Pulsation tomography of rapidly oscillating Ap stars: Resolving the third dimension in peculiar pulsating stellar atmospheres
2007 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 473, no 3, 907-922 p.Article in journal (Refereed) Published
Aims. We present detailed analysis of the vertical pulsation mode cross-section in ten rapidly oscillating Ap (roAp) stars based onspectroscopic time-series observations. The aim of this analysis is to derive from observations a complete picture of how the amplitudeand phase of magnetoacoustic waves depend on depth.Methods. We use the unique properties of roAp stars, in particular chemical stratification, to resolve the vertical structure of p-modes.Our approach consists of characterising pulsational behaviour of a carefully chosen, but extensive sample of spectral lines. We analysethe resulting amplitude-phase diagrams and interpret observations in terms of pulsation wave propagation.Results. We find common features in the pulsational behaviour of roAp stars. Within a sample of representative elements the lowestamplitudes are detected for Eu (and Fe in 33 Lib and in HD 19918), then pulsations go through the layers where Hα core, Nd,and Pr lines are formed. There RV amplitude reaches its maximum, and after that decreases in most stars. The maximum RV of thesecond REE ions is always delayed relative to the first ions. The largest phase shifts are detected in Tb and Th lines. Pulsationalvariability of the Th lines is detected here for the first time. The Y lines deviate from this picture, showing even lower amplitudesthan Eu lines but half a period phase shift relative to other weakly pulsating lines. We measured an extra broadening, equivalent toa macroturbulent velocity from 4 to 11–12 km s−1 (where maximum values are observed for Tb and Th lines), for pulsating REElines. The surface magnetic field strength is derived for the first time for three roAp stars: HD 9289 (2 kG), HD 12932 (1.7 kG), andHD 19918 (1.6 kG).Conclusions. The roAp stars exhibit similarity in the depth-dependence of pulsation phase and amplitude, indicating similar chemicalstratification and comparable vertical mode cross-sections. In general, pulsations waves are represented by a superposition of therunning and standing wave components. In the atmospheres of roAp stars with the pulsation frequency below the acoustic cut-offfrequency, pulsations have a standing-wave character in the deeper layers and behave like a running wave in the outer layers. CoolerroAp stars develop a running wave higher in the atmosphere. In stars with pulsation frequency close to the acoustic cut-off one,pulsation waves have a running character starting from deep layers. The transition from standing to running wave is accompanied byan increase in the turbulent broadening of spectral lines.
Place, publisher, year, edition, pages
2007. Vol. 473, no 3, 907-922 p.
stars: atmospheres, stars: chemically peculiar, stars: magnetic fields, stars: oscillations
IdentifiersURN: urn:nbn:se:uu:diva-14066DOI: 10.1051/0004-6361:20077230ISI: 000249958000030OAI: oai:DiVA.org:uu-14066DiVA: diva2:41836