MODEL OF A KUIPER BELT SMALL-GRAIN POPULATION AND RESULTING FAR-INFRARED EMISSION

We have calculated a simple model of the expected Kuiper Belt (KB) sma ll grain population and the thermal emission that would arise from suc h grains. We have also sought observational evidence for this emission . The model assumed equilibrium between grain creation by collisional fragmentation of comets and removal by Poynting-Robertson radiation dr ag, radiation pressure-driven ejection, mutual collisions, and sublima tion. The model far-IR intensity scales as the square of total KB mass . Comparison of our model with observations of the zodiacal dust rules out emission from trans-Neptunian dust representing more than about 0 .3 M(+) of KB comets. This agrees with recent HST reports of a populat ion of comet-sized bodies in the KB which has a minimum mass of about 0.04 M(+), although that population can be extrapolated to include as much as 1 M(+) in the volume of our model. The model KB dust fractiona l bolometric luminosity (L(dust)/L(star)) would have about 10(-2) and 10(-4) of the values for the grain disks around Vega and beta Pic, res pectively. A preliminary search in COBE DIRBE data reveals nonuniform bands near the ecliptic of cold (T = 20-30 K) emission prominent at wa velengths of 140 and 240 mu m but not prominent relative to zodiacal e mission at shorter (IRAS) wavelengths. Most of this emission is probab ly not from solar system material.