The Size Distribution of the Neptune Trojans and the Missing Intermediate Sized Planetesimals

TL;DR

This study used deep telescopic surveys to analyze Neptune Trojans, revealing a scarcity of intermediate-sized objects and supporting the idea that planetesimals formed directly from small to large sizes, not through collisional processes.

Contribution

It provides the first ultra-deep survey of Neptune Trojans, identifying a size distribution rollover and supporting the primordial origin of planetesimals.

Findings

Scarcity of Neptune Trojans smaller than 45 km radius.

Size distribution follows a steep power-law for larger objects.

Evidence for missing intermediate-sized planetesimals across the solar system.

Abstract

We present an ultra-deep survey for Neptune Trojans using the Subaru 8.2-m and Magellan 6.5-m telescopes. The survey reached a 50% detection efficiency in the R-band at 25.7 magnitudes and covered 49 square degrees of sky. This depth corresponds to Neptune Trojans that are about 16 km in radius (assuming an albedo of 0.05). A paucity of smaller Neptune Trojans (radii < 45 km) compared to larger ones was found. The brightest Neptune Trojans appear to follow a steep power-law slope (q = 5+-1) similar to the brightest objects in the other known stable reservoirs such as the Kuiper Belt, Jupiter Trojans and main belt asteroids. We find a roll-over for the Neptune Trojans that occurs around a radii of r=45+-10 km (23.5+-0.3 mags), which is also very similar to the other stable reservoirs. All the observed stable regions in the the solar system show evidence for Missing Intermediate Sized Planetesimals (MISPs). This indicates a primordial and not collisional origin, which suggests planetesimal formation proceeded directly from small to large objects. The scarcity of intermediate and smaller sized Neptune Trojans may limit them as being a strong source for the short period comets.