The Reflectance of Cold Classical Trans-Neptunian Objects in the Nearest Infrared

TL;DR

This study presents reflectance spectra of six cold classical Trans-Neptunian Objects, revealing a common spectral flattening above 0.8 microns likely due to surface organics, and identifies an interloper based on spectral linearity.

Contribution

First detailed reflectance spectra of six cold classical TNOs in the 0.55-1.0 micron range, analyzing surface composition and dynamical classification.

Findings

Five TNOs show spectral flattening above 0.8 micron.

No evidence of an absorption band near 1.0 micron.

One TNO's spectrum suggests it is a hot classical interloper.

Abstract

Recent photometric surveys of Trans-Neptunian Objects (TNOs) have revealed that the cold classical TNOs have distinct z-band color characteristics, and occupy their own distinct surface class. This suggested the presence of an absorption band in the reflectance spectra of cold classicals at wavelengths above 0.8 micron. Here we present reflectance spectra spanning 0.55-1.0 micron for six TNOs occupying dynamically cold orbits at semimajor axes close to 44 au. Five of our spectra show a clear and broadly consistent reduction in spectral gradient above 0.8 micron that diverges from their linear red optical continuum and agrees with their reported photometric colour data. Despite predictions, we find no evidence that the spectral flattening is caused by an absorption band centered near 1.0 micron. We predict that the overall consistent shape of these five spectra is related to the presence of similar refractory organics on each of their surfaces, and/or their similar physical surface properties such as porosity or grain size distribution. The observed consistency of the reflectance spectra of these five targets aligns with predictions that the cold classicals share a common history in terms of formation and surface evolution. Our sixth target, which has been ambiguously classified as either a hot or cold classical at various points in the past, has a spectrum which remains nearly linear across the full range observed. This suggests that this TNO is a hot classical interloper in the cold classical dynamical range, and supports the idea that other such interlopers may be identifiable by their linear reflectance spectra in the range 0.8-1.0 micron.