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Wesenheit function for Galactic Cepheids: Application to the projection factors

Published online by Cambridge University Press:  26 February 2013

Chow-Choong Ngeow
Graduate Institute of Astronomy, National Central University, Jhongli City, 32001, Taiwan email:
Hilding Neilson
Argelander Institute for Astronomy, Auf dem Huegel 71, 53121 Bonn, Germany
Nicolas Nardetto
Laboratoire Lagrange, UMR7293, UNSA/CNRS/OCA, 06300 Nice, France
Massimo Marengo
Department of Physics and Astronomy, Iowa State University, Ames, IA 50010, USA
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Galactic Cepheids are necessary tools for calibrating the period–luminosity relation, but distances to individual Galactic Cepheids are difficult to measure precisely and their application is limited to a small number of techniques, such as direct parallax measurements, main-sequence fitting to open clusters that host Cepheids, and Baade–Wesselink (BW)-type methods. Here, we re-examine the application of Wesenheit functions in determining distances to more than 300 Galactic Cepheids by taking advantage of the fact that the Wesenheit function is extinction-free by definition. Wesenheit distances are used to calibrate the projection (p) factor for Galactic Cepheids that also have BW distances. Based on ~ 70 Cepheids, we find that the period–p-factor relation may exhibit a nonlinear trend with a considerable scatter. We found discrepant p factors for δ Cephei in the literature. This may be due to inconsistent measurements of its angular diameter using different empirical techniques. We discuss the reason for the inconsistency in angular-diameter measurements and offer a possible remedy.

Contributed Papers
Copyright © International Astronomical Union 2013


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