Investigating Surface Variability of Main-Belt Asteroid (203) Pompeja through Spectropolarimetry

Main-belt asteroid (203) Pompeja exhibits unusual and variable spectral behaviour. Observations during its 2021 apparition revealed an extremely steep red spectral slope in the visible and near-infrared, closely resembling trans-Neptunian objects (TNOs) rather than typical main-belt asteroids [1]. However, observations in 2022 showed a more moderate spectral slope across the same wavelength range, suggesting that Pompeja’s spectral appearance varies across its surface and depends on its rotational phase and viewing geometry [2,3] (see Figure 1c). Lightcurve data and shape modelling indicate that the primary difference in observing geometry between the extreme and moderate spectral slope observations is the sub-solar latitude at the time of the observation, implying heterogeneous surface properties [2,4].Main-belt asteroid (269) Justitia shares the extremely red spectral slope observed in Pompeja during 2021. Given their spectral similarities to TNOs, it has been hypothesised that both asteroids originated in the outer Solar System and later migrated inwards. The peculiarity of these objects has also been noted in [5] and [6], with the latter assigning both Pompeja and Justitia to the rare Z-type taxonomic class. Additionally, Justitia exhibits unique polarimetric behaviour, characterised by an inversion angle of ~15°, a trait associated with the uncommon F-type class [7] (see Figure 1b). Despite this, Justitia’s shallower negative polarisation branch distinguishes it from both F-types and other known taxonomic classes, marking it as an anomalous and intrinsically interesting object.In 2023, we conducted spectropolarimetric observations of Pompeja using FORS2 @ VLT, spanning the 0.4-0.9μm wavelength range and 1.8-20.3° phase angle range. We converted these measurements to FORS2 broadband filters bhigh, vhigh, R_SPECIAL and I_BESS (B, V, R, I) using the transmission curves of each filter and used solar analogue data to obtain the reflectance spectra of the asteroid. The main results, presented in Figure 1, reveal that (a) the shape of the polarisation spectra remains largely consistent across phase angles; (b) Pompeja’s polarimetric behaviour as a function of phase angle aligns more closely with that of low-albedo asteroids than with Justitia; and (c) the measured reflectance spectra indicate a moderate spectral slope similar to that found in [3], implying the observations were taken with the extreme sub-solar latitude region was not in view.Here, we will discuss these results and their implications in further detail. This study represents the first part of a two-part polarimetric investigation, with follow-up observations planned for 2025/2026 when the extreme sub-solar latitudes will be observable again [4]. These future measurements aim to determine whether Pompeja’s polarimetric behaviour varies with viewing geometry, further elucidating the nature of its surface properties.Figure 1: (a) spectropolarimetric measurements of (203) Pompeja; (b) BVRI polarimetric measurements of (203) Pompeja compared to V-band polarimetric measurements of (269) Justitia [7]; (c) normalised reflectance spectra of (203) Pompeja from one spectropolarimetry epoch compared to that of 2021 [2] and 2022 [3]. [1] Hasegawa, et al. 2021, ApJL, 916, L6; [2] Hasegawa, et al. 2022, ApJL, 939, L9; [3] Humes, et al. 2024, PSJ, 5, 80; [4] Humes, et al. 2024, PSJ, 5, 271; [5] Cellino, et al. 2020, A&A, 642, A80; [6] Mahlke et el. 2022, A&A, 665; [7] Gil-Hutton & García-Migani 2017, A&A, 607, A103.