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Abstract: The rapidly oscillating Ap stars are of importance for studying theatmospheric structure of stars where the process of chemical element diffusionis significant. We have performed a survey for rapid oscillations in a sampleof 9 luminous Ap stars, selected from their location in the colour-magnitudediagram as more evolved main-sequence Ap stars that are inside the instabilitystrip for rapidly oscillating Ap roAp stars. Until recently this region wasdevoid of stars with observed rapid pulsations. We used the VLT UV-VisualEchelle Spectrograph UVES to obtain high time resolution spectroscopy to makethe first systematic spectroscopic search for rapid oscillations in this regionof the roAp instability strip. We report 9 null-detections with upper limitsfor radial-velocity amplitudes of 20 - 65 m-s and precisions of 7 - 20 m-s forcombinations of Nd and Pr lines. Cross-correlations confirm these null-results.At least six stars are magnetic and we provide magnetic field measurements forfour of them, of which three are newly discovered magnetic stars. It is foundthat four stars have magnetic fields smaller than ~ 2 kG, which according totheoretical predictions might be insufficient for suppressing envelopeconvection around the magnetic poles for more evolved Ap stars. Suppression ofconvection is expected to be essential for the opacity mechanism acting in thehydrogen ionisation zone to drive the high-overtone roAp pulsationsefficiently. Our null-results suggest that the more evolved roAp stars mayrequire particularly strong magnetic fields to pulsate. Three of the studiedstars do, however, have magnetic fields stronger than 5 kG.

Author: L. M. Freyhammer, D. W. Kurtz, M. S. Cunha, G. Mathys, V. G. Elkin, J.D. Riley

Source: https://arxiv.org/

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