# Collisional Elongation: Possible Origin of Extremely Elongated Shape of 1I/`Oumuamua

## TL;DR

This study proposes that the extremely elongated shape of interstellar object 1I/`Oumuamua could result from specific planetesimal collision impacts, supported by numerical simulations of such impacts under various conditions.

## Contribution

It provides detailed impact conditions and simulations demonstrating how collisions can produce highly elongated planetesimal shapes, explaining 1I/`Oumuamua's morphology.

## Key findings

- Impacts with specific mass ratios and angles produce elongated remnants.
- Weakly turbulent protoplanetary disks favor formation of elongated shapes.
- Small planetesimals (<7 km) are likely involved in such impact scenarios.

## Abstract

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 degrees, v_imp \leq 40 degrees, and theta_imp \leq 30 degrees 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 ~ 7 km to ensure small impact velocity.