Non-LTE Monte Carlo radiative transfer. I. The thermal properties of Keplerian disks around classical Be stars

We present a three- dimensional non- LTE Monte Carlo radiative transfer code that we use to study the temperature and ionization structure of Keplerian disks around classical Be stars. The method we employ is largely similar to the Monte Carlo transition probability method developed by Lucy. Here we present a simplification of his method that avoids the use of the macroatom concept. Our investigations of the temperature structure of Be star disks show that the disk temperature behavior is a hybrid between the behavior of young stellar object ( YSO) disks and hot star winds. The optically thick inner parts of Be star disks have temperatures that are similar to YSO disks, while the optically thin outer parts are like stellar winds. Thus, the temperature at the disk midplane initially drops, reaching a minimum at 3 - 5 stellar radii, after which it rises back to the optically thin radiative equilibrium temperature at large distances. On the other hand, the optically thin upper layers of the disk are approximately isothermal - a behavior that is analogous to the hot upper layers of YSO disks. Interestingly, unlike the case of YSO disks, we find that disk flaring has little effect on the temperature structure of Be star disks. We also find that the disks are fully ionized, as expected, but that there is an ionization minimum in the vicinity of the temperature minimum. The deficit of photoionization at this location makes it the most likely site for the low ionization state lines ( e. g., Fe (II)) that produce the shell features observed in Be stars. Finally, we find that despite the complex temperature structure, the infrared excess is well approximated by an equivalent isothermal disk model whose temperature is about 60% of the stellar temperature. This is largely because at long wavelengths, the effective photosphere of the disk is located in its isothermal regions.