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

TL;DR

This work tackles the challenge of identifying old, dispersed asteroid families by augmenting dynamical clustering with Gaia DR3 visible spectra. By building a color taxonomy and a spectral template from known L-types, the authors uncover an L-type family in the central Main Belt, led by (460) Scania, and confirm its existence with a V-shape analysis and an age of about $1.1 \\pm \\ 0.6$ Gyr. Independent numerical simulations with Yarkovsky forces reproduce the observed dispersion and suggest a breakup ~1 Gyr ago, possibly with anisotropic initial ejection. Polarimetry and spin-obliquity data further corroborate a Barbarian-like composition and a coherent drift pattern, while comparisons to literature reveal HCM's limitations in detecting such ancient families; the study highlights the value of integrating dynamical and physical data and motivates further spectroscopic campaigns.

Abstract

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 374-1034 nm, representing a large sample that is well suited to studies of asteroid families. Using Gaia spectroscopic data, we investigate 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 underscore the need for an independent dynamical analysis of this region. We determine 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. We identify 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. 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.

Figures (10)

• Figure 1: Template spectrum of the L- type family (in black). The reflectances have been computed by averaging the Gaia spectra of (460) Scania, in blue, (2085) Henan, in orange, and (2354) Lavrov, in green. The area in light blue represents the region where the spectra of the family members identified in our analysis fall within one standard deviation.
• Figure 2: Distribution in the phase space of the proper elements of the L- types (blue circles) and all the other objects observed by Gaia (grey circles). The four largest L- types are marked by diamonds. The panels are, from left to right, ($a$, $e$), ($a$, $\sin(i)$), ($e$, $\sin(i)$) and ($a$, $H$).
• Figure 3: Family members (in blue) in the ($a$, $1/D$) plane fitted by V-shapes corresponding to different ages. The green continuous line corresponds to 1.5 Gyr and the red dashed line to 1.0 Gyr.
• Figure 4: Result of the V-shape-searching method. The value of $N_{in}^2/N_{out}$ is plotted as a function of the slope ($K$) and semimajor axis of the vertex of a V-shape ($a_c$). The parameter $C$, which quantifies the width of the V shape in the ($a$, $H$) space, is computed from $C = 1/K \sqrt{p_V} / 1329$, where the geometrical visible albedo $p_V$ is set to 0.2.
• Figure 5: Comparison between the proper elements of the family members observed by Gaia (grey circles) and the mean proper elements of the synthetic family members integrated in REBOUND (blue stars). The panels are, respectively, from left to right, ($a$, $e$), ($a$, $\sin(i)$), ($e$, $\sin(i)$), and ($a$, $1/D$).