Observational calibration of the projection factor of Cepheids

III. The long-period Galactic Cepheid RS Puppis^⋆

¹ Unidad Mixta Internacional Franco-Chilena de Astronomía (CNRS UMI 3386), Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
e-mail: pkervell@das.uchile.cl
² LESIA (UMR 8109), Observatoire de Paris, PSL Research University, CNRS, UPMC, Univ. Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
e-mail: pierre.kervella@obspm.fr
³ Department of Astronomy & Astrophysics, 525 Davey Lab., Pennsylvania State University, University Park, PA 16802 USA
⁴ European Southern Observatory, 3107 Alonso de Córdova, Casilla 19001, Santiago 19, Chile
⁵ Konkoly Observatory, MTA CSFK, Konkoly Thege M. út 15–17, 1121 Budapest Hungary
⁶ Institut Supérieur de l’Aéronautique et de l’Espace, 10 avenue Édouard Belin, 31400 Toulouse, France
⁷ Physics and Astronomy Department, The Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218, USA
⁸ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
⁹ IRAP, UMR 5277, CNRS, Université de Toulouse, 14 avenue Édouard Belin, 31400 Toulouse, France
¹⁰ CFHT Corporation, 65-1238 Mamalahoa Hwy, Kamuela, Hawaii 96743, USA
¹¹ Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Chile
¹² Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06000 Nice, France
¹³ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warszawa, Poland

Received: 6 December 2016
Accepted: 11 January 2017

Abstract

The projection factor (p-factor) is an essential component of the classical Baade-Wesselink (BW) technique, which is commonly used to determine the distances to pulsating stars. It is a multiplicative parameter used to convert radial velocities into pulsational velocities. As the BW distances are linearly proportional to the p-factor, its accurate calibration for Cepheids is of critical importance for the reliability of their distance scale. We focus on the observational determination of the p-factor of the long-period Cepheid RS Pup (P = 41.5 days). This star is particularly important as this is one of the brightest Cepheids in the Galaxy and an analog of the Cepheids used to determine extragalactic distances. An accurate distance of 1910 ± 80 pc (± 4.2%) has recently been determined for RS Pup using the light echoes propagating in its circumstellar nebula. We combine this distance with new VLTI/PIONIER interferometric angular diameters, photometry, and radial velocities to derive the p-factor of RS Pup using the code Spectro-Photo-Interferometry of Pulsating Stars (SPIPS). We obtain p = 1.250 ± 0.064 ( ± 5.1%), defined for cross-correlation radial velocities. Together with measurements from the literature, the p-factor of RS Pup confirms the good agreement of a constant $\overline{\mathit{p}}\mathrm{=}\mathrm{1.293}\mathrm{\pm }\mathrm{0.039}\mathrm{(}\mathrm{\pm }\mathrm{3.0}\mathrm{\%}\mathrm{)}$ model with the observations. We conclude that the p-factor of Cepheids is constant or mildly variable over a broad range of periods (3.7 to 41.5 days).