ULTRAVIOLET EXTINCTION TO 10.8 INVERSE MICRONS

UV extinction curves that represent observed variations in diffuse clouds (zeta Oph, xi Per, and sigma Sco) have been derived by combining IUE and Voyager data and have been modeled by a multicomponent size distribution of silicate and carbonaceous giants. We have used theoretical model atmospheres as our reddening-free standards, and, in the case of zeta Oph, also compare with the use of a stellar standard. An attempt was made to classify the UV spectra via theoretical spectral indices, but still missing line opacity appears to limit the use of this method at the present. The modeling includes five possible grain ingredients, large (0.015-0.25 mu m) and small (0.005 mu m) silicate grains, similarly large and small carbonaceous grains, and PAHs. The observed extinction curves are fitted by the model via nonlinear chi(2)-minimization, varying the fractional amount of cosmic Si and C that are locked up in each ingredient. The observed variation from ''steep'' to ''flat'' UV extinction curves is found to correspond to a modest model variation in the lower limit of the power-law size distribution of the large grains. Graphite is found to be a necessary grain ingredient that cannot be replaced by the laboratory amorphous carbon materials. Carbonaceous mantles on the larger silicate grains provide no good fits to the observed curves. Using absorption properties of neutral laboratory PAHs limits PAHs to a few percent of the cosmic C abundance for acceptable fits that also give values for R(V) close to the observed values.