The discovery of remarkable 5 km s-1 pulsational radial velocity variations in the roAp star HD 99563*

Abstract

In a high-resolution spectral survey of nearly half the 34 known rapidly oscillating Ap (roAp) stars, using the Ultraviolet-Visual Echelle Spectrograph on the Very Large Telescope, we have discovered remarkably large amplitude pulsations in the roAp star HD 99563 with some spectral lines showing radial velocity amplitudes up to 5 km s−1 (10 km s−1 peak-to-peak) with a pulsation period of 10.7 min. As for many other roAp stars, we find the largest pulsation amplitudes for lines of some rare earth elements and in the core of the Hα line. The highest amplitudes of 5 km s−1 are seen in rather weak lines of Eu ii and Tm ii. Stronger lines of Pr iii and Nd iii have pulsation amplitudes in the range 0.7 to 3.5 km s−1 for different lines. In the narrow Hα core, the average amplitude is 2.6 km s−1, but, as is the case for other lines, the amplitude and phase vary strongly with line depth (atmospheric height), with the amplitude of the radial velocity variations of the line bisector reaching a maximum of 4.3 km s−1 at the bottom of the core. Some other elements show pulsation amplitudes 0.1 to 0.7 km s−1. Variations in velocity amplitude and phase for several spectral lines were studied using line-bisector measurements to obtain information about the vertical structure of the pulsation modes and the stellar atmosphere.