Understanding the dynamical structure of pulsating stars

HARPS spectroscopy of the δ Scuti stars ρ Puppis and DX Ceti ^⋆

¹ Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, Nice, France
e-mail: Nicolas.Nardetto@oca.eu
² INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
³ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁴ Université de Toulouse, UPS-OMP, IRAP, 65000 Tarbes, France
⁵ CNRS, IRAP, 57 avenue d’Azereix, BP 826, 65008 Tarbes, France

Received: 24 July 2013
Accepted: 5 December 2013

Abstract

Context. High-resolution spectroscopy is a powerful tool to study the dynamical structure of a pulsating star’s atmosphere.

Aims. We aim at comparing the line asymmetry and velocity of the two δ Sct stars ρ Pup and DX Cet with previous spectroscopic data obtained on classical Cepheids and β Cep stars.

Methods. We obtained, analysed and discuss HARPS high-resolution spectra of ρ Pup and DX Cet. We derived the same physical quantities as used in previous studies, which are the first-moment radial velocities and the bi-Gaussian spectral line asymmetries.

Results. The identification of f = 7.098 d^-1 as a fundamental radial mode and the very accurate Hipparcos parallax promote ρ Pup as the best standard candle to test the period–luminosity relations of δ Sct stars. The action of small-amplitude nonradial modes can be seen as well-defined cycle-to-cycle variations in the radial velocity measurements of ρ Pup. Using the spectral-line asymmetry method, we also found the centre-of-mass velocities of ρ Pup and DX Cet, V_γ = 47.49 ± 0.07 km s^-1 and V_γ = 25.75 ± 0.06 km s^-1, respectively. By comparing our results with previous HARPS observations of classical Cepheids and β Cep stars, we confirm the linear relation between the atmospheric velocity gradient and the amplitude of the radial velocity curve, but only for amplitudes larger than 22.5 km s^-1. For lower values of the velocity amplitude (i.e., <22.5 km s^-1), our data on ρ Pup seem to indicate that the velocity gradient is null, but this result needs to be confirmed with additional data. We derived the Baade-Wesselink projection factor p = 1.36 ± 0.02 for ρ Pup and p = 1.39 ± 0.02 for DX Cet. We successfully extended the period–projection factor relation from classical Cepheids to δ Scuti stars.