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Comet 67P/Churyumov–Gerasimenko (67P) become observable for the first time in 2021 since the Rosetta rendezvous in 2014–2016. Here, we present pre-perihelion polarimetric measurements of 67P from 2021 performed with the Very Large Telescope (VLT), as well as post-perihelion polarimetric measurements from 2015 to 2016 obtained with the VLT and the William Herschel Telescope. This new data covers a phase angle range of ∼4○−50○ and presents polarimetric measurements of unprecedentedly high S/N ratio. Complementing previous measurements, the polarimetric phase curve of 67P resembles that of other Jupiter family comets and high-polarization, dusty comets. Comparing pre- and post-perihelion data sets, we find only a marginal difference between the polarimetric phase curves. In our imaging maps, we detect various linear structures produced by the dust in the inner coma of the comet. Despite this, we find a homogeneous spread of polarization around the photocentre throughout the coma and tail, in contrast to previous studies. Finally, we explore the consequences of image misalignments on both polarimetric maps and aperture polarimetric measurements. [ABSTRACT FROM AUTHOR]

This study presents the first optical (420–930 nm) spectropolarimetric study of a sample of large (≳100 km in diameter) C-complex asteroids in observing circumstances for which light scattered by asteroid surfaces undergoes a so-called negative polarization. We analyzed a total of 64 C-complex asteroids of different subclasses, using archival polarimetric and reflectance data to refine the link between polarimetric parameters and surface properties of the asteroids. We find a consistent difference in the polarization spectra between asteroids containing phyllosilicates and those without, which is correlated with the overall morphology of the reflectance spectrum. The inverse relationship between reflectance and the polarization degree, known as the Umov law, is not very sharp in our sample of asteroids. These objects exhibit broad similarities in polarization-phase curves; nonetheless, we do observe a gradual enhancement of the negative polarization branch (both in depth and width) in the ascending order of F → B → T → Ch types (Spearman correlation coefficient ρ = 0.70-0.07+0.06 and p-value p = 0.0001), along with an increase in the reflectance curvature around 500 nm. A weaker inverse correlation between the inversion angle and diameter of the asteroids in our sample has been found with ρ = -0.42-0.07+0.08 and p = 0.01, apparently driven by the distribution of C-type asteroids (ρ = -0.51-0.11+0.13 and p = 0.04). Our observations suggest that at least for large C-complex asteroids: 1) a common mechanism underlies the diversity in polarimetric and spectroscopic properties at optical wavelengths; 2) the observed trends would be explained by the surface composition of the asteroids, particularly with respect to the optical heterogeneity on the wavelength scale caused by carbon's varying levels of optical influence; and 3) aqueous alteration may play a significant role in regulating this operational effect of carbon. [ABSTRACT FROM AUTHOR]