Letter to the Editor

The orbital parameters of the δ Cep inner binary system determined using 2019 HARPS-N spectroscopic data

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
e-mail: nicolas.nardetto@oca.eu
² Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warszawa, Poland
³ LESIA (UMR 8109), Observatoire de Paris, PSL, CNRS, UPMC, Univ. Paris-Diderot, 5 place Jules Janssen, Meudon, France
⁴ Instituto de Astrofísica, Departamento de Ciencias Físicas, Facultad de Ciencias Exactas, Universidad Andrés Bello, Fernández Concha 700, Las Condes, Santiago, Chile
⁵ Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Chile
⁶ Unidad Mixta Internacional Franco-Chilena de Astronomía (CNRS UMI 3386), Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁷ European Southern Observatory, Alonso de Córdova 3107, Casilla, 19001 Santiago 19, Chile
⁸ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
⁹ Leibniz Institute for Astrophysics, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 13 February 2024
Accepted: 22 March 2024

Abstract

Context. An inner companion has recently been discovered orbiting the prototype of classical Cepheids, δ Cep, whose orbital parameters are still not fully constrained.

Aims. We collected new precise radial velocity measurements of δ Cep in 2019 using the HARPS-N spectrograph mounted at the Telescopio Nazionale Galileo. Using these radial velocity measurements, we aimed to improve the orbital parameters of the system.

Methods. We considered a template available in the literature as a reference for the radial velocity curve of the pulsation of the star. We then calculated the residuals between our global dataset (composed of the new 2019 observations plus data from the literature) and the template as a function of the pulsation phase and the barycentric Julian date. This provides the orbital velocity of the Cepheid component. Using a Bayesian tool, we derived the orbital parameters of the system.

Results. Considering priors based on already published Gaia constraints, we find for the orbital period a maximum a posteriori probability of P_orb = 9.32_−0.04^+0.03 years (uncertainties correspond to the 95% highest density probability interval), and we obtain an eccentricity e = 0.71_−0.02^+0.02, a semimajor axis a = 0.029_−0.003^+0.002 arcsec, and a center-of-mass velocity V₀ = −17.28_−0.08^+0.08 km s⁻¹, among other parameters.

Conclusions. In this short analysis we derive the orbital parameters of the δ Cep inner binary system and provide a cleaned radial velocity curve of the pulsation of the star, which will be used to study its Baade–Wesselink projection factor in a future publication.