A search for rapid pulsations among nine luminous Ap stars

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 the Very 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 20-65 m s −1 and precisions of σ = 7-20 m s −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.

Magnetic, chemically peculiar (CP2) stars in the SuperWASP survey

“…However, our data show no eclipses, which is also supported by an analysis of Hipparcos data. In addition, the search for roAp characteristics gave a null result (Freyhammer et al 2008).…”

The discovery of a 21-kG magnetic field in the Ap star BD+0°4535

“…Moreover, HD 154708, like many other cool Ap stars, shows rapid oscillation (Kurtz et al 2006) characteristic of the roAp stars. Another recent discovery is of a very large surface field of 30 kG in HD 75049 (Freyhammer et al 2008); further observations and a simple dipole model suggest that the polar field strength in this star is of the order of 42 kG, making the star a rival of HD 215441 (Elkin et al 2009).…”

On the spectroscopic nature of the cool evolved Am star HD 151878★

“…Freyhammer et al (2008a) searched for pulsations in nine evolved cool Ap stars located inside the theoretical instability strip, the majority having estimated temperatures below 8100 K. With only slightly better precision and upper limits on radial velocity amplitudes as in this study, these authors found nine null results. Out of seven stars, only three stars had magnetic fields significantly stronger than 2 kG, which possibly explains most of their null results for such evolved stars.…”

“…Freyhammer et al (2008a) searched for rapid pulsations among a group of nine evolved Ap stars inside the roAp instability strip in the HR diagram. Surprisingly, they did not detect any in the time‐resolved radial velocity measurements for these stars, and showed that three to five of the stars had magnetic field strengths considerably in excess of 2 kG.…”