WR 121 obscured by a dust cloud: the key to understanding occasional ``eclipses'' of ``dusty" Wolf-Rayet WC stars?
We observed the Wolf-Rayet star WR 121 (= AS 320, WC9) coming out of aminimum with a depth of 0fm8 to its normal brightness in a dozen days.The nature of this event was analyzed by means of Walraven five-colourphotometry. The colour changes are similar to those caused byinterstellar dust. Hot dust is known to be formed continuously aroundthis carbon-rich Wolf-Rayet star. Therefore, we suggest that the fadingof WR 121 was caused by the temporary condensation of an optically thickline-of-sight dust cloud, comparable to what occurs around R CoronaeBorealis stars. We suggest that occasional ``eclipses'' shown by other``dusty" Wolf Rayet stars (WR 113, WR 103) are also caused by suchtemporary condensing dust clouds. In addition, we present observationsof a new ``eclipse'' of WR 103. This brightness dip was more shallowthan the earlier ``eclipses'' and the star reddens during the descent.This also supports the model of a condensing dust cloud. >Frommodeling the shapes of the various ``eclipses'' we find that thecondensation takes place at radii ranging from 80 to 800Rsun, i.e. between the stellar surface and the permanent dustshell that is inferred from the infrared excess. >From the colourchanges we estimate the sizes of the particles in the clouds to be oforder 0.1 mu m and using the depth of the darkening we derive a dustmass condensation rate per column in the range of 3 -- 20 10(-10) kgm(-2) s(-1) . The dust mass flux per solid angle turns out to becomparable to that of the shell. Moreover, we find two possible trendswithin our small set of ``eclipses'': (1) the closer the condensationoccurs to the star, the larger the dust mass flux is, and (2) the closerthe condensation, the larger the particles are. These correlations arediscussed within the framework of the model. Based on observationscollected at the European Southern Observatory (ESO), La Silla, Chile
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