Magnetic field structure of slow CP-rotators: HD 9996 and HD 188041

This is the final paper on a study of the magnetic field structure of CP stars with long rotation periods. It is first demonstrated that the orientation and strength of the magnetic field have no effect on their rotation velocity. The orientation of the dipole structures in slow magnetic rotators is shown to be random, as it is in fast rotators. The hypothesis that magnetic stars are slowed down under the influence of the magnetic field is called into question. The origin of CP stars is probably related to their initial slow rotation. __________ Translated from Astrofizika, Vol. 51, No. 2, pp. 295–303 (May 2008).     

Composite model for the magnetic field of HD 21699

A model is constructed for the magnetic field of the He-weak CP star HD 21699. This star has the field structure of a dipole displaced by 0.4 radii from the center perpendicular to its axis. The magnetic poles appear to be close to one another on the surface; they are separated by 55°, not by 180° as in the case of a central dipole. The phase dependences of the equivalent widths of He and Si lines have extrema at the phases corresponding to passage through the visible meridian of zero magnetic field between the magnetic poles. At the magnetic poles, the intensity of the helium lines is maximal and of the silicon lines, minimal. The silicon abundance is maximal in the regions where the magnetic field is predominantly tangential to the star’s surface. Because of averaging over the visible hemisphere and owing to the closeness of the magnetic poles, only one wave of variation in the intensity of the spectral lines of these chemical elements, one wave of photometric variability, and an average surface magnetic field Bs are observed. __________ Translated from Astrofizika, Vol. 50, No. 3, pp. 441–451 (August 2007).     

General properties of magnetic CP stars

We present the review of our previous studies related to observational evidence of the fossil field hypothesis of formation and evolution of magnetic and non-magnetic chemically peculiar stars. Analysis of the observed data shows that these stars acquire their main properties in the process of gravitational collapse. In the non-stationary Hayashi phase, a magnetic field becomes weakened and its configuration complicated, but the fossil field global orientation remains. After a non-stationary phase, relaxation of young star’s tangled field takes place and by the time of joining ZAMS (Zero Age Main Sequence) it is generally restored to a dipole structure. Stability of dipole structures allows them to remain unchanged up to the end of their life on the Main Sequence which is 10⁹ years at most.     

Features of multi-dipole magnetic field structures in CP stars

Ten of the sixty investigated magnetic stars have two- or three-dipole structures. From the viewpoint of the relic hypothesis a wide variety of magnetic field structures and strengths allows to assume that in the initial phases of formation of magnetic stars, their fields were even more entangled and heterogeneous than now. This may be due to the complex structure of protostellar clouds, the consequence of non-stationary processes during the collapse, and, probably, the result of subsequent accretion interactions. The expected variation of the large-scale structure with age is lost at the background of a wide variety of structures, depending on the initial conditions. Complex structures occur both in the stars at ZAMS, and in the stars leaving the Main Sequence. As a result of quadratic dependence of the magnetic structure lifetime on their characteristic dimensions, large-scale configurations can exist for times comparable to the lifetime of stellar magnetic field, i.e. τ ≥ 10⁹ yrs. One of the common properties of multi-dipole stars is that the centers of the dipoles are predominantly located in the equatorial plane of rotation. In the majority of studied objects magnetic dipoles (i.e. the regions with the maximum field) are shifted from the center of the star by the distance greater than the radius of the convective core (approximately 0.1R*). This may indicate that the poloidal field is not compatible with the convective core and is not generated therein. Large distances between the monopoles, comparable to the radii of the stars are typical. This may be a sign indicating that inside the stars the field structure is slightly different from the dipole, what implies that the dipole is not a mathematical point, but rather some highly magnetized volume inside the star, comparable to a magnetized rod.