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In a molecular cloud dust opacity typically dominates over gas opacity, yetin the vicinities of forming stars dust is depleted, and gas is the soleprovider of opacity. In the optically thin circumstellar environments theradiation temperature cannot be assumed to be equal to the gas temperature,hence the two-temperature Planck means are necessary to calculate the radiativeequilibrium.  By using the two-temperature mean opacity one does obtain the properequilibrium gas temperature in a circumstellar environment, which is in achemical equilibrium. A careful consideration of a radiative transfer problemreveals that the equilibrium temperature solution can be degenerate in anoptically thin gaseous environment.  We compute mean gas opacities based on the publicly available code DFSYNTHEby Kurucz and Castelli. We performed the calculations assuming localthermodynamic equilibrium and an ideal gas equation of state. The values werederived by direct integration of the high-resolution opacity spectrum.  We produced two sets of gas opacity tables: Rosseland means andtwo-temperature Planck means (the tables available viahttp://cdsweb.u-strasbg.fr/ as well as viahttp://www.mpia-hd.mpg.de/homes/malygin). For three metallicities [Me/H] =0.0,+/-0.3 we covered the parameter range 3.48 <= log T_rad[K] <= 4.48 inradiation temperature, 2.8 <= log T_gas[K]} <= 6.0 in gas temperature, and -10<= log P[dyn cm^-2] <= 6 in gas pressure. We show that in the optically thincircumstellar environment for a given stellar radiation field and local gasdensity there are several equilibrium gas temperatures possible.  We conclude that, in general, equilibrium gas temperature cannot bedetermined without treating the temperature evolution.

Mean gas opacity for circumstellar environments and equilibrium temperature degeneracy