Near-ultraviolet absorption distribution of primitive asteroids from photometric surveys

II. Collisional families

¹ Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Sagamihara, Kanagawa, Japan
² Planetary Science Institute (PSI), Tucson, AZ, USA
³ Instituto de Astrofísica de Canarias (IAC), University of La Laguna, La Laguna, Tenerife, Spain
⁴ Department of Astrophysics, University of La Laguna, La Laguna, Tenerife, Spain
⁵ Astronomical Institute of the Romanian Academy, 5 Cuţitul de Argint, 040557 Bucharest, Romania
⁶ Florida Space Institute, University of Central Florida, Orland, FL, USA

^★ Corresponding author; tatsumi.eri@jaxa.jp

Received: 9 May 2024
Accepted: 12 November 2024

Abstract

Context. Primitive asteroids consisting of mainly phyllosilicates and opaque minerals have great variation at near-ultraviolet (NUV) wavelengths (0.35–0.5 μm). The absorption in NUV could be indicative of phyllosilicates that reflect their formation environments such as the distribution of water, temperature, and pressure. The asteroid collisional families are the fragments of large primordial bodies that record the early Solar System environments.

Aims. Our objective is to investigate the reflectance spectrophotometry of primitive asteroid families in NUV to visible (VIS) wavelengths to constrain the internal structure and formation of primordial bodies.

Methods. The NUV-VIS reflectance spectrophotometry of 38 primitive asteroid families was investigated using two spectrophotometric surveys, the Eight Color Asteroid Survey (ECAS) and the Sloan Digital Sky Survey (SDSS). We classified the members of the primitive asteroid families based on Tholen’s taxonomy. After grouping these families into eight overarching types, we discussed the compositions of primitive asteroid families based on the NUV, 0.7 μm, 3 μm absorptions, and the near-infrared (NIR) spectral slopes.

Results. We have found a correlation between the 0.7 μm absorption band and the NUV absorption among the asteroid families, suggesting that both features are caused by the charge transfer of interlayer iron in phyllosilicates. This implies that NUV absorption can be a valuable indicator of Fe-rich phyllosilicate abundance. Furthermore, we have revealed correlations between the NUV absorption, VIS slope, albedo, and the NIR slope. Primitive asteroid families with strong NUV absorption exhibit a high albedo and a low NIR slope (1.25–2.14 μm). The Pallas family deviates from this general trend due to its exceptionally high albedo. This anomaly, combined with the Pallas family’s unique density and the deep and sharp 3 μm absorption, suggests that the Pallas family could be a potential source of CR chondrites. Overall, our results demonstrate that NUV absorption aligns well with established indicators of phyllosilicate presence (0.7 μm and 3 μm absorptions). The largest bodies in the high-NUV absorption families show a sharp 3-μm feature, while the red-dominant families show a w-shaped 3-μm feature. Notably, two young endmember families (Theobalda, F-dominant; Veritas, G-dominant) highlight that NUV absorption is not solely linked to aging or space weathering, but likely reflects inherent compositional differences. The Polana–Eulalia complex family and the Theobalda family, dominated by F types (>80%), exhibit minimal to non-NUV absorption, suggesting that their primordial bodies contained little Fe-rich phyllosilicates, such as CI drated carbonaceous chondrites. Conversely, the Veritas family, with over 80% of C and G types, displays stron. indicative of an Fe-rich phyllosilicate-rich parent body, such as CM chondrites.