Effect of molecules and grains on Rosseland mean opacities

It is pointed out that at temperatures below 4000 K bound-bound transitions in molecules, which are not included in the Cox-Stewart calculations, become important. At temperatures below about 1500 K, scattering and absorption from solid grains also figure prominently. These temperatures are encountered in the envelopes of stars with effective temperatures less than about 5000 K and during the collapse of protostellar and planetary clouds. The Rosseland mean opacity for a solar composition is presented here in tabular form. Also given is an analysis of the significance of different absorbers as a function of temperature. The effects of lowering the metal abundances and changing the carbon abundance (to simulate an S star or carbon star) are also considered. It is also shown that rather subtle changes in the chemical composition can lead to dramatic changes in the opacity through alterations in the chemical equilibrium of molecules.