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

The rapidly oscillating Ap (roAp) star 10 Aquilae (10 Aql) shows one of the lowest photometric pulsation amplitudes and is characterized by an unusual spectroscopic pulsational behaviour compared to other roAp stars. In summer 2006 this star became target of an intense observing campaign, that combined ground-based spectroscopy with space photometry obtained with the MOST (Microvariability & Oscillations Stars) satellite. More than 1000 spectra were taken during seven nights over a time-span of 21 d with high-resolution spectrographs at the 8-m European Southern Observatory (ESO) Very Large Telescope (VLT) and 3.6-m Telescopio Nazionale Galileo (TNG) 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. Taking advantage of the clear oscillation patterns seen in a number of rare earth ions and using the contemporaneous MOST photometry to resolve aliasing in the radial velocity measurements, we improve also the determination of pulsation frequencies. 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 H alpha core is formed, reach a maximum of 200-300 m s(-1) in the layers probed by Ce, Sm, Dy lines and then decrease to 20-50 m s(-1) in the layers where Nd III and Pr III lines are formed. A unique pulsation feature of 10 Aql is a second pulsation maximum indicated by Tb III lines which form in the uppermost atmospheric layers and oscillate with amplitudes of up to 350 m s(-1). The dramatic decline of pulsations in the atmospheric layers probed by the strong Pr III and Nd III lines accounts for the apparent peculiarity of 10 Aql when compared to other roAp stars. The phase-amplitude diagrams and bisector measurements of the Nd III 5102 angstrom 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. Finally, we report the discovery of a puzzling asymmetry of the strong Nd III lines with their blue wing extending up to -50 km s(-1), which is about 25 times the estimated value of nu(e) sin i.