Magnetic field measurements of sharp-lined Ap stars

TL;DR

This study uses high-resolution spectropolarimetry to measure magnetic fields in sharp-lined Ap stars, finding some with very weak magnetic fields and analyzing their rotation periods to better understand their magnetic and rotational properties.

Contribution

It provides new magnetic field measurements for sharp-lined Ap stars, including some with very weak fields, and assesses their rotation periods to inform models of magnetic field stability and stellar evolution.

Findings

Two stars exhibit weak magnetic fields (~-45 G and 21 G)

Most studied stars have long rotation periods

Additional slow rotator studies are needed for understanding Ap star evolution

Abstract

Previous observations suggested that Ap and Bp stars exhibit a bimodal distribution of surface magnetic field strengths and that actually only few or no stars exist with magnetic dipole field strengths below 300 G down to a few Gauss. As the number of Ap and Bp stars currently known to possess weak magnetic fields is not large, it is necessary to carry out additional spectropolarimetric studies of Ap and Bp stars to prove whether the assumption of the existence of a critical value for the stability of magnetic fields is realistic. In this study, we present high-resolution HARPSpol magnetic field measurements for a sample of Ap stars with sharp spectral lines with a view to characterize the strengths of their magnetic fields. Out of the studied seven sharp-lined stars, two stars, HD 174779 and HD 203932, exhibit a rather weak longitudinal magnetic field with $\left< B_{\rm z}\right>=-45\pm3$ G and $\left< B_{\rm z}\right>=21\pm4$ G, respectively. Additionally, TESS observations were used to test previous conclusions on the differentiation of rotation periods of Ap and Bp stars. Apart from HD 189832 and HD 203932, all other studied sharp-lined stars have long rotation periods. Since an explanation for the slow rotation of Ap stars is currently missing, additional studies of slowly rotating Ap and Bp stars are necessary to improve our understanding of the formation and evolution of Ap and Bp stars.