Thermal Modeling of Comet-Like Objects from AKARI Observation

TL;DR

This study models the thermal properties of two comet-like objects using AKARI data, revealing their sizes, albedos, and thermal inertias, and suggesting no clear distinction from asteroids based on these parameters.

Contribution

It provides the first thermophysical modeling of these objects using AKARI data, exploring the relationship between size and thermal inertia in comet-like bodies.

Findings

4015WH has a size of 3.74-4.39 km and low albedo.

HR30 has a size of 23.9-27.1 km with higher thermal inertia.

Both objects' properties align with previous studies, blurring the line between comets and asteroids.

Abstract

We investigated the physical properties of the comet-like objects 107P/(4015) Wilson--Harrington (4015WH) and P/2006 HR30 (Siding Spring; HR30) by applying a simple thermophysical model (TPM) to the near-infrared spectroscopy and broadband observation data obtained by AKARI satellite of JAXA when they showed no detectable comet-like activity. We selected these two targets since the tendency of thermal inertia to decrease with the size of an asteroid, which has been demonstrated in recent studies, has not been confirmed for comet-like objects. It was found that 4015WH, which was originally discovered as a comet but has not shown comet-like activity since its discovery, has effective size $ D= $ 3.74--4.39 km and geometric albedo $ p_V \approx $ 0.040--0.055 with thermal inertia $ \Gamma = $ 100--250 J m$ ^{-2} $ K$ ^{-1} $ s$ ^{-1/2}$. The corresponding grain size is estimated to 1--3 mm. We also found that HR30, which was observed as a bare cometary nucleus at the time of our observation, have $ D= $ 23.9--27.1 km and $ p_V= $0.035--0.045 with $ \Gamma= $ 250--1,000 J m$ ^{-2} $ K$ ^{-1} $ s$ ^{-1/2}$. We conjecture the pole latitude $ - 20^{\circ} \lesssim \beta_s \lesssim +60^{\circ}$. The results for both targets are consistent with previous studies. Based on the results, we propose that comet-like objects are not clearly distinguishable from asteroidal counterpart on the $ D $--$ \Gamma $ plane.