An ancient L- type family associated to (460) Scania in the Middle Main Belt as revealed by Gaia DR3 spectra

Asteroid families are groups of asteroids sharing similar orbits and surface properties resulting from the disruption of a parent body. Asteroid families are usually identified using hierarchical clustering algorithms (HCM) in the proper element phase space. The HCM has several limitations, such as being unable to separate overlapping families, identify interlopers, or detect old families. Spectroscopic data can overcome these issues, although they are less abundant than proper element measurements. The Gaia Data Release 3 (DR3) significantly improved spectroscopic analysis by providing over 60,000 asteroid reflectance spectra.
We have already proven that asteroid families can be identified using Gaia DR3 spectra alone by detecting the Tirela/Klumpkea and Watsonia families [1], two L-type families known for their connection to Barbarian asteroids. Barbarians, characterized by unique polarimetric properties [2], might represent an old population of asteroids preserving some of the properties of the early protoplanetary disk.

In this study, we focus on a low-inclination family in the middle belt, which, due to its large spread, challenges pure HCM-based identification. This region is also known to host Barbarians. By classifying Gaia spectra with the color taxonomy from [1], we identified a new L-type family with (460) Scania as its largest member. The distribution of the family in the (a, 1/D) plane, the so-called V-shape, is reported in Figure 1. This distribution was analyzed with the V-shape detection method [3], which confirmed that the observed structure is not a statistical artifact but a real family. Its age, estimated with the V-shape fitting method [4], is around 1.0 Gyr. The second-largest remnant, (1007) Pawlowia, is unusual because its size is comparable to the largest remnant. Further observations are needed to clarify its nature.

The V-shape was numerically reproduced by simulating a fragmentation event using the N-body integrator REBOUND [5]. The distribution of the synthetic fragments closely matches the observed one, as reported in Figure 2.

Barbarian asteroids exhibit negative polarization at large phase angles, unlike normal asteroids, which show a positive transition at the same phase angles. Among the family members, polarimetric data are available only for (460) Scania, (2085) Henan, and (2354) Lavrov. The latter has limited data, while the first two are likely Barbarians.
The V-shape reflects the Yarkovsky-driven drift in semi-major axis, which depends on spin orientation: prograde rotators drift outward, retrograde inward. A strong correlation was observed between spin obliquity and position within the family’s V-shape.

To verify that the L-type family was not a result of a misclassification of Gaia spectra, we compared it with existing taxonomic data. The memberships are in good agreement for large asteroids, but discrepancies emerged at smaller sizes due to missing or ambiguous classifications. Figure 3 compares the size distributions from Gaia, literature, and their combination, along with the geometric model [6], whose slope agrees with the observed distributions, but does not resolve the issue of the two similarly sized largest remnants.

In conclusion, our analysis likely identifies an L-type family in the middle Main Belt, potentially linked to Barbarian asteroids. We stress here that although our spectroscopy method addresses some HCM limitations, it still presents some biases, as it assumes compositional homogeneity within the family, which is the commonly accepted paradigm. In addition, the Gaia color taxonomy has some limitations, since it tends to overclassify objects into the C and S classes and may misclassify faint objects with low SNR spectra. Future Gaia releases, combined with data from other surveys, will offer a larger and improved spectroscopic sample, helping to refine our understanding of this family and the whole asteroid population.

Figure 1: Figure 1: The family members in the (a, 1/D) plane fitted by V-shapes corresponding to different ages.

Figure 2: Comparison between the proper elements of the family members observed by Gaia (grey circles) and the proper elements of the synthetic family members integrated in REBOUND (blue stars).

Figure 3: Size distributions corresponding to family memberships and the geometric model.

References

[1] Balossi, R., Tanga, P., Sergeyev, A., Cellino, A., Spoto, F. (2024), Gaia DR3 asteroid reflectance spectra: L-type families, memberships, and ages, A&A, 688, A221.
[2] Cellino, A., Belskaya, I. N., Bendjoya, Ph., et al. (2006), The strange polarimetric behavior of Asteroid (234) Barbara, Icarus, 180, 565–567.
[3] Bolin, B. T., Delbo, M., Morbidelli, A., Walsh, K. J. (2017), Yarkovsky V-shape identification of asteroid families, Icarus, 282, 290–312.
[4] Spoto, F., Milani, A., Knežević, Z. (2015), Asteroid family ages, Icarus, 257, 275–289.
[5] Rein, H., Liu, S.-F. (2012), REBOUND: an open-source multi-purpose N-body code for collisional dynamics, A&A, 537, A128.
[6] Tanga, P., Cellino, A., Michel, P., et al. (1999), On the size distribution of asteroid families: The role of geometry, Icarus, 141, 65–78.