Understanding the dynamical structure of pulsating stars: The center-of-mass velocity and the Baade-Wesselink projection factor of the beta-Cephei star alpha-Lupi

TL;DR

This study uses high-resolution spectroscopy to determine the center-of-mass velocity and projection factor of the beta-Cephei star alpha-Lup, providing insights into its atmospheric dynamics and refining distance measurement techniques.

Contribution

It presents the first spectroscopic determination of alpha-Lup's center-of-mass velocity and a semi-theoretical projection factor, linking atmospheric velocity gradients to radial velocity amplitudes.

Findings

Center-of-mass velocity Vγ = 7.9 ± 0.6 km/s

Velocity gradient in alpha-Lup's atmosphere is null

Derived projection factor p = 1.43 ± 0.01

Abstract

High-resolution spectroscopy of pulsating stars is a powerful tool to study the dynamical structure of their atmosphere. Lines asymmetry is used to derive the center-of-mass velocity of the star, while a direct measurement of the atmospheric velocity gradient helps determine the projection factor used in the Baade-Wesselink method of distance determination. We aim at deriving the center-of-mass velocity and the projection factor of the beta-Cephei star alpha-Lup. We present HARPS high spectral resolution observations of alpha-Lup. We calculate the first-moment radial velocities and fit the spectral line profiles by a bi-Gaussian to derive line asymmetries. Correlations between the gamma-velocity and the gamma-asymmetry (defined as the average values of the radial velocity and line asymmetry curves respectively) are used to derive the center-of-mass velocity of the star. By combining our spectroscopic determination of the atmospheric velocity gradient with a hydrodynamical modelof the photosphere of the star, we derive a semi-theoretical projection factor for alpha Lup. We find a center-of-mass velocity of Vgamma = 7.9 +/- 0.6 km/s and that the velocity gradient in the atmosphere of alpha Lup isnull. We apply to alpha Lup the usual decomposition of the projection factor into three parts, p = p0 fgrad fog (originally developed for Cepheids), and derive a projection factor of p = 1.43 +/-0.01. By comparing our results with previous HARPS observations of classical Cepheids, we also point out a linear relation between the atmospheric velocity gradient and the amplitude of the radial velocity curve. Moreover, we observe a phase shift (Van Hoof effect), whereas alpha Lup has no velocity gradient.