Abstrakt
We studied asteroid clusters suggesting a possibility of at least two disruption events in their recent history (<= 5Myr). We searched for new members of known asteroid pairs and clusters and we verified their membership using backward orbital integrations.
We found four asteroid clusters, namely the clusters of (11842) Kap'bos, (14627) Emilkowalski, (63440) 2001 MD30 and (157123) 2004 NW5 that show at least two secondary separation events that occurred at significantly different times. We considered a possible formation mechanism for these clusters: The parent of an asteroid cluster was spun up to its critical rotation frequency, underwent a rotation fission and was slowed down by escape of the newly formed secondary/ies.
Then the YORP effect spun up the primary again and it reached its critical rotation frequency and underwent another fission. We created a simple model to test whether the scenario of two rotation fission events of a parent primary induced via the YORP effect is possible for the four clusters.
We obtained a good agreement between the model and the cluster properties for the clusters of Kap'bos and (63440). For the cluster of Emilkowalski, our model explained the unusually slow rotation of the primary.
However, the time needed for the primary to reach its critical frequency after the first fission event was predicted to be too long by a factor of several. We suspect, considering also its D type taxonomic classification and the existence of a dust band associated with the cluster, that the asteroid Emilkowalski may actually be a cometary nucleus.
Regarding the cluster of (157123), the final rotational frequency of the primary after the last fission event predicted by our model is in a good agreement with the observed rotation frequency of (157123). However, a separation of the older secondary is not possible in our model due to the deficiency of free energy needed for an escape of the large secondary.
This could be due to an error in the H value of the secondary or the possibility that we did not find the real primary of this cluster.y