  With the Infrared Space Observatory, we conducted 3x3-pixel imaging
photometry of twelve luminosity class III stars, which were previously presumed
to have dust particles around them, at far infrared wavelengths (60 and 90 um).
Eleven out of twelve targets show a peak of excess (above photosphere) far
infrared emission at the location of the star, implying that the dust particles
are truly associated with stars. To estimate the size of the excess emission
source, the flux ratio of center to boundary pixels of the 3x3 array was
examined. The radius of the dust emission is found to be ~3000 to ~10000 AU for
a thin shell distribution, and ~5000 to ~25000 AU for a uniform distribution.
We consider three models for the origin of the dust: disintegration of comets,
sporadic dust ejection from the star, and emission from nearby interstellar
cirrus. The data seem to rule out the first model (as far as the "Kuiper--belt"
like particles are assumed to be large blackbody grains), but do not enable us
to choose between the other two models.
