The atmosphere, the p-factor and the bright visible circumstellar environment of the prototype of classical Cepheids δ Cep

Nicolas Nardetto; Ennio Poretti; Antoine Mérand; Richard I. Anderson; Andrei Fokin; Pascal Fouqué; Alexandre Gallenne; Wolfgang Gieren; Dariusz Graczyk; Pierre Kervella; Philippe Mathias; Denis Mourard; Hilding Neilson; Grzegorz Pietrzynski; Bogumil Pilecki; Monica Rainer; Jesper Storm

doi:10.1051/epjconf/201715207003

Proceedings

Open Access

EPJ Web of Conferences 152, 07003 (2017)
https://doi.org/10.1051/epjconf/201715207003

The atmosphere, the p-factor and the bright visible circumstellar environment of the prototype of classical Cepheids δ Cep

Nicolas Nardetto¹^*, Ennio Poretti², Antoine Mérand³, Richard I. Anderson⁴, Andrei Fokin⁵, Pascal Fouqué⁶, Alexandre Gallenne³^,7, Wolfgang Gieren⁷^,8, Dariusz Graczyk⁷^,8^,9, Pierre Kervella¹⁰^,11, Philippe Mathias¹²^,13, Denis Mourard¹, Hilding Neilson¹⁴, Grzegorz Pietrzynski⁹, Bogumil Pilecki⁹, Monica Rainer² and Jesper Storm¹⁵

¹ Laboratoire Lagrange, UMR7293, Univ. de Nice Sophia-Antipolis, CNRS, Obs.de la Côte d’Azur, France
² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate ( LC), Italy
³ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
⁴ Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD, 21218, USA
⁵ Institute of Astronomy of the Russian Academy of Sciences, 48 Pjatnitskaya Str., 109017, Moscow, Russia
⁶ Observatoire Midi-Pyrénées, UMR 5572, Université Paul Sabatier-Toulouse, France
⁷ Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
⁸ Millennium Institute of Astrophysics, Santiago, Chile
⁹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Warszawa, Poland
¹⁰ LESIA (UMR 8109), Obs. de Paris, PSL, CNRS, UPMC, Univ. Paris-Diderot, Meudon, France
¹¹ 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
¹² Univ. de Toulouse, UPS-OMP, Institut de recherche en Astrophysique et Planétologie, Toulouse, France
¹³ CNRS, UMR5277, Institut de recherche en Astrophysique et Planétologie, Toulouse, France
¹⁴ Department of Astronomy & Astrophysics, University of Toronto, Toronto, ON, M5S 3H4, Canada
¹⁵ Leibniz Institute for Astrophysics, An der Sternwarte 16, 14482, Potsdam, Germany

^* Nicolas.Nardetto@oca.eu

Published online: 8 September 2017

Abstract

Even ≃ 16000 cycles after its discovery by John Goodricke in 1783, δ Cep, the prototype of classical Cepheids, is still studied intensively in order to better understand its atmospheric dynamical structure and its environment. Using HARPS-N spectroscopic measurements, we have measured the atmospheric velocity gradient of δ Cep for the first time and we confirm the decomposition of the projection factor, a subtle physical quantity limiting the Baade-Wesselink (BW) method of distance determination. This decomposition clarifies the physics behind the projection factor and will be useful to interpret the hundreds of p-factors that will come out from the next Gaia release. Besides, VEGA/CHARA interferometric observations of the star revealed a bright visible circumstellar environment contributing to about 7% to the total flux. Better understanding the physics of the pulsation and the environment of Cepheids is necessary to improve the BW method of distance determination, a robust tool to reach Cepheids in the MilkyWay, and beyond, in the Local Group.

© The Authors, published by EDP Sciences, 2017

This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.