EPJ Web of Conferences (Jan 2017)

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

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

DOI
https://doi.org/10.1051/epjconf/201715207003
Journal volume & issue
Vol. 152
p. 07003

Abstract

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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.