VEGA/CHARA interferometric observations of Cepheids

I. A resolved structure around the prototype classical Cepheid δ Cep in the visible spectral range^⋆

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, UMR7293, Nice, France
e-mail: Nicolas.Nardetto@oca.eu
² European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
³ Leibniz Institute for Astrophysics, An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
⁵ Millenium Institute of Astrophysics, Santiago, Chile
⁶ Observatoire Midi-Pyrénées, Laboratoire d’Astrophysique, UMR 5572, Université Paul Sabatier – Toulouse 3, 14 avenue Édouard Belin, 31400 Toulouse, France
⁷ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warszawa, Poland
⁸ LESIA (UMR 8109), Observatoire de Paris, PSL, CNRS, UPMC, Univ. Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
⁹ Unidad Mixta Internacional Franco-Chilena de Astronomía, CNRS/INSU, France (UMI 3386) and Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
¹⁰ Department of Astronomy & Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON, M5S 3H4, Canada
¹¹ Laboratoire Dynamique Moléculaire et Matériaux Photoniques, UR11ES03, Université de Tunis/ESSTT, Tunisie
¹² Université de Toulouse, UPS-OMP, Institut de recherche en Astrophysique et Planétologie, Toulouse, France
¹³ CNRS, UMR5277, Institut de recherche en Astrophysique et Planétologie, 14 avenue Édouard Belin, 31400 Toulouse, France
¹⁴ Université Grenoble Alpes, IPAG, 38000 Grenoble, France
¹⁵ CNRS, IPAG, 38000 Grenoble, France
¹⁶ INAF–Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate ( LC), Italy
¹⁷ Université de Lyon, Université Lyon 1, École Normale Supérieure de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France
¹⁸ Georgia State University, PO Box 3969, Atlanta GA 30302-3969, USA
¹⁹ CHARA Array, Mount Wilson Observatory, 91023 Mount Wilson CA, USA

Received: 18 December 2015
Accepted: 6 June 2016

Abstract

Context. The B-W method is used to determine the distance of Cepheids and consists in combining the angular size variations of the star, as derived from infrared surface-brightness relations or interferometry, with its linear size variation, as deduced from visible spectroscopy using the projection factor. The underlying assumption is that the photospheres probed in the infrared and in the visible are located at the same layer in the star whatever the pulsation phase. While many Cepheids have been intensively observed by infrared beam combiners, only a few have been observed in the visible.

Aims. This paper is part of a project to observe Cepheids in the visible with interferometry as a counterpart to infrared observations already in hand.

Methods. Observations of δ Cep itself were secured with the VEGA/CHARA instrument over the full pulsation cycle of the star.

Results. These visible interferometric data are consistent in first approximation with a quasi-hydrostatic model of pulsation surrounded by a static circumstellar environment (CSE) with a size of θ_CSE = 8.9 ± 3.0 mas and a relative flux contribution of f_CSE = 0.07 ± 0.01. A model of visible nebula (a background source filling the field of view of the interferometer) with the same relative flux contribution is also consistent with our data at small spatial frequencies. However, in both cases, we find discrepancies in the squared visibilities at high spatial frequencies (maximum 2σ) with two different regimes over the pulsation cycle of the star, φ = 0.0 − 0.8 and φ = 0.8−1.0. We provide several hypotheses to explain these discrepancies, but more observations and theoretical investigations are necessary before a firm conclusion can be drawn.

Conclusions. For the first time we have been able to detect in the visible domain a resolved structure around δ Cep. We have also shown that a simple model cannot explain the observations, and more work will be necessary in the future, both on observations and modelling.