Pulsations in the atmosphere of the rapidly oscillating Ap star 10 Aquilae★

Abstract

The rapidly oscillating Ap (roAp) star 10 Aql shows one of the lowest photometric pulsation amplitudes and is characterized by an unusual spectroscopic pulsational behavior compared to other roAp stars. More than 1000 spectra were taken during 7 nights over a time span of 21 days with high-resolution spectrographs at the 8-m ESO VLT and 3.6-m TNG telescopes giving access to radial velocity variations of about 150 lines from different chemical species. A comparison of pulsation signatures in lines formed at different atmospheric heights allowed us to resolve the vertical structure of individual pulsation modes in 10 Aql which is the first time for a multiperiodic roAp star. The inferred propagation of pulsation waves in 10 Aql is qualitatively similar to other roAp stars: pulsation amplitudes become measurable in the layers where Y and Eu are concentrated, increase in layers where the Halpha core is formed, reach a maximum of 200-300 m/s in the layers probed by Ce, Sm, Dy lines and then decrease to 20-50 m/s in the layers where NdIII and PrIII lines are formed. A unique pulsation feature of 10 Aql is a second pulsation maximum indicated by TbIII lines which form in the uppermost atmospheric layers and oscillate with amplitudes of up to 350 m/s. The dramatic decline of pulsations in the atmospheric layers probed by the strong PrIII and NdIII lines accounts for the apparent peculiarity of 10 Aql when compared to other roAp stars. The phase-amplitude diagrams and bisector measurements of the NdIII 5102 A line reveal a rapid change of phase and amplitude with height for all three main pulsation modes, indicating the presence of a pulsation node in the stellar atmosphere. (abridged)