Evidence for a Color Dependence in the Size Distribution of Main-Belt Asteroids

We present the results of a project to detect small (~1 km) main-beltasteroids with the 3.6 meter Canada-France-Hawaii Telescope (CFHT). We observedin 2 filters (MegaPrime g' and r') in order to compare the results in eachband. Owing to the observational cadence we did not observe the same asteroidsthrough each filter and thus do not have true colour information. Howeverstrong differences in the size distributions as seen in the two filters pointto a colour-dependence at these sizes, perhaps to be expected in this regimewhere asteroid cohesiveness begins to be dominated by physical strength andcomposition rather than by gravity. The best fit slopes of the cumulative sizedistributions (CSDs) in both filters tend towards lower values for smallerasteroids, consistent with the results of previous studies. In addition to thistrend, the size distributions seen in the two filters are distinctly different,with steeper slopes in r' than in g'. Breaking our sample up according tosemimajor axis, the difference between the filters in the inner belt is foundto be somewhat less pronounced than in the middle and outer belt, but the CSDof those asteroids seen in the r' filter is consistently and significantlysteeper than in g' throughout. The CSD slopes also show variations withsemimajor axis within a given filter, particularly in r'. We conclude that thesize distribution of main belt asteroids is likely to be colour dependent atkilometer sizes and that this dependence may vary across the belt.Comment: 28 pages, 5 figures, submitted to the Astronomical Journa