Interstellar graphite grain temperature and infrared optical constants

The temperature of graphite grains in the interstellar medium has been computed with different assumptions on the extrapolation of its optical constants into infrared. It is found that values computed up to now are generally underestimated by 10-20% in normal interstellar conditions. For extreme conditions (very dark clouds) errors by a factor of two are possible and more attention has to be paid in dealing with problems related to molecule formation. (Auth.)