A search for rapid pulsations among 9 luminous Ap stars

TL;DR

This study conducted a systematic spectroscopic survey of 9 luminous Ap stars within the roAp instability strip to detect rapid pulsations, finding no pulsations but providing new magnetic field measurements and insights into the magnetic conditions for pulsation.

Contribution

First systematic high-resolution spectroscopic search for rapid oscillations in evolved Ap stars within the instability strip, with new magnetic field measurements and null results informing pulsation theories.

Findings

No rapid pulsations detected with upper limits of 20-65 m/s.

Discovered three new magnetic Ap stars with fields >5 kG.

Some stars have magnetic fields too weak to suppress convection, affecting pulsation likelihood.

Abstract

The rapidly oscillating Ap 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 9 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 rapidly oscillating Ap (roAp) stars. Until recently this region was devoid of stars with observed rapid pulsations. We used the VLT UV-Visual Echelle Spectrograph (UVES) 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 9 null-detections with upper limits for radial-velocity amplitudes of 20 - 65 m/s and precisions of 7 - 20 m/s 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 ionisation 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.