  More than a half of asteroids in the main belt have irregular shapes with the
ratios of the minor to major axis lengths less than 0.6. One of the mechanisms
to create such shapes is collisions between asteroids. The relationship between
shapes of collisional outcomes and impact conditions such as impact velocities
may provide information on the collisional environments and its evolutionary
stages when those asteroids are created. In this study, we perform numerical
simulations of collisional destruction of asteroids with radii 50 km and
subsequent gravitational reaccumulation using Smoothed Particle Hydrodynamics
for elastic dynamics with self-gravity, a model of fracture of rock, and a
model of friction of completely damaged rock. We systematically vary the impact
velocity from 50 m/s to 400 m/s and the impact angle from 5 degrees to 45
degrees. We investigate shapes of the largest remnants resulting from
collisional simulations. As a result, various shapes (bilobed, spherical, flat,
elongated, and hemispherical shapes) are formed through equal-mass and
low-velocity (50 - 400 m/s) impacts. We clarify a range of the impact angle and
velocity to form each shape. Our results indicate that irregular shapes,
especially flat shapes, of asteroids with diameters larger than 80 km are
likely to be formed through similar-mass and low-velocity impacts, which are
likely to occur in the primordial environment prior to the formation of
Jupiter.
