- The paper demonstrates that gravitational collapse of slowly rotating pebble clouds leads to the formation of flattened, ellipsoidal planetesimals resembling observed contact binaries and lobed comets.
- It employs an adapted Monte-Carlo method solving the Boltzmann-Enskog equation to simulate complex granular dynamics, including particle collisions and self-gravity.
- The findings provide a theoretical basis for understanding the disc-like shapes of small Solar System bodies, highlighting the influence of initial angular momentum on planetesimal formation.
Formation of Flattened Planetesimals by Gravitational Collapse
This paper investigates the formation of planetesimals through the gravitational collapse of rotating pebble clouds, providing insights into the shapes observed in bodies such as comets and Kuiper Belt Objects (KBOs). The paper utilizes numerical simulations grounded in granular dynamics principles to assess how these structures transition from loosely bound pebble swarms into dense, solid bodies.
Methodology
The authors employed a Monte-Carlo method adapted to solve the Boltzmann-Enskog equation to simulate the dynamical behavior of pebble clouds. This approach accounts for both particle collisions and self-gravity effects within a discretized spatial grid. The model accommodates approximately 10<sup\>18</sup> millimeter-sized pebbles, making direct simulation computationally infeasible; thus, the method abstracts them into computational particles to follow the evolution of particle distribution function influenced by collisions and gravity.
Key assumptions in the simulations include:
- Initial pebbles uniformly distributed and their velocities following a Maxwellian distribution.
- Variations in angular momentum characterized as a fraction of the critical angular momentum for forming ellipsoidal bodies.
The paper adjusts the initial conditions, particularly the angular momentum content, to explore their influence on the resulting planetesimal shape.
Numerical Results
The simulation outcomes demonstrate that the gravitational collapse of slowly rotating pebble clouds leads to naturally flattened ellipsoidal bodies. These results align closely with observations of certain Solar System bodies, particularly contact binaries like Arrokoth and the lobes of comets such as 67P/Churyumov-Gerasimenko.
Key findings include:
- Planetesimals arising from clouds with lower angular momentum (L/L<sub>J</sub> < 1) tend to form single elongated or flattened bodies.
- In clouds with higher angular momentum, the formation process might lead to binary systems, though this paper's fixed grid approach limits insights into binary orbital characteristics.
Implications and Future Directions
The paper's results provide a robust foundation supporting the hypothesis that physical properties observed in minor Solar System bodies can be traced back to their formative conditions involving pebble cloud collapse. The shape analysis suggests morphological characteristics, such as the disc-like nature seen in `Oumuamua, could originate from high angular momentum conditions in such collapse scenarios.
The broader implications of this work point towards reevaluating formation scenarios for other irregular bodies within our Solar System. There is potential for further research integrating higher resolutions and varying initial conditions, potentially influenced by surrounding gas dynamics, to explore binary formation in more detail.
The paper opens up pathways for exploring the connection between primordial formation environments and present-day morphology of small bodies, bridging theoretical models with increasingly detailed observational data. This underscores the need to consider fine-scale dynamics and complex environmental interactions in understanding planetesimal evolution and formation in early protoplanetary disks.