1995A&A...296..761P

The semi-analytic model of Bjorkman & Cassinelli for the equatorial `discs' around Be stars is re-examined and shown to be capable of producing higher equator-to-pole density contrasts when the effects of the dynamical decoupling of the stellar radiation field from the outflowing gas are incorporated. The enhancement, as measured by the normal component of the momentum fed into the disc, is in the region of a factor of 2. Inclusion of the effect also lowers the threshold stellar rotation rate for the formation of a wind-compressed disc from v_rot/v_crit≃0.4 to 0.25. Two physical mechanisms that may give rise to this decoupling, which are most effective in B star winds, are presented and their relative merits compared. This comparison is performed with reference to available UV and X-ray observations. Ion-stripping (Springmann & Pauldrach 1992A&A...262..515S) could be the cause of the low observed maximum outflow velocities, but it cannot simultaneously explain the ubiquitous high-temperature (>10⁶K) X-ray emission. Shock disruption of the wind ionization balance (Castor 1987, in Instabilities in Luminous Early-Type, eds. H.J.G.L.M. Lamers & C.W.H. dw Loore, D. Reidel, The Netherlands, p159) may also terminate the outward radiative acceleration in the winds of B stars. This effect suggests a simple method for setting an upper limit to the X-ray emission from weak shock-disrupted winds which compares reasonably favourably with X-ray luminosities derived from ROSAT data.