In Barbarian territory: Spectral bimodality in L-type asteroids and their link to CO-CV chondrites

Introduction: L-type asteroids represent a rare taxonomic class that is spectrally similar to CO and CV carbonaceous chondrites [1]. Their spectra often suggest highabundances of refractory materials like calcium-aluminium-rich inclusions (CAIs), implying their parent bodies were among the earliest chondrites to form in the Solar System [2]. CO and CV chondrites in turn contain the highest vol% of refractory inclusions in the meteorite collection and are predominantly of petrographic type 3, with varying degrees of thermal metamorphsim [3]. Understanding the distribution and spectral characteristics of L-types is thus crucial for deciphering early Solar System composition and evolution. However, their rarity and spectral ambiguity with other classes have hindered comprehensive characterisation [4, 5]. Our study aimed to identify new L-types within known associated asteroid families, provide detailed UV-VisNIR spectral characterisation using VLT/XSHOOTER [6], assess the spectral diversity within the class, and investigate links to chondritic meteorites.

Methods: We obtained high-quality UV-VisNIR (0.30µm to 2.48µm) reflectance spectra of nine asteroids belonging to five families previously associated with L-types (Aquitania, Brangäne, Henan, Tirela, Watsonia [7, 8, 9]) using VLT/XSHOOTER. We classified them using different taxonomies [10, 11, 12] and compared them quantitatively with laboratory reflectance spectra of ordinary, CO, and CV chondrites [13, 14].

Results: Our observations successfully identified five new L-type asteroids, expanding the sample of this class with VisNIR wavelength coverage by ~30%. The remaining targets were classified as S-types. The new L-type spectra confirm the considerable spectral diversity previously noted for this population [12], often falling taxonomically between or on the edges of established classes (e.g., L, M, S) depending on the classification system and spectral features considered.

Combining our new data with existing VisNIR spectra of L-types and related "Barbarian" asteroids (polarimetrically defined and likely compositionally linked), we investigated the overall spectral distribution of this population. A potential bimodal distribution emerged, separating the L-type/Barbarian asteroids into two groups, tentatively named L_L and L_M. As illustrated by the mean spectra and albedos (see Figure 1), these groups primarily differ in their 2µm absorption band depth and geometric albedo: the L_L group exhibits generally higher albedos and a more pronounced 2µm feature, while the L_M group shows lower albedos and weaker spectral features, resembling M-type asteroids in overall shape but presenting subtle features. Notably, members from the same dynamical families (e.g., Aquitania, Brangäne) are found in both the L_L and L_M groups.

A similar analysis of laboratory spectra of CO and CV chondrites revealed analogous spectral diversity. CO chondrites, in particular, displayed a similar bimodal separation in spectral features and albedo (denoted here as “CO_r” and “CO_p” groups), mirroring the L_L/L_M asteroid split (Figure 1). CV chondrites also showed significant spectral variation across their subclasses (CV_Red, CV_OxA, CV_OxB [15]), correlating feature strength with albedo. Comparisons suggest that the L_L asteroids share spectral characteristics (stronger features, higher albedo) with CO_r and CV_OxA chondrites, while the L_M asteroids align better with the lower albedo, weaker-featured CO_p, CV_Red, and CV_OxB chondrites, once expected space weathering effects (primarily reddening) are considered [16, 17].

Figure 1: Reflectance spectra and albedo of CO-CV classes and subgroups and the two identified L-type populations. Shown are the mean values and the standard deviation in all wavelengths bins and the albedo distribution. The spectra are vertically offset for comparability.

Conclusions: The expanded census and detailed characterisation of L-type asteroids reveal a spectrally diverse population, likely sampling ancient parent bodies related to CO and CV chondrites. The observed spectral bimodality (L_L/L_M), mirrored in laboratory data for CO and CV chondrites, strongly suggests significant heterogeneity within the parent bodies of multiple asteroid families. This heterogeneity may stem from differing degrees of thermal metamorphism across the original planetesimals. The presence of members from the same dynamical family in both spectral groups further supports the concept of compositionally heterogeneous, yet related, L-type parent bodies.

The distinct UV-visible slope of L-types compared to S-types promises L-type identification in future Gaia data releases (e.g., DR4, mid 2026). The recently launched SPHEREx mission, providing near-infrared spectra, will be crucial for robustly testing the proposed bimodality, refining mineralogical interpretations, and fully characterising the distribution and nature of these enigmatic Solar System relics.

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Acknowledgements: The authors acknowledge important contributions by A. Aleón-Toppani to this work. This work was supported by the Programme National de Planétologie (PNP) of CNRS-INSU co-funded by CNES.