Nuclear Magnitudes and the Size Distribution of Jupiter Family Comets

We present a new catalog of absolute nuclear magnitudes of Jupiter family (JF) comets, which is an updated version of our previous catalog (Tancredi et al. 2000). The full version of the catalog can be accessed on line at http://www.fisica.edu.uy/∼gonzalo/catalog. From the new catalog we find a linear cumulative luminosity function (CLF) of slope 0.54 ± 0.03 for JF comets with q < ∼ 2.5 AU. By considering this CLF combined with the few measured geometric albedos with their respective uncertainties, and assuming a canonical albedo of 0.035± 0.012 for those comets with undetermined albedos, we derive a cumulative size distribution that follows a power-law of index −2.7 ± 0.3. The slope is similar to that derived from some theoretical collisional models and from some populations of solar system bodies like the trans-Neptunian objects. We also discuss and compare our size distribution with those by other authors that have recently appeared in the literature. Some striking differences in the computed slopes are explained in terms of biases in the studied samples, the different weights given to the brightest members of the samples, and discrepancies in the values of a few absolute nuclear magnitudes. We also compute sizes and fractions of active surface area of JF comets from their estimated absolute nuclear magnitudes and their water production rates. With the outgassing model that we use, about 60% of the computed fractions f of active surface area are found to be smaller than 0.2, with one case (28P/Neujmin 1) of no more than 0.001, which suggests that JF comets may transit through stages of very low activity, or even dormancy. There is an indication that JF comets with radii RN > ∼ 3 km have active fractions f < ∼ 0.01, which might be due to the rapid formation of insulating dust mantles on larger nuclei.