Loss of angular momentum of magnetic Ap stars in the pre-main sequence phase
Received 3 August 1999 / Accepted 8 October 1999
A model for rotation evolution of an intermediate mass star with the primordial magnetic field in the pre-main sequence (PMS) phase was developed. It takes into account the accretion of matter along the magnetic field lines, the stellar field-disk interaction and a magnetized wind. Variations of stellar moment of inertia were included based on evolutionary models of PMS evolution of such stars. Stellar mass and magnetic moment were assumed constant during the PMS evolution. Values of the parameters describing the strength of the magnetic field, accretion rate and mass loss rate were taken from observations. In addition, the life time of the disk was varied. An equation describing the evolution of the rotation rate of a magnetic PMS star was derived and solved for different stellar masses.
The results indicate that the interaction of the stellar ymagnetic field with circumstellar environment wipes out quickly a memory of the initial rotation period. The ZAMS period depends solely on the details of this interaction. Accretion spins up a star early in its PMS life and if the disk disappears right after that the star may keep its faster rotation until ZAMS and appear there as a Be star.
A wide variety of parameters describing the evolution of stellar AM results in typical ZAMS rotation periods of magnetic stars several times longer than of normal stars. This agrees well with the observations. Under special circumstances a star can reach an exceptionally long rotation period of several years (up to 100 years). This requires a long PMS life time, an existence of a disk for only a part of the PMS phase and the wind in the strong magnetic field existing for the rest of the PMS life. The observations confirm indeed that extremely slowly rotating Ap stars are lower mass stars with strong magnetic fields.
Key words: stars: chemically peculiar stars: emission-line, Be stars: magnetic fields stars: pre-main sequence stars: rotation
© European Southern Observatory (ESO) 2000
Online publication: December 8, 1999