An ancient L-type family associated with (460) Scania in the middle main belt as revealed by Gaia DR3 spectra

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Bd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
² University of Leicester, School of Physics and Astronomy, University Road, LE1 7RH, Leicester, UK
³ INAF – Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese (TO), Italy
⁴ Minor Planet Center, Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138, USA

^★ Corresponding author: roberto.balossi@oca.eu

Received: 15 May 2025
Accepted: 21 September 2025

Abstract

Context. Asteroid families are typically identified using hierarchical clustering methods (HCM) in the proper element phase space. However, these methods struggle with overlapping families, interlopers, and the detection of older structures. Spectroscopic data can help overcome these limitations. The Gaia Data Release 3 (DR3) contains reflectance spectra at visible wavelengths for 60 518 asteroids over the range between 374 and 1034 nm, representing a large sample that is well suited to studies of asteroid families.

Aims. Using Gaia spectroscopic data, we investigated a region in the central main belt centered around 2.72 AU, known for its connection to L-type asteroids. Conflicting family memberships reported by different HCM implementations underscored the need for an independent dynamical analysis of this region.

Methods. We determined family memberships by applying a color taxonomy derived from Gaia data and by assessing the spectral similarity between candidate members and the template spectrum of each family.

Results. We identified an L-type asteroid family in the central main belt, with (460) Scania as its largest member. Analysis of the family’s V shape indicates that it is relatively old, with an estimated age of approximately 1 Gyr, which likely explains its non-detection by the HCM. The family’s existence is supported by statistical validation, and its distribution in proper element space is well reproduced by numerical simulations. Independent evidence from taxonomy, polarimetry, and spin-axis obliquities consistently supports the existence of this L-type family.

Conclusions. This work highlights the value of combining dynamical and physical data to characterize asteroid families and raises questions about the origin of L-type families, potentially linked to primordial objects retaining early protoplanetary disk properties. Further spectroscopic data are needed to clarify these families.