Are the surface abundance structures stable in rapidly rotating Ap star 56 Ari?

TL;DR

This study uses Doppler Imaging to analyze the surface abundance and magnetic structures of the rapidly rotating Ap star 56 Ari, finding that its surface features are stable over decades despite observed period changes, challenging existing theories.

Contribution

It provides the first long-term Doppler Imaging analysis of 56 Ari, demonstrating the stability of surface abundance structures over 28 years despite period variability.

Findings

Surface abundance spots are stable over 28 years.

Period changes are not caused by surface magnetic or abundance reconfigurations.

The stability suggests alternative mechanisms for period variability.

Abstract

The surface magnetic and abundance inhomogeneities in chemically peculiar Ap/Bp stars are coupled and responsible for their rotationally modulated variability. Within the framework of fossil field hypothesis these inhomogeneities are considered to be essentially stable over the Main Sequence (MS) timescale. However, a small group of Ap/Bp stars show rotational period changes, which are currently not well understood. We present results of Doppler Imaging (DI) of rapidly rotating Ap star 56 Ari for which changes in period were previously detected. Reconstruction of the surface distribution of silicon in 56 Ari reveals its complex spot pattern, which is responsible for the rotationally light variability and correlated with magnetic field modulation. Comparison of abundance maps obtained over the unprecedentedly long for such studies interval from 1986 to 2014 confirms stability and rigid rotation of the spot pattern. Thus, the period change in 56 Ari is not caused by rearrangement of the surface magnetic structures and/or atomic diffusion operating on short time scale. It is also unlikely to be explained by the visibility changes of the spots due to free-body precession of stellar rotational axis. In the end of the paper we briefly discuss possible alternative explanations of period variability.