L. M. Freyhammer, D. W. Kurtz, M. S. Cunha, G. Mathys, V. G. Elkin
Abstract
The rapidly oscillating Ap (roAp) stars are of importance for studying the atmospheric structure
of stars where the process of chemical element diffusion is significant. We have performed
a survey for rapid oscillations in a sample of nine luminous Ap stars, selected from their
location in the colour–magnitude diagram as more evolved main-sequence Ap stars that are
inside the instability strip for roAp stars. Until recently this region was devoid of stars with
observed rapid pulsations.We used theVery Large Telescope UV–Visual Echelle Spectrograph
to obtain high time resolution spectroscopy to make the first systematic spectroscopic search
for rapid oscillations in this region of the roAp instability strip.We report nine null detections
with upper limits for radial velocity amplitudes of 20–65ms−1 and precisions of σ = 7–
20ms−1 for combinations of Nd and Pr lines. Cross-correlations confirm these null results.
At least six stars are magnetic and we provide magnetic field measurements for four of them,
of which three are newly discovered magnetic stars. It is found that four stars have magnetic
fields smaller than ∼2 kG, which according to theoretical predictions might be insufficient
for suppressing envelope convection around the magnetic poles for more evolved Ap stars.
Suppression of convection is expected to be essential for the opacity mechanism acting in
the hydrogen ionization zone to drive the high-overtone roAp pulsations efficiently. Our null
results suggest that the more evolved roAp stars may require particularly strong magnetic fields
to pulsate. Three of the studied stars do, however, have magnetic fields stronger than 5 kG.
Monthly Notices of the Royal Astronomical Society
Volume 385, Page 1402
2008 February
