Primitive asteroids in the main belt, Cybele, and Hilda populations from Gaia DR3

Primitive asteroids include C-, P-, and D-classes, known to be dark and having spectra mostly featureless. They differ in the spectral slope, which ranges from moderate values for C-types, and progressively increases in P- and D-types, the latter being the reddest. While C- and P-types are commonly observed in the asteroid main belt, D-types are commonly found further from the Sun, in the Cybele, Hilda, and Jupiter Trojans regions, and very few are reported in the main belt. This study aims at characterizing the abundance of primordial and red asteroids, belonging to the P-, D-, and Z-classes in the Mahlke et al. (2022) taxonomy, in the 2-5.2 AU region using the third data release by the Gaia mission (DR3) spectral catalog, which includes more than 60000 spectrophotometric data of asteroids. We have applied the following methodology to identify primordial asteroids in the catalog: (1) selection of objects with signal to noise ratio (SNR) greater than 20; 2) albedo value less than 12%; (3) chi-squared fit to automatically identify potential D-, Z-, and P-types using Bus-DeMeo and Mahlke taxonomy; (4) visual inspection of every spectrum to confirm the taxonomic classification. Referring to Mahlke taxonomy, we have found 318 new D-types across the main belt, as well as 124 Z-types, which is a considerable increase from previous studies (Mahlke et al., 2022 and DeMeo et al., 2014; Humes et al., 2024b; Gartrelle et al., 2021b), and is in agreement with theoretical estimations (DeMeo et al., 2014; Vokrouhlický et al., 2016). We look for correlations among physical and orbital parameters, and we computed the spectral slope in the visible range (0.55 – 0 . 81 μ m ). We also have identified 265 P-types in the main belt. For the Cybele and Hilda asteroids, we characterize the taxonomic class of all the bodies with SNR higher than 20 in the Gaia catalog, for a total sample of 193 and 180 asteroids, respectively. For D- and Z-types in the main belt, we have found a correlation between size and semimajor axis, meaning that the D-types closer to the Sun are smaller and also brighter, which could uncover their implantation processes, such as collisions or Yarkovsky effect. No families dominated by D- or Z-types have been identified so far. These red asteroids were likely formed further from the Sun and implanted during the giant planets’ migrations. For both the Cybele and Hilda, we found a bimodal spectral slope distribution as already reported in the literature. These groups are dominated by P- and D-types. Furthermore, for both populations, despite not being the most abundant group, P-types show a larger range of diameters and include the biggest bodies, hinting at a more robust material. We have identified 42 D- and 18 Z-asteroids having orbits with high inclination and eccentricity, and Tisserand parameter lower than 3, which could have cometary origins, in the main belt, Cybele, and mostly in the Hilda. We also compared the mean D- and Z-types spectra with those of the two Martian moons, Phobos and Deimos. Deimos is spectrally closer to Z-types, while Phobos red unit to D-types, confirming the possibility that Mars’ satellites might be captured asteroids.