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Toward understanding the origin of asteroid geometries-Variety in shapes produced by equal-mass impacts

More than a half of the asteroids in the main belt have irregular shapes with ratios of the minor to major axis lengths of less than 0.6. One of the mechanisms that create such shapes is collisions between asteroids. The relationship between the 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 selfgravity, a model of rock fractures, and a model of friction in completely damaged rock. We systematically vary the impact velocity from 50 to 400 m s−1 and the impact angle from 5◦ to 45◦. 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−1) 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.