An Update of the Correlation between Polarimetric and Thermal Properties of Cometary Dust

TL;DR

This study investigates the correlation between polarimetric and thermal properties of cometary dust, revealing that dust size and porosity, rather than composition, primarily influence these characteristics, especially in the K-band.

Contribution

It demonstrates a positive correlation between polarimetric excess in the K-band and silicate emission features, emphasizing the importance of dust size and porosity over composition in modeling cometary dust.

Findings

Positive correlation between K-band polarimetric excess and silicate emission.

Gas-rich comets have weaker silicate features than dust-rich ones.

Dust size and porosity are key factors in dust properties, more than composition.

Abstract

We present a possible correlation between the properties of scattered and thermal radiation from dust and the principal dust characteristics responsible for this relationship. To this end, we use the NASA/PDS archival polarimetric data on cometary dust in the Red (0.62--0.73 $\mu$m) and K (2.00--2.39 $\mu$m) domains to leverage the relative excess of the polarisation degree of a comet to the average trend at the given phase angle ($P_{\rm excess}$) as a metric of the dust's scattered light characteristics. The flux excess of silicate emissions to the continuum around 10 $\mu$m ($F_{\rm Si}/F_{\rm cont}$) is adopted from previous studies as a metric of the dust's MIR feature. The two metrics show a positive correlation when $P_{\rm excess}$ is measured in the K domain. No significant correlation was identified in the Red domain. The gas-rich comets have systematically weaker $F_{\rm Si}/F_{\rm cont}$ than the dust-rich ones, yet both groups retain the same overall tendency with different slope values. The observed positive correlation between the two metrics indicates that composition is a peripheral factor in characterising the dust's polarimetric and silicate emission properties. The systematic difference in $F_{\rm Si}/F_{\rm cont}$ for gas-rich versus dust-rich comets would rather correspond with the difference in their dust size distribution. Hence, our results suggest that the current MIR spectral models of cometary dust should prioritise the dust size and porosity over the composition. With light scattering being sensitive to different size scales in two wavebands, we expect the K-domain polarimetry to be sensitive to the properties of dust aggregates, such as size and porosity, which might have been influenced by evolutionary processes. On the other hand, the Red-domain polarimetry reflects the characteristics of sub-$\mu$m constituents in the aggregate.