Gravitational Darkening of Classical Be Stars

Be stars are rapidly rotating B-type stars that possess a gaseous disk formed by matter released from the rapidly rotating, central star. The disk produces the characteristic observational features of these objects: hydrogen emission lines, particularly Hα, infrared excess, and continuum polarization. Gravitational darkening is a phenomenon associated with stellar rotation. It causes a reduction of the stellar effective temperature towards the equator and a redirection of energy towards the poles. Rotation also distorts the star, increasing its equatorial radius. It is an important physical effect in these star-disk systems because the photoionizing radiation from the star is essentially the sole energy source for the disk. The effect of gravitational darkening on models of the thermal structure of Be star disks is systematically studied for a wide range of Be star spectral types, rotation rates, and disk densities. Additionally, the effects of rapid rotation on model Hα lines, spectral energy distributions (SED) and photometric colours are investigated. To achieve this, gravitational darkening has been added to the bedisk and beray codes to produce circumstellar disk models and synthetic observables that include the change in the effective temperature with latitude and the non-spherical shape of the star. The effect of gravitational darkening on disk temperature is generally significant for rotation rates above 80 % of critical rotation. The bulk of the disk material becomes cooler with rotation. For example in a dense disk surrounding a B0V central star rotating at 95 % of its critical velocity, the density-averaged disk temperature is ≈ 2500 K cooler than its non-rotating counterpart. The effect of gravity darkening on observables such as the Hα line, spectral energy distributions, and colours is found to be primarily through the change in the stellar surface brightness with rotation