The Thermal Structure of the Circumstellar Disk Surrounding the Classical Be Star gamma Cassiopeia

TL;DR

This study models the thermal structure of the circumstellar disk around gamma Cassiopeia using advanced atomic physics, matching observations with a solar composition gas in hydrostatic equilibrium.

Contribution

First to compute the thermal structure of a Be star disk with a realistic solar chemical composition using detailed atomic heating and cooling processes.

Findings

Predicted disk temperature matches observations.

Disk density profile consistent with hydrostatic equilibrium.

Revealed effects of solar composition on disk thermal structure.

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.