The detection of the very low amplitude 16.2-min pulsation in individual lines of first ionization stage of rare earth elements in the roAp star β CrB

Abstract

Abstract With high precision spectroscopic data obtained with Ultraviolet and Visual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) we detect radial velocity pulsation with a period of 16.2 min and with amplitudes of about 30 m s−1 in individual lines of the first ionization stage of rare earth elements, and in Hα, with typical precision in the range 2–8 m s−1 for a single line, and better than 1 m s−1 for some ensembles of lines. The second ionization stages of rare earth elements show weak or no detectable pulsation, while lines of Fe i or Fe ii show no signal at high precision. An abundance analysis shows the rare earth elements to be overabundant by about 1 dex, rather than the 2–3 dex often seen in other rapidly oscillating Ap (roAp) stars, and it shows the presence of the ionization disequilibrium for Nd and Pr that is typical of most roAp stars. The pulsational radial velocity curve reaches maximum for the lines of the rare earth elements before it does for Hα which is opposite to the case for most other roAp stars with implications for the stratification level of the rare earth elements. Similarities between β CrB and the longest period roAp star, HD 116114, suggest that many more low-amplitude roAp stars await detection, with implications for understanding the pulsation driving mechanism and mode selection mechanism in these stars.