Ancient Asteroids: An observing campaign reveals the spin states of asteroids that belong to the most ancient collisional families of our Solar System

Abstract

The first moments of our Solar System are determined by the gas phase of the protoplanetary disk, where the first solid bodies, the planetesimals, were formed. While planetesimals accreted to form planets, a lot of them were also removed during the violent phases of dynamical interaction with the growing planets, planetary migration and instability. However, few leftover planetesimals are still present today in the solar system as dwarf planets or even smaller bodies. Those of the inner Solar System formed an asteroid main belt. Reconstructing the original state of main belt is thus a crucial problem of planetary science to study the formation of the planets. The main belt collisions disrupted several of those original planetesimals creating new asteroids that initially stayed relatively close to the parent body, creating the so-called asteroid collisional families. But members of families disperse over time due to the Yarkovsky effect and interaction with orbital resonances. Traditional identification methods are unable to effectively recognize Gyr- and older asteroid families, which family members are very dispersed. A novel technique takes the advantage of the correlation between semimajor axis and size imprinted on family members by the Yarkovsky effect to identify the most ancient collisional families of the Solar System. The method has already successfully identified three primordial families which likely formed before the giant planet orbital instability and could be as old as the Solar System and an ancient one that is ~3 Gyr-old. There is evidence from observations and theoretical evolution models that there could be more old families than the ones currently detected. Here we report on the assessment of the reliability of these families by means of data independent of those used for the family detection: namely, we used the direction of the spin state of families' members. These data have been collected by the international observing campaign "Ancient Asteroids" involving both professionals and amateur astronomers. During the last 3 years, photometric observations of more than 120 asteroids were performed. The obtained lightcurves were combined with dense photometric data available in the literature, as well as with sparse data from space missions and global sky surveys. The results of this research confirm the existence of a 4 Gry-old low-albedo collisional family in the inner Main-Belt and uses the spin states as a crucial physical parameter for the reassessment of the members that belong to Athor and Zita families, which members are overlapping with similar spectral and albedo characteristics.