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

• Main Authors: 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
• Format: Article
• Language: English
• Published: EDP Sciences 2017-01-01
• Series: EPJ Web of Conferences
• Online Access: https://doi.org/10.1051/epjconf/201715207003
• ISSN: 2100-014X

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.