NEOWISE data and Thermophysical Modeling of 98943 Torifune (2001 CC21)

TL;DR

This work applies an ellipsoidal thermophysical model to all available NEOWISE observations of 98943 Torifune (2001 CC21) to derive a radiometric diameter of $D = 337_{-27}^{+33}$ m with prograde rotation ($24_{-9}^{+6}$ degrees obliquity). A separate TPM analysis of Spitzer IRS data yields a larger diameter of about $D o 476$ m, highlighting model-dependent differences in radiometric sizing. When NEOWISE and Spitzer data are combined, the fit requires an exceptionally large thermal inertia ($ ext{Γ} o 2924^{+39 ext{%}}_{-105 ext{%}}$ MKS) and shows a bimodal posterior for diameter, indicating tension among datasets and priors. These results underscore the challenges in constraining the sizes of small, irregular NEOs and forecast how ground-truth observations from the Hayabusa2 flyby will calibrate solar-system size estimation techniques for sub-kilometer bodies.

Abstract

The Hayabusa2# flyby target 98943 Torifune (2001 CC21) has an uncertain size based on an uncertain albedo and uncertain absolute magnitude. We have collected all the NEOWISE observations of 2001 CC21 from Nov 2021 through Feb 2024, a total of 132 frames, and analyzed this data to estimate an infrared radiometric diameter. We analyze the multi-epoch 3.4 and 4.6 micron NEOWISE data using an ellipsoidal rotating, cratered ThermoPhysical Model (TPM) to obtain estimates for the diameter, rotation pole, shape, and thermal inertia. 2001 CC21 is quite faint at 4.6 microns when Delta is about 0.7 AU, so the resulting diameter is substantially smaller than the 700 meters derived from the H magnitude and L spectral type. Recent polarimetric data has also suggested a smaller diameter, but not quite as small as the diameter derived from the thermal IR data. A fit to an ellipsoidal TPM model gives a volume equivalent sphere diameter of 337-27+33 meters [posterior median and central 68% confidence interval]. Prograde rotation with an obliquity of 24-9+6 deg is preferred. We also applied this TPM to the Spitzer data presented by Fornasier etal. (2024) and obtain a diameter of 476 +/- 9% meters which is consistent with the NEATM modeling presented by Fornasier etal. but with more realistic errorbars. Finally, fitting the NEOWISE and Spitzer data together requires unexpectedly large thermal inertias and gives a bimodal posterior diameter distribution.

Figures (7)

• Figure 1: Orbit of 2001 CC$_{21}$ relative to the Earth (blue dot) in a frame rotating to keep the Sun on the $-y$ axis. The orange circle at $y = -1$ AU indicates the Sun's direction. The plot covers the period from MJD 57754 (1 Jan 2017) in the lower left to 60484 (23 Jun 2024). Lines indicating the current scan paths of the NEOWISE mission are shown. Tiny labels give the MJD for each observing epoch..
• Figure 2: Correlation between the parameters of the ellipsoidal TPM model fit to the NEOWISE data. The maps and scatter diagrams show a random sample of 1000 models selected from the MCMC, while the histograms show the entire chain. The range for the scatter diagrams and histograms cover a "$\pm 2\sigma$" range for each parameter that is actually given by $(p_{16}+p_{84})/2 \pm (p_{84}-p_{16})$ where $p_{16}$ and $p_{84}$ are the $16^{1h}$ and $84^{th}$ percentile values of the parameter $p$. These ranges are shown in the upper right corner. However the range is $[0..1]$ for $f_c$, $(b/a)^3$, or $(c/b)^4$. The maps show the rotation poles of the models in ceelestial coordinates. The points on the maps are colored to show how a given parameter correlates with the pole position, so a red dot on the map shows a model with a low value of the parameter.
• Figure 3: Histogram of the cosine of the obliquity. The prior would be uniform in these coordinates. The black histogram shows the posterior distribution using the NEOWISE data alone.. The gray histogram shows the posterior when using both the NEOWISE and the Spitzer data.
• Figure 4: The spectra of the ellipsoidal model with the highest posterior likelihood compared to the NEOWISE data.
• Figure 5: Spectral energy distributions for the Spitzer observation and the many epochs of NEOWISE observations predicted by the maximum posterior model found during the MCMC. The thermal inertia for this model is $\Gamma_1 = 2111$ MKS units. The rotation pole is at $(\alpha, \delta) = (281^\circ, 45^\circ)$. The diameter of a sphere with the same volume is $D = 2(abc)^{1/3} = 474$ meters.