DUST PROPERTIES AROUND EVOLVED STARS FROM FAR-INFRARED SIZE LIMITS

We have obtained high angular resolution far-infrared scans of six stars surrounded by circumstellar dust shells believed to result from mass loss by the central star. None of the dust shells was clearly resolved at either 50-mu-m or 100-mu-m; the upper limits are in the range 4" to 10". These size limits place constraints on the far-IR dust emissivity and dust density distribution. For one of the objects, AFGL 2343, larger than normal grains are almost certainly required. For several other stars the size limits are much more consistent with dust having an emissivity law no steeper than lambda--1 in the 1-100-mu-m spectral region. For IRC + 10216 we confirm an earlier suggestion that, assuming reasonable grain properties, a smooth radial dust distribution is not consistent with our scans and the energy distribution of the object.