Thermal Modeling of Comet-Like Objects from AKARI Observation

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.