  Asteroid pairs had a single progenitor that split due to rotational-fission
of a weak, rubble-pile structured body. By constructing shape models of
asteroid pairs from multiple-apparition observations and using a lightcurve
inversion technique, we mapped the gravitational and rotational accelerations
on the surfaces of these asteroids. This allows us to construct a map of local
slopes on the asteroids' surfaces. In order to test for frictional failure, we
determine the maximum rotation rate at which an area larger than half the
surface area of the secondary member (assumed to be the ejected component) has
a slope value greater than 40 degrees, the angle of friction of lunar regolith,
where loose material will begin sliding. We use this criterion to constrain the
failure stress operating on the body, just before disruption at the commonly
observed spin barrier of 2.2 h. Our current sample includes shape models of
eleven primary members of asteroid pairs, observed from the Wise Observatory in
the last decade. In the studied parameter space we find that the shape models
only reach the spin barrier when their bulk density is larger than the ~2 gr
cm-3 measured for the rubble pile structured 25143 Itokawa, suggesting that
km-sized asteroid pairs are dense compared to sub-km bodies. Assuming ejection
of secondary components that are larger than those observed (up to the maximal
size allowing separation), can also increase the spin barrier of the asteroids,
thus supporting the previously suggested scenario of continuous disruption of
the secondary. In addition, cohesion levels of hundreds of Pascals are also
required to prevent these shape models from disrupting at spin rates slower
than the usual spin barrier.
