The search for roAp stars: null results and new candidates from Stroemgren-Crawford photometry

TL;DR

This study used Stroemgren-Crawford photometry to search for new rapidly oscillating Ap stars but found no new candidates, highlighting the observational boundaries of the instability strip and identifying promising candidates for future research.

Contribution

The paper provides a comprehensive photometric survey of known and candidate roAp stars, including new measurements, and discusses the observational limits of the instability strip boundaries.

Findings

No new roAp stars were detected in the survey.

The blue border of the roAp instability strip is well defined observationally.

4646 candidates within the instability strip are identified for future studies.

Abstract

The rapidly oscillating Ap (roAp) stars exhibit pulsational photometric and/or radial velocity variations on time scales of several minutes, which are essential to test current pulsation models as well as our assumptions of atmospheric structure characteristics. In addition, their chemical peculiarity makes them very interesting for probing stellar formation and evolution in the presence of a global magnetic field. To date, a limited number of only 61 roAp stars are known to show photometric variability. On the other hand, a literature survey yields 619 unique stars that have unsuccessfully been searched for variability of this kind. Stroemgren-Crawford uvbybeta photometry of stars from both subgroups were used to investigate whether there is a selection bias of the investigated stars. We also present new photometric measurements (202 hours in 59 different nights) of 55 roAp candidates.We did not detect any new roAp star. Although our detection limits are comparable to other surveys, we also did not find pulsations in the known roAp star HD 12098, which may be a consequence of temporal amplitude changes. On the other hand, we do find some evidence for photometric variability of beta CrB at its spectroscopically derived pulsation period. From the uvbybeta photometry we conclude that the blue border of the roAp instability strip appears observationally well defined, whereas the red border is rather poorly known and studied. Within these boundaries, a total of 4646 candidates were identified which appear worthwhile to be investigated for short-term pulsational variability.