Ancient and primordial collisional families as the main sources of X-type asteroids of the inner main belt

Marco Delbo,Alessandro Morbidelli,Chrysa Avdellidou

doi:10.1051/0004-6361/201834745

Abstract

Aims. The near-Earth asteroid population suggests the existence of an inner main belt source of asteroids that belongs to the spectroscopic X complex and has moderate albedos. The identification of such a source has been lacking so far. We argue that the most probable source is one or more collisional asteroid families that have escaped discovery up to now. Methods. We apply a novel method to search for asteroid families in the inner main-belt population of asteroids belonging to the X complex with moderate albedo. Instead of searching for asteroid clusters in orbital element space, which could be severely dispersed when older than some billions of years, our method looks for correlations between the orbital semimajor axis and the inverse size of asteroids. This correlation is the signature of members of collisional families that have drifted from a common centre under the effect of the Yarkovsky thermal effect. Results. We identify two previously unknown families in the inner main belt among the moderate-albedo X-complex asteroids. One of them, whose lowest numbered asteroid is (161) Athor, is ~3 Gyr old, whereas the second one, whose lowest numbered object is (689) Zita, could be as old as the solar system. Members of this latter family have orbital eccentricities and inclinations that spread them over the entire inner main belt, which is an indication that this family could be primordial, that is, it formed before the giant planet orbital instability. Conclusions. The vast majority of moderate-albedo X-complex asteroids of the inner main belt are genetically related, as they can be included into a few asteroid families. Only nine X-complex asteroids with moderate albedo of the inner main belt cannot be included in asteroid families. We suggest that these bodies formed by direct accretion of the solids in the protoplanetary disc, and are thus surviving planetesimals.

Highlights

IntroductionThe most energetic impacts can eject asteroid fragments at speeds larger than the gravitational escape velocity of the parent body
Selection of asteroids We restricted our search for the intermediate albedo component of the X complex and we select all asteroids of the inner main belt with 0.1 ≤ pV ≤ 0.3 belonging to the X complex and with spectral taxonomic classes of X, Xc, Xe, Xk, Xt, M, E, as described in Sect
2.28 au and 2.26 au and corresponds to the V shape of the Baptistina asteroid family (Nesvorný et al 2015; Bottke et al 2007; Masiero et al 2012b), while the first and second peaks represent previously unknown V shapes (Fig. A.1 gives a graphical representation of the V shapes)

Summary

Introduction

The most energetic impacts can eject asteroid fragments at speeds larger than the gravitational escape velocity of the parent body. This process can form families of daughter asteroids initially placed on orbits that group near that of the parent asteroid (Zappala et al 1984). Asteroid families can be recognised as clusters of bodies in proper orbital element space – proper semimajor axis, proper eccentricity, and proper inclination (a, e, i) – with significant contrast with respect to the local background (Milani et al 2014; Nesvorný et al 2015). The hierarchical clustering method (HCM; Zappalà et al 1990; Nesvorný et al 2015, and references therein) is typically used for the identification of these asteroid clusters.

Methods

Results

Discussion

Conclusion