EQUATORIAL DISK FORMATION AROUND ROTATING STARS DUE TO RAM PRESSURE CONFINEMENT BY THE STELLAR WIND

We present a simple approximation that permits us to obtain the axisym metric two-dimensional supersonic solution of a rotating radiation-dri ven stellar wind from the Friend & Abbott one-dimensional model of the equatorial flow. Our solution predicts the formation of a dense equat orial disk if the rotation rate of the star is above a threshold value , which depends on the ratio of the terminal speed of the wind to the escape speed of the star. Along the upper main sequence (earlier than B2), both the observed and theoretical values for this ratio decrease monotonically toward later spectral types. For early O stars, the disk can only form if the rotation speed is in excess of 90% of the critic al (breakup) speed. For B2 stars, the disk forms at rotation speeds ab ove 50%-60% of the critical rotation speed, depending on the adopted t erminal speed (observational vs. theoretical estimates). This correspo nds to a rotation speed V(rot) > 230-300 km s-1. Later than B2, the th eoretical terminal speed ratio increases, and at B9 the disk forms whe n the rotation speed exceeds 73% of the critical value. The change in the disk formation threshold as a function of spectral type qualitativ ely explains the frequency distribution of Be stars and indicates a ma ximum probability around B2. The disk is formed because the supersonic wind that leaves the stellar surface at high latitudes travels along trajectories that carry it down to the equatorial plane, where the mat erial passes through a standing oblique shock on top of the disk. The ram pressure of the polar wind thus confines and compresses the disk. For Be stars, the disk is predicted to be quite thin (almost-equal-to 0.5-degrees opening angle) and has a density enhancement rho(eq)/rho(p ole) 10(3). This compression is large enough to potentially explain th e discrepancy between the inferred UV and IR mass-loss rates of Be sta rs. Adjacent to the disk, the standing shock heats the flow that enter s the equatorial region to temperatures of 10(5)-10(6) K before the ma terial finally mixes with the disk. This temperature is sufficient to produce superionization in the winds of Be stars, and the shock locati on explains observations indicating that C IV is concentrated toward t he equator. In addition, the shock temperature indicates that Be stars will be EUV and soft X-ray emitters.