High time resolution spectroscopy and magnetic variability of the cool Ap star HD 965

Abstract

We present the results of an investigation of the magnetic Ap star HD 965 with high spectral and time resolution. We determine precise radial velocities using spectra obtained with the Ultraviolet‐Visual Echelle Spectrograph (UVES) on the European Southern Observatory Very Large Telescope. Special attention is given to spectral lines of rare-earth elements which in rapidly oscillating Ap (roAp) stars exhibit the strongest radial velocity variations with pulsation period. Careful time series analysis did not detect any convincing evidence of pulsation in HD 965 with an upper limit in amplitude of 15‐20 m s −1 . All properties of HD 965 are similar to the majority of roAp stars. A likely reason for the apparent lack of pulsation could be connected with the geometrical structure of the magnetic field and the aspect of the star at the time of observation. Longitudinal magnetic field measurements for HD 965 showed that our UVES spectral observations were carried out when the longitudinal field was near zero and therefore, according to the oblique rotator model, near a time when the star was viewed from the magnetic equator. For a dipole oscillation aligned with the magnetic field, as is typical of roAp stars, no variation can be detected at this aspect. We may, therefore, expect to detect rapid oscillations in HD 965 in the future, when the star will present one of the magnetic poles. Ke yw ords: stars: individual: HD 965 ‐ stars: magnetic fields ‐ stars: oscillations ‐ stars: variables: other.