Research of primordial asteroids in the main belt using the Gaia spectral catalog

Introduction :

Spectrally red and featureless asteroids, taxonomically classified as D-, P-, T-, and Z-type in the Bus-DeMeo and Mahlke’s taxonomies [1, 2], have traditionally been found in Jupiter’s Trojans and are considered to have an organic-rich surface. They are thought to have formed in the outer Solar System [3]. Very few have been so far discovered in the main belt, including the inner part [4, 5, 6]. Using Gaia’s spectral catalog, we have found and classified, both using Bus-DeMeo and Mahlke’s taxonomies, more than 900 primordial asteroids and conducted a statistical study on them.

Methods :

We have used the DR3 spectral catalog of solar system objects of the Gaia mission, referencing 60,518 spectra of asteroids in the visible [7]. Each spectrum is the mean of several observations at different epochs of a given asteroid, and it consists of 16 spectrophotometric points covering the 0.37-1.03 micron wavelength range.

First, we computed the signal-to-noise ratio (SNR) for all the asteroids of the DR3 catalog, we selected the main belt asteroids and discarded all the objects for which the SNR was < 30. We applied an automatic taxonomy classification tool between each asteroid and the mean spectra of the two taxonomies, spotting the two « best fits » for the classes of interest (in Bus-DeMeo taxonomy D and T-types and in Mahlke P, D, and Z). Then, to better constrain the taxonomic classification and to solve ambiguities with X-complex asteroids, we applied selection criteria based on the albedo and the slope.

We considered only low albedo asteroids (geometric albedo < 12% ). Secondly, we looked for objects having a spectral slope (in the 0.550-0.814 micron) in the 2-7 %/1000 Å range for the P-types, and higher than 7 %/1000 Å for the D and Z-types.

After the preliminary classification and selection criteria, we visually inspected each potential primordial asteroid in order to remove potential known problematic points [7] and do a final classification in both Bus-DeMeo and Malhke’s taxonomies.

We added to the final dataset 13 asteroids previously classified as D-types in the literature [6].

Results :

Fig1: Position of the dataset in the main belt. Blue dots represent the D and Z types, the red triangle the P-types, and the green stars the PD-types. The three vertical lines delimit the inner, middle, and outer main belt.

Using the Malhke taxonomy, we have found 446 D-types, 270 P-types, 193 Z-types, and nine PD-types (T-types for Bus-DeMeo taxonomy). This last class represents extremely red asteroids. D and Z-types represent 1,8% of Gaia spectral catalog considering asteroids with SNR > 30 as similarly done in this analysis, P-types represent 0,7%, and PD-types 0,02%. If we only consider Z-types, they represent 0,54% of the dataset, which is less than what was previously estimated: 1.1% [2].

Primordial asteroids are observed mainly in the outer belt (207 D-types, 91 Z-types, and 146 T-types), as expected (Fig. 1), but they are also present in the inner belt, even if in lower amounts (65 D-types, 14 Z-types, and 32 T-types). This indicates important migration processes in the Solar System with implantation of primordial asteroids from the outer solar system up to the inner main belt.

We computed the correlations between the spectral slope, diameter, albedo and orbital elements for D/ Z and P-types. For the D/Z types, we found a weak anti-correlation between the albedo and the semi-major axis, the farther away from the Sun they are, the darker they are this could attributed to space weathering effects [8]. Furthermore, there are more smaller P, D and Z-types in the inner main belt. They could have been driven to their current position by the Yarkovski force as proposed by [4].

We also compared the slope distribution of D-types in the main belt with different dynamical classes of TNOs and comets and found similarities with the less red population of centaurs, SDO, and detached objects. Yet, if those D-types were in fact implanted from the TNOs population, it does not explain why we don’t find redder objects in the D/Z-types asteroids.

Acknowledgement : This work has received support from France 2030 through the project named Académie Spatiale d'Île-de-France (https://academiespatiale.fr/) managed by the National Research Agency under bearing the reference ANR-23-CMAS-0041, as well as the Centre National d’Etude Spatial (CNES).

References :

[1] DeMeo et al. (2009), Icarus, 202, 160-180

[2] Mahlke et al. (2022) , A&A, 665, A26

[3] Levison et al. (2009), Nature, 460, 7253, 364-366.

[4] DeMeo et al. (2014), Icarus, 229, 392-399.

[5] Gartrelle et al. (2021), Icarus, 363.

[6] Humes et al. (2024), PSJ, 5,3, 80.

[7] Gaia Collaboration (2023), A&A, 674, A35.

[8] Lantz et al. (2017), Icarus, 285, 43-57.