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

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 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 Xcomplex with moderate albedo. Instead of searching for asteroid clusters in orbital elements 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, which 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 Gyrs-old, whereas the second one, whose lowest numbered object is (689) Zita, can 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, i.e. 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 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 disk, and are thus surviving planetesimals.

Efficiency characterization of the V-shape asteroid family detection method

“…Figure 5 shows an increase of loss within the families when considering collisions that is primarily due to the depletion of the smallest members of the family. This is intuitive and observed in very old families that are mainly composed by larger objects (Delbó et al, 2017(Delbó et al, , 2019. Regardless of considering or not collisional evolution, the density of a family in the quasi-proper 1∕ space decreases to ∼90% within the first 100-300 Myr (figure 5 right panel).…”

“…As anticipated in the previous section, edge effect is mostly a result of having a small number of objects outside the V related to those inside the V (where → 0 would lead to 2 ∕ → ∞). Therefore, edge effect could result as a strong signal in a K vs diagram (commonly used in many previous works as a tool to find family signals; Bolin et al, 2017Bolin et al, , 2018aDelbó et al, 2017Delbó et al, , 2019, even if no family was present. However, we have to point out that: 1) The edge effect is highly dependent on the searching method applied, i.e., whether both-sides or left/right-side are used.…”

“…Of course, depending on the density of the background and how isolated the family is in the data set considered, the edge of the family can be so clear that even a narrower sliver can detect its signal. A good example of that is the case for the ancient families found by Delbó et al (2017Delbó et al ( , 2019, where the selection of the data set used allowed the families found to be recognized even by eye. This can also, of course, be problematic when the search becomes so wide that it encounters the edges of a region of the asteroid belt, thus adding to the complications of finding very old families.…”

“…This technique, not considered in the present work, was developed by Bolin et al (2018b) and consists in giving more importance for the large diameter objects above the nominal V, rather than accounting for every object as we do. Therefore, although the strategy used by Delbó et al (2017Delbó et al ( , 2019 is correct and should be used in every case where physical properties are well known, as well as that from Bolin et al (2018b) (certainly a much better approach to characterize the precise slope of the family/signal found by any method or data selection), our method intention is to characterize the best set of parameter that most efficiently return a signal that well represents a family slope and center, while being more general and less dependent on data selection. As a final note, recall that what is shown in figure 10 is the optimal value for detection with efficiency larger than 50% for a both side scoring approach (Delbó et al, 2017, for example considered a left side scoring approach; see section 3.3).…”

Machine-learning identification of asteroid groups

“…based on cladistic classifications have been recently proposed for the identification of asteroid families among Jupiter Trojans Holt et al (2017). Other approaches, based on the V-shape identification in domains of (a, 1/D), with a the semimajor axis and D the asteroid diameter, have been recently used to identify very old families not easily detectable using standard HCM (Bolin et al 2017(Bolin et al , 2018Delbó et al 2017;Delbó, Avdellidou & Morbidelli 2019).…”