A majority part of magnetic chemically peculiar (mCP) stars of the upper main sequence exhibits strictly periodic light, magnetic, radio, and spectral variations that can be fully explained by the model of a rigidly rotating main-sequence star with persistent surface structures and stable global magnetic field frozen into the body of the star. Nevertheless, there is an inhomogeneous group consisting of a few mCP stars whose rotation periods vary on timescales of decades, while the shapes of their phase curves remain nearly unchanged. Alternations in the rotational period variations, proven in the case of some of them, offer new insight on this theoretically unpredicted phenomenon. We present a novel and generally applicable method of period analysis based on the simultaneous exploitation of all available observational data containing phase information. This phenomenological method can monitor gradual changes in the observed instantaneous period very efficiently and reliably. We present up to date results of the period monitoring of V901~Ori, CU~Vir, $\sigma$\,Ori\,E, and BS~Cir, known to be mCP stars changing their observed periods and discuss the physics of this unusual behaviour. To compare the period behavior of those stars, we treated their data with an orthogonal polynomial model, which was specifically developed for this purpose. We confirmed period variations in all stars and showed that they reflect real changes in the angular velocity of outer layers of the stars, fastened by their global magnetic fields. However, the nature of the observed rotational instabilities has remained elusive up to now. The discussed group of mCP stars is inhomogeneous to such extent that each of the stars may experience a different cause for its period variations.
Mon, 27 Feb 17
Comments: 36 pages, 14 figures