X-ray emission from magnetically torqued disks of Oe/Be stars.
LI Q., CASSINELLI J.P., BROWN J.C., WALDRON W.L. and MILLER N.A.
Abstract (from CDS):
The near-main-sequence B stars show a sharp dropoff in their X-ray-to-bolometric luminosity ratio in going from B1 to later spectral types. Here we focus attention on the subset of these stars that are also Oe/Be stars, to test the concept that the disks of these stars form by magnetic channeling of wind material toward the equator. Calculations are made of the X-rays expected from the magnetically torqued disk (MTD) model for Be stars discussed by Cassinelli et al., Maheswaran, and Brown et al. In this model, the wind outflow from Be stars is channeled and torqued by a magnetic field such that the flows from the upper and lower hemispheres of the star collide as they approach the equatorial zone. X-rays are produced by the material that enters the shocks above and below the disk region and radiatively cools and compresses while moving toward the MTD central plane. The model predictions are compared with ROSAT observations obtained for an O9.5 star, ζ Oph, by Berghöfer et al. and for seven Be stars from Cohen et al. Two types of fitting models are used to compare predictions with observations of X-ray luminosity versus spectral type. Extra consideration is also given here to the well-studied Oe star ζ Oph, for which we have Chandra observations of the X-ray line profiles of the triad of He-like lines from the ion Mg XI. Thus, the X-ray properties add to the list of observables that can be explained within the context of the MTD concept. This list already includes the Hα equivalent widths and white-light polarization of Be stars.