Characterizing the dust in active asteroids by modeling their photometric and polarimetric images

In this presentation, we explore photometric and polarimetric observations of the asteroids that became active either naturally or as a result of space experiments.

As an example of naturally active asteroids, we consider asteroid (248370) QN173, leveraging quasi-simultaneous data on color and polarization distribution along its tail reported in [1]. Through computer modeling, we analyze these data to unveil the characteristics of the dust particles. Utilizing irregular solid particles resembling material found on C-type asteroids, we employ the surface-integral-equation (SIE) method for particle sizes r ≤ 3 microns and the SIRIS4 code based on geometric optics approximation for particles larger than r > 3 microns. Our modeling reveals the size distribution of particles and their variation with distance to the asteroid and phase angle. Our findings suggest that in July 2021, at phase angle of 23°, dust particles exhibited a power-law size distribution with the smallest particles of radius 2.5 microns near the asteroid, decreasing to 1.6 microns at the distance of 60,000 km. In October 2021, at a phase angle of 8°, the size distribution near the asteroid had a power of 3.0 with the smallest particles of radius ~ 2.5 microns, while at 60,000 km distance, the power was 4.0 with the smallest particles of radius 0.8 microns. These results align with the dynamics of dust particles influenced by radiation pressure.

We also examine the dust ejected by asteroid Dimorphos following the impact of the DART spacecraft. Employing similar modeling techniques, we reproduce VLT FORS2 observations of polarization [2] and VLT MUSE observations of color [3] obtained for the same dates. The observations showed the absence of any trends in color and polarization with the distance from the impact; see Figure 1 which shows the polarization and color distribution in the tail on October 25, 2022. This indicated the dominance of particles larger than 100 microns that scattered light in the geometric optics regime. This limited our capability to study the variations in the dust properties along the observed tail. To extract more information about the ejecta particles, we considered several dates of observations, exploring the change in the dust size distribution with the time after impact. Utilizing the characteristics of size distributions obtained from the modeling, we analyze HST WFC3 images  [4]  for the same dates. This allows us to estimate the dust column density at varying distances from the asteroid facilitating estimation of the mass of the dust in the tail.

Acknowledgment. This work was supported by NASA DART Participating Scientist grant #80NSSC21K1131.

 

           

Figure 1. Polarization (left) and color (right) along the DART ejecta tail on October 25, 2022.

References

1. Ivanova, O., Licandro, J., Moreno, F., Luk’yanyk, I., Markkanen, J., Tomko, D., Husárik, M., Cabrera-Lavers, A., Popescu, M., Shablovinskaya, E. and Shubina, O., 2023. Long-lasting activity of asteroid (248370) 2005 QN173. Monthly Notices of the Royal Astronomical Society, 525(1), pp.402-414.

2. Gray, Z., Bagnulo, S., Granvik, M., Cellino, A., Jones, G.H., Kolokolova, L., Moreno, F., Muinonen, K., Muñoz, O., Opitom, C. and Penttilä, A., 2024. Polarimetry of Didymos–Dimorphos: Unexpected Long-term Effects of the DART Impact. The Planetary Science Journal, 5(1), p.18.

3. Opitom, C., Murphy, B., Snodgrass, C., Bagnulo, S., Green, S.F., Knight, M.M., de Léon, J., Li, J.Y. and Gardener, D., 2023. Morphology and spectral properties of the DART impact ejecta with VLT/MUSE. Astronomy & Astrophysics, 671, p.L11.

4. Li, J.Y., Hirabayashi, M., Farnham, T.L., Sunshine, J.M., Knight, M.M., Tancredi, G., Moreno, F., Murphy, B., Opitom, C., Chesley, S. and Scheeres, D.J., 2023. Ejecta from the DART-produced active asteroid Dimorphos. Nature, 616(7957), pp.452-456.