A search for rapid pulsations among 9 luminous Ap stars

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.

Accepted by MNRAS main journal

References in corpus (8)

• Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
• The Core-Wing Anomaly of Cool Ap Stars: Abnormal Balmer Profiles
• On the detection of chemically peculiar stars using Delta a photometry
• Evolution of magnetic fields in stars across the upper main sequence: I. Catalogue of magnetic field measurements with FORS1 at the VLT
• Improved pulsating models of magnetic Ap stars I: exploring different magnetic field configurations
• Pulsation in the atmosphere of the roAp star HD 24712. I. Spectroscopic observations and radial velocity measurements
• The Nainital-Cape Survey-III : A Search for Pulsational Variability in Chemically Peculiar Stars
• The pre-main sequence star HD 34282: A very short period delta Scuti-type pulsator

Cited by in corpus (12)

• Rotation and pulsation in Ap stars: first light results from TESS sectors 1 and 2
• Discovery of 17 new sharp-lined Ap stars with magnetically resolved lines
• New Photometrically Variable Magnetic Chemically Peculiar Stars in the ASAS-3 Archive
• The Nainital-Cape Survey-III : A Search for Pulsational Variability in Chemically Peculiar Stars
• A 3-D study of the photosphere of HD99563: I. Pulsation analysis
• TESS Cycle 1 observations of roAp stars with 2-min cadence data
• Time-resolved photometric and spectroscopic analysis of a luminous Ap star HD103498
• Magnetic, chemically peculiar (CP2) stars in the SuperWASP survey
• A search for rapid pulsations in the magnetic cool chemically peculiar star HD3980
• The search for roAp stars: null results and new candidates from Stroemgren-Crawford photometry
• On the spectroscopic nature of the cool evolved Am star HD151878
• Numerical simulations of line-profile variation beyond a single-surface approximation for oscillations in roAp stars