Wolf-Rayet stars. VI. The nature of the optical and infrared continuum

Results of IR photometry between 1.6 and 11.3 microns are combined with results of scanner photometry between 3300 and 11,100 A to produce composite energy distributions for Wolf-Rayet stars. Reddening corrections are subtracted from the distributions to obtain IR-excess difference spectra which are fitted to either free-free emission or blackbody-like continua. WN stars are found to exhibit only free-free emission, while only WC stars show effects due to dust. Red and near-IR spectra are obtained for WC9 stars, and evidence is presented in favor of visual extinction by circumstellar dust around the star Ve2-45. The IR excesses in WC stars are interpreted as thermal emission by graphite grains; on this basis, radii and dust masses of circumstellar shells are estimated. Based on all these results, an evolutionary scheme is suggested for WC9 stars