COSINE (Cometary Object Study Investigating their Nature and Evolution) II: Colors and Sizes of Comet Nuclei

Comet nuclei preserve a record of planetesimal formation, but their sizes are difficult to measure because the nucleus is usually blended with its coma. This study presents a catalog of nucleus sizes for 523 comets: 235 long-period comets (LPCs) and 288 short-period comets (SPCs), drawn from the 14.5-year WISE/NEOWISE archive and supplemented by Spitzer photometry.

A coma-nucleus decomposition of the radial profiles yields conservative upper limits for all 523 comets and diameters with uncertainties for 301 of them: 107 LPCs and 194 SPCs. The median diameters are 5.3 km for LPCs and 2.5 km for SPCs. Nucleus colors across the four WISE bands follow the heliocentric trend expected from the transition between reflected and thermal emission, with no measurable difference between the two dynamical classes.

The cumulative size-frequency distribution of SPCs has a slope of gamma = 1.45 (+0.15, -0.13) at 68% confidence over D = 2-15 km, steepening to 2.05 (+0.47, -0.38) for the perihelion q < 2 au subset. The LPC distribution is smoothly concave down: a broken power law places a break near 35 km, with slopes of 0.92 (+0.13, -0.12) below and 3.68 (+1.96, -1.24) above, while the q < 2 au subset gives 1.39 (+0.35, -0.28) over D = 4-20 km. All distributions flatten below the fitted ranges and level off near 1 km.

The authors discuss the implications of these results and their connection with models of the destabilized trans-Neptunian population.