Pulsation properties of the rapidly oscillating Ap star 10 Aquilae (HD 176232)

Abstract

We have studied the pulsation behaviour of radial velocities of individual spectral lines of different chemical elements for the well known, bright rapidly oscillating Ap (roAp) star 10 Aql (HD 176232) based on high-quality Very Large Telescope spectra. We found that only lines of rare earth elements and the narrow Hα core show detectable rapid variations of radial velocity with the known photometric pulsation period, 11.7 min. The highest pulsation amplitudes of more than 0.5 km s−1 are found for rather weak lines of Tb iii and Dy iii. The pulsational phases obtained for lines of Tb iii and Dy iii differ by π rad, perhaps an indication of a radial node between their line-forming layers. Weak lines of Sm ii also display amplitudes near 0.5 km s−1. Surprisingly, the lines of Pr iii show the lowest detected pulsation amplitude in strong contrast with the majority of roAp stars for which this ion has some of the highest radial velocity amplitudes. Lines of Nd iii with different intensities have pulsation amplitudes different to the weaker lines that are formed more deeply in the atmosphere having higher amplitudes. Line bisectors for strong Nd iii line profiles show significant changes of phase, and even phase jumps for some lines, indicating complex variations in the pulsation phase as a function of atmospheric depth. The low-measured rotational velocity of this star indicates that there is little Doppler resolution of the line profiles, hence that the large range of pulsation amplitudes and phases of radial velocities determined for line bisectors are primarily caused by atmospheric depth effects.