Dust particle size distributions around oxygen-rich mass-losing red giants

We investigate the size distributions of circumstellar grains around seven oxygen-rich mass-losing red giants by using published data for the spatial extents of the circumstellar OH masers to compare with measures of the near-infrared sizes and thus to constrain the circumstellar extinction curves between the ultraviolet and the infrared. We find that the extinction increases toward the ultraviolet with the implication that a large fraction of the grains must be ejected with radii much less than 0.3 mu m. For an assumed size distribution of circumstellar particles similar to that in the interstellar medium such that n(a)da varies as n(0)a(-3.0) exp (-a/a(0)), where a is the radius of a spherical grain, two different arguments show that the upper limits or inferred values of a(0) are comparable to 0.14 mu m, the value of this parameter which describes the size distribution of interstellar grains in diffuse clouds. These results are consistent with the standard hypothesis that interstellar grains grow and evolve from circumstellar dust. Around IRC +10420, there appears to be an additional population of big grains, with radii greater than or equal to 0.5 mu m and carrying similar to 20% of the dust mass. These large particles may be formed because they have time to grow in the rotating equatorial disk postulated to exist around this star, and they may be similar to some of the isotopically anomalous inclusions found in meteorites.