The Thermal Structure of the Circumstellar Disk Surrounding the Classical Be Star γ Cassiopeiae

Abstract

We have computed radiative equilibrium models for the gas in the circumstellar envelope surrounding the hot, classical Be star $\gamma $Cassiopeia. This calculation is performed using a code that incorporates a number of improvements over previous treatments of the disk's thermal structure by \citet{mil98} and \citet{jon04}; most importantly, heating and cooling rates are computed with atomic models for H, He, CNO, Mg, Si, Ca, & Fe and their relevant ions. Thus, for the first time, the thermal structure of a Be disk is computed for a gas with a solar chemical composition as opposed to assuming a pure hydrogen envelope. We compare the predicted average disk temperature, the total energy loss in H$\alpha$, and the near-IR excess with observations and find that all can be accounted for by a disk that is in vertical hydrostatic equilibrium with a density in the equatorial plane of $\rho(R)\approx 3$ to $5\cdot 10^{-11} (R/R_*)^{-2.5} \rm g cm^{-3}$. We also discuss the changes in the disk's thermal structure that result from the additional heating and cooling processes available to a gas with a solar chemical composition over those available to a pure hydrogen plasma.