Thermal Modeling of Comet-like Objects from AKARI Observation

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 to the near-infrared spectroscopy and broadband observation data obtained by the AKARIsatellite of JAXA when they showed no detectable comet-like activity. We selected these two targets because 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 after its discovery, has effective size D = 3.74-4.39 km and geometric albedo p_V ≈ 0.040-0.055 with thermal inertia Γ=100-250 J m^-2 K^-1 s^-1/2. The corresponding grain size is estimated as 1-3 mm. We also found that HR30, which was observed as a bare cometary nucleus at the time of our observation, has D=23.9-27.1 km and p_V =0.035-0.045 with Γ =250-1000 J m^-2 K^-1 s^-1/2. We conjecture the pole latitude -20° ≲ β_s ≲ +60°. 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 their asteroidal counterparts on the D-Γ plane.