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We have studied the IRAS colors of the ring nebula RCW 58 surrounding the Wolf-Rayet star HD 96548 (= WR 40; type WN 8) by analyzing the IRAS survey data with the Groningen Exportable High-Resolution Analysis system (GEISHA) and by using the Chopped Photometric Channel high-resolution imaging at 50-mu-m. Additional JHKLM photometry of the exciting star of RCW 58, and also of HD 50896 (WR 6, type WN 5), exciting the bubble S308, and of HD 192163 (WR 136, type WN 6), exciting the bubble nebula NGC 6888, have been obtained. We have estimated the emission from dust in all three bubbles by subtracting the stellar contributions to the IRAS images and correcting the IRAS fluxes for atomic emissions. Except for the 12-mu-m filter, the atomic contributions do not comprise the bulk of the observed fluxes. As compared to the interstellar medium (ISM) and clouds, the dust emissions are very weak in 12-mu-m relative to 25-mu-m but have F(60)/(100) > 1, compared to less-than-or-equal-to 0.2 for the ISM, showing that the grains are heated by the enhanced radiation field in the vicinity of the central star. The dust emission is analyzed by means of several models. For RCW 58, the diffuse dust model of Mathis, Rumpl, & Nordsieck, with optical constants as modified by Draine & Lee and grains in the size range 0.005-0.25-mu-m, produces fairly good agreement with the observations, except that the required radiation field is higher (almost-equal-to 100 eV cm-3) than one would expect from the mean distance of the bubble from the star (almost-equal-to 44 eV cm-3) with the stellar temperature we adopt. It is possible to fit the spectrum with the expected radiation field if a combination of small grains (0.002-mu-m-0.008-mu-m) are in the inner part of the nebula and somewhat larger ones (0.005-mu-m-0.05-mu-m) farther out. For S308 and NGC 6888, if there is enough radiation to produce the proper F(25)/F(60) from MRN grains the F(60)/F(100) flux ratio is too large. However, small grains transiently heated by random absorptions of stellar photons produce too large a F(60)/F(100) ratio at the observed F(25)/F(60). For these objects we find fits to both IRAS ratios by assuming that there are small grains transiently heated in the inner parts of the bubbles and standard interstellar grains in the outer portions. The masses of material in the nebulae are consistent with most of the matter being stellar for RCW 58 and a substantial amount of material being contributed by the swept-up ISM for S308 and NGC 6888. However, the masses are very sensitive to both the photometry and to the assumptions of the nature of the small grains near the star. Quantitative estimates of the mass are only order of magnitude. Graphs are given which allow an easy estimation of the nebular mass for other situations for nebular excitation. The principal parameter is the dilution factor of the stellar radiation. |
