THE LOW FILLING FACTOR OF DUST IN THE GALAXY

We have examined the neighborhood of 745 luminous stars in the IRAS Skyflux plates for the presence of dust heated by the nearby star. This dust may be distinguished from the ubiquitous cool circus by its higher temperature and thus enhanced 60 mum emission. We have found 123 dust clouds around only 106 of the stars with a volume filling factor of 0.006 and an intercloud separation of 46 pc. Nowhere do we find a region where the dust is smoothly distributed through the volume of space heated by the star and hence we place an upper limit of 0.05 cm-3 on the equivalent gas density in the intercloud regions. From the lack of infrared emission near the star, we find that less than 1% of the stellar luminosity is reprocessed within 10 pc of the star. The clouds, themselves, have an average density of 0.22 cm-3 (assuming a standard gas-to-dust ratio) and a radius of 1.9 pc, albeit with wide variations in their properties. We have tentatively identified these clouds with the warm, ionized medium of McKee & Ostriker. We have found two different scale heights of 140 and 540 pc for the number of clouds around different groups of stars which we have interpreted as evidence for different distributions of dust in and out of the Galactic disk. The dust at higher altitudes also appears to be more uniformly distributed with Galactic latitude.