Abstract
A small percentage (~25%) of discovered asteroids has been associated with more than 100 known asteroid families, generated from the fragmentation of a common parent body. The determination of which asteroids of the remaining population are members of undiscovered families would constrain the original planetesimal population. Such a classification of the asteroids seems to be the key for the understanding of the dynamical and the collisional evolution of the asteroid population and the correlation between of them. Asteroid families are typically identified through clustering methods based on orbital elements. However, alternative methods, such as the V-shape method, which analyses correlations between asteroid fragment sizes and their distance from the family centre, have proven effective in identifying very old and diffuse families. This PhD thesis focuses on the study of the spin states of asteroids in the inner main belt, that they belong to ancient collisional families identified b ...
A small percentage (~25%) of discovered asteroids has been associated with more than 100 known asteroid families, generated from the fragmentation of a common parent body. The determination of which asteroids of the remaining population are members of undiscovered families would constrain the original planetesimal population. Such a classification of the asteroids seems to be the key for the understanding of the dynamical and the collisional evolution of the asteroid population and the correlation between of them. Asteroid families are typically identified through clustering methods based on orbital elements. However, alternative methods, such as the V-shape method, which analyses correlations between asteroid fragment sizes and their distance from the family centre, have proven effective in identifying very old and diffuse families. This PhD thesis focuses on the study of the spin states of asteroids in the inner main belt, that they belong to ancient collisional families identified by V-shape method. Namely, three families were studied: a low-albedo primordial family, and the Athor and Zita families. The low-albedo primordial family is characterised by featureless spectra, where only the inward side of the family is identified so far. The age of this family is estimated to be about 4 Gyr, meaning it could be as old as the Solar System itself. The Athor and Zita families contain asteroids that belong spectroscopically to the X-complex. The Athor family is about 3 Gyr, and the Zita family is potentially as old as the Solar System. Both the Athor and Zita families are believed to be sources of near-Earth X-complex asteroids. Additionally, the Athor family has been linked to low-iron enstatite (EL) meteorites. The main objective of this research is to analyse the spin states of members of these families to confirm their common origin. The primary method employed is based on the established theory of asteroid family evolution, which predicts a predominance of retrograde-rotating asteroids on the inward side of a family's V-shape and prograde-rotating asteroids on the outward side, a pattern driven by the Yarkovsky effect. The study utilises dense and sparse-in-time photometric data, obtained through a dedicated observing campaign called Ancient Asteroids, specifically initiated for this PhD research, and archival sources, to construct asteroid rotational light curves. The light curve inversion method is then applied to determine the sidereal period, 3D convex shape, and spin axis orientation of numerous asteroids in all three families. The Ancient Asteroids observing campaign stands as a testament to the power of international collaboration, uniting amateur and professional astronomers around the globe, collecting data for 120 asteroids, producing almost 700 lightcurves used in this research constructing of over 115 high-resolution asteroid models. The results reveal distinct spin state distributions consistent with the theoretical predictions. For the low-albedo primordial family, the study found that 31 out of 46 confirmed asteroid members are retrograde and 15 are prograde. This predominance of retrograde compared to prograde asteroids is very unlikely (1.29% probability) to be due to sampling a distribution of objects with an equal probability of being prograde and retrograde. The Athor family exhibits a significant excess of retrograde asteroids on the inward side and prograde asteroids on the outward side of its V-shape, a pattern even more pronounced when considering only the family's core members. Similarly, the Zita family displays a high percentage of retrograde asteroids on its inward side. Furthermore, while the Zita family shows a more balanced distribution on its outward side, analysis using Kernel density estimation (KDE) reveals a peak for prograde asteroids in this region, supporting the expected pattern. These findings offer compelling evidence for the common origin of members within each family and validate the effectiveness of the V-shape method in identifying ancient and dispersed families. The spin state analysis also reveals potential cases of spin-orbit resonance among family members, a phenomenon requiring further investigation. In conclusion, the comprehensive analysis of spin states presented in this thesis provides strong support for the existence and characteristics of a low-albedo primordial family, and the Athor and Zita families. The findings confirm their ancient origins and potential contribution to the near-Earth asteroid population for the latter two, while also highlighting the role of the Yarkovsky effect in shaping the rotational properties of family members. This work advances our understanding of the Solar System's evolution and underscores the value of spin state analysis in unravelling the history of asteroid families.
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