Near-infrared spectra of 21 Wolf-Rayet stars: Wind clumping and mass-loss rates

We present 0.9-2.4 mu m spectra of 21 galactic Wolf-Rayet (WR) stars that were obtained by using an echelle spectrograph attached to the 1.5 m infrared telescope in Tokyo. The observed Wolf-Rayet stars contain 12 WNE, 3 WNL, and 6 WC stars. The near-infrared continuum spectra can be fitted by the superposition of stellar blackbody radiation with a stellar effective temperatures, T-* of 30-60 kK and free-free radiation from the stellar wind with an electron temperature, T-e, of similar to 15 kK. However, these calculated spectra show an excess at cm wavelength compared with the cm data for almost all stars. This may be interpreted by an infrared excess flux possibly due to the clumping of the stellar wind gas. Assuming an "enhancement factor" that depends on the distance from the center of the stars as similar to r(-omega), we can reproduce both our near-infrared spectra and the cm data by this clump-corrected free-free radiation with mass-loss rates similar to those derived by Nugis et al. On the other hand, we also estimated the mass-loss rates from our He I 1.083 and He II 1.012 fluxes without a consideration of clumping, although a more detailed consideration should be necessary in order to estimate more reliable values of the mass-loss rates from the He line fluxes.