Light curve observations of a recently discovered interstellar object 1I/ ‘ Oumuamua suggest that this object has an extremely elongated shape with the axis ratio 0.3 or smaller .
Planetesimal collisions can produce irregular shapes including elongated shapes .
In this paper , we suggest that the extremely elongated shape of 1I/ ‘ Oumuamua may be the result of such an impact .
To find detailed impact conditions to form the extremely elongated objects , we conduct numerical simulations of planetesimal collisions using Smoothed Particle Hydrodynamics method for elastic dynamics with self-gravity and interparticle friction .
Impacts into strengthless target planetesimals with radius 50 { m } are conducted with various ratios of impactor mass to target mass q , friction angles \phi _ { { d } } , impact velocities v _ { { imp } } , and impact angles \theta _ { { imp } } .
We find that impacts with q \geq 0.5 , \phi _ { { d } } \geq 40 ^ { \circ } , v _ { { imp } } \leq 40 { cm / s } , and \theta _ { { imp } } \leq 30 ^ { \circ } produce remnants with the ratio of intermediate to major axis length less than 0.3 .
This impact condition suggests that the parent protoplanetary disk in the planetesimal collision stage was weakly turbulent ( \alpha < 10 ^ { -4 } for the inner disk ) and composed of planetesimals smaller than \sim 7 { km } to ensure small impact velocity .