$ Centauri A and B - A comparison of the asteroseismic and VINCI/VLTI views , 2003, astro-ph/0303634.

[8]  Stephen T. Ridgway,et al.  FLUOR fibered beam combiner at the CHARA array , 2003, SPIE Astronomical Telescopes + Instrumentation.

[9]  Laszlo Sturmann,et al.  An Update of the CHARA Array , 2003, SPIE Astronomical Telescopes + Instrumentation.

[10]  M. Skrutskie,et al.  Astrometry with the Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator δ Cephei , 2002, astro-ph/0206214.

[11]  H. Ford,et al.  Final Results from the Hubble Space Telescope Key Project to Measure the Hubble Constant , 1998, astro-ph/9801080.

[12]  J.-M. Mariotti,et al.  Deriving object visibilities from interferograms obtained with a fiber stellar interferometer , 1997 .

[13]  K. A. Venn,et al.  On Spectral Line Formation and Measurement in Cepheids: Implications to Distance Determination , 1995 .

[14]  T. G. Barnes,et al.  Observational studies of Cepheids. III: Catalog of light curve parameters , 1984 .

[15]  S. Parsons.  The Conversion Factor from Radial to Pulsational Velocity and the Radii of Classical Cepheids , 1972 .

[16]  W. Baade,et al.  Über eine Möglichkeit, die Pulsationstheorie der δ Cephei‐Veränderlichen zu prüfen , 1926 .