The long-period Galactic Cepheid RS Puppis^⋆,⋆⋆

III. A geometric distance from HST polarimetric imaging of its light echoes

¹ LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris-Diderot, PSL Research University, 5 place Jules Janssen, 92195 Meudon, France
e-mail: pierre.kervella@obspm.fr
² Department of Astronomy and Astrophysics, 525 Davey Lab., Pennsylvania State University, University Park, PA 16802, USA
³ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
⁴ Konkoly Observatory, 1525 Budapest XII, PO Box 67, Hungary
⁵ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
⁶ Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Chile
⁷ Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park, 146 Brownlow Hill Liverpool, L3 5RF, UK
⁸ IRAP, UMR 5277, CNRS, Université de Toulouse, 14 av. E. Belin, 31400 Toulouse, France
⁹ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland

Received: 13 June 2014
Accepted: 29 July 2014

Abstract

As one of the most luminous Cepheids in the Milky Way, the 41.5-day RS Puppis is an analog of the long-period Cepheids used to measure extragalactic distances. An accurate distance to this star would therefore help anchor the zero-point of the bright end of the period-luminosity relation. But, at a distance of about 2 kpc, RS Pup is too far away for measuring a direct trigonometric parallax with a precision of a few percentage points with existing instrumentation. RS Pup is unique by being surrounded by a reflection nebula whose brightness varies as pulses of light from the Cepheid propagate outward. We present new polarimetric imaging of the nebula obtained with HST/ACS. The derived map of the degree of linear polarization p_L allows us to reconstruct the three-dimensional structure of the dust distribution. To retrieve the scattering angle from the p_L value, we consider two different polarization models, one based on a Milky Way dust mixture and one assuming Rayleigh scattering. Considering the derived dust distribution in the nebula, we adjust a model of the phase lag of the photometric variations over selected nebular features to retrieve the distance of RS Pup. We obtain a distance of 1910 ± 80 pc (4.2%), corresponding to a parallax of π = 0.524 ± 0.022 mas. The agreement between the two polarization models that we considered is good, but the final uncertainty is dominated by systematics in the adopted model parameters. The distance we obtain is consistent with existing measurements from the literature, but light echoes provide a distance estimate that is not subject to the same systematic uncertainties as other estimators (e.g., the Baade-Wesselink technique). RS Pup therefore provides an important fiducial for calibrating the systematic uncertainties of the long-period Cepheid distance scale.