SEISMOLOGY OF BE STARS

摘要

Be stars are main sequence or slightly evolved, usually rapidly rotating B stars that show a near infrared excess and Balmer emission lines attributed to an equa-torially concentrated envelope, fed by discrete mass loss events. In the H-R diagram, early Be stars are located at the lower border of the instability strip of the β Cep stars, while mid- and late-Be stars are mixed with SPB stars. Be stars are not observed to rotate near the break-up velocity and the causes of the non-regular mass loss in these stars are still unknown. Non-radial pulsations (NRP) and stellar activity of magnetic origin have been proposed as mechanisms that could give rise to the additional amount of angular momentum needed to eject material. Multiple periods detected in photospheric line profile variations of early Be stars and modeling of these variations strongly support that such stars do pulsate non-radially (see Baade 2000). In the case of the Be star μ Cen, the beating phenomenon within the main group of NRP modes with low, identical degree l and azimuthal order m (l = m = 2) seems to determine the times of mass loss events in the star. Whether this fact is valid for all Be stars has to be investigated. An oblique magnetic dipole field has recently been detected in the Be star ω Ori, which is also found to pulsate non-radially. The period observed in the magnetic measurements and in the wind modulation differs from the NRP period and corresponds to the rotational period. Though high rotation is thought to strongly alter the pulsation characteristics, magnetic fields also contribute to modify the spacing in the frequency spectrum of the pulsations of Be stars. Eddington will provide a very important and attractive opportunity to study the competing influence of rotation and magnetism on the pulsation characteristics of hot stars such as Be stars, as well as to better understand the build-up of the disk which surrounds these objects.