Probing the surface environment of large T-type asteroids

Context. The thermal and radiative environments encountered by asteroids have shaped their surface features. Recent observations have focused on asteroids in the main belt and showed indications for ices and organics in the interiors of the asteroids that were likely significant on prebiotic Earth. They stand out in reflectance spectra as darker, redder colours than most colocated asteroids. Aims. We probe the surface environment of large (>80 km in diameter) T-type asteroids. This taxonomic type is relatively ill-constrained as an independent group. We discuss their place of origin based on our probing. Methods. We performed spectroscopic observations of two T-type asteroids, (96) Aegle and (570) Kythera, over the L band (2.8-4.0 mu m) using the Subaru telescope. The spectra of other T-type asteroids are available in the literature, as are survey datasets. Based on this, we strove to find commonalities and global trends in this group. We also used the archival polarimetric data of the asteroids and meteorite spectra from laboratory experiments to constrain their surface texture and composition. Results. Our targets exhibit red L-band continuum slopes, (0.30 +/- 0.04) mu m(-1) for (96) Aegle and (0.31 +/- 0.03) mu m(-1) for (570) Kythera, that are similar to those of (1) Ceres and 67P/Churyumov-Gerasimenko, and have an OH-absorption feature with band centres <2.8 mu m. For (96) Aegle, we find an indication of a shallow N-H band near 3.1 mu m and a C-H band of organic materials over 3.4-3.6 mu m, whereas no diagnostic bands of water ice and other volatiles exceeding the noise of the data were seen for both asteroids. The large T-type asteroids except for (596) Scheila display spectral shapes similar to those of to our targets. About 50% of large T-type asteroids contain an absorption band near 0.6-0.65 mu m that is likely associated with hydrated minerals. For T-type asteroids (except for Jupiter Trojans) of all sizes, we found a weak correlation that the smaller the diameter and the closer to the Sun, the redder the visible (0.5-0.8 mu m) slope. Conclusions. The 2.9 mu m band depths of large T-type asteroids suggest that they might have experienced aqueous alteration comparable to Ch-type asteroids, but that it was more intense than for most of the main-belt asteroids. The polarimetric phase curve of the T-type asteroids is well described by a particular surface structure. The 0.5-4.0 mu m reflectance spectra of large T-type asteroids appear most similar to those of CI chondrites with grain sizes of similar to 25-35 mu m. Taken as a whole, we propose that large T-type asteroids may have been dislodged roughly around 10 au in the early Solar System.