Secular stability of surface abundance structures in the rapidly rotating Ap star 56 Ari

The combination of the surface magnetic and abundance inhomogeneities in chemically peculiar Ap/Bp stars is responsible for their rotationally modulated variability. Within the framework of the 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 that are not fully understood. Here, we present results of Doppler imaging (DI) of the rapidly rotating Ap star 56 Ari whose changes in period had previously been detected. A reconstruction of the surface distribution of silicon in 56 Ari reveals its complex spot pattern, which is responsible for the rotationally light variability and is correlated with a magnetic field modulation. Comparisons of abundance maps obtained over the unprecedentedly long (for such studies) interval from 1986 to 2014 confirms the stability and rigid rotation of the spot pattern. Thus, we find the period change in 56 Ari is not caused by a rearrangement of the surface magnetic structures and/or atomic diffusion operating on short timescale. It is also unlikely to be explained by the visibility changes of the spots due to the free-body precession of the stellar rotational axis. We briefly discuss possible alternative explanations of the period variability.