Keck, Gemini, and Palomar 200-inch visible photometry of red and very-red Neptunian Trojans

TL;DR

This study expands the visible colour dataset of Neptunian Trojans, revealing a distinct colour distribution that suggests the red to very-red transition zone in the protoplanetary disk was likely within 30-35 au.

Contribution

It provides new visible photometry for 18 NTs, doubling the sample size, and analyzes their colour distribution to infer implications for the early Solar System's disk structure.

Findings

Approximately 4 NTs are very red with g-i > 1.2 mags.

NTs have a statistically distinct colour distribution from other trans-Neptunian objects.

Presence of very red NTs suggests the red-VR transition zone was inside 30-35 au.

Abstract

Neptunian Trojans (NTs), trans-Neptunian objects in 1:1 mean-motion resonance with Neptune, are generally thought to have been captured from the original trans-Neptunian protoplanetary disk into co-orbital resonance with the ice giant during its outward migration. It is possible, therefore, that the colour distribution of NTs is a constraint on the location of any colour transition zones that may have been present in the disk. In support of this possible test, we obtained $g$, $r$, and $i$-band observations of 18 NTs, more than doubling the sample of NTs with known visible colours to 31 objects. Out of the combined sample, we found $\approx$4 objects with $g$-$i$ colours of $>$1.2 mags placing them in the very red (VR) category as typically defined. We find, without taking observational selection effects into account, that the NT $g$-$i$ colour distribution is statistically distinct from other trans-Neptunian dynamical classes. The optical colours of Jovian Trojans and NTs are shown to be less similar than previously claimed with additional VR NTs. The presence of VR objects among the NTs may suggest that the location of the red to VR colour transition zone in the protoplanetary disk was interior to 30-35 au.