The nature of the dust around R Coronae Borealis stars - Isolated amorphous carbon or graphite fractals?

Wright (1989) has proposed that the R CrB phenomenon is caused by the occurrence of graphite fractal grains, each composed of numerous, less than 15-nm-radius graphite spheres. It is shown that both the observational data and models of dust formation are more in accord with an earlier model by Hecht et al. (1984): that the R CrB type dust is composed of isolated small (less than 100-nm-radius) glassy or amorphous carbon grains. It cannot be ruled out that a significant number of fractal grains exist, provided that they have dielectric properties different from bulk graphite and closer to amorphous carbon, and that the individual grains in the fractals are below 1 nm in size. However, the existence of such fractal grains around R CrB stars might be ruled out by the lack of a 2400-A feature in the interstellar extinction curve. 30 refs.