# Exciting spiral arms in protoplanetary discs from flybys

**Authors:** Jeremy L. Smallwood, Chao-Chin Yang, Zhaohuan Zhu, Rebecca G. Martin, Ruobing Dong, Nicolás Cuello, Andrea Isella  
**arXiv:** [2303.05753](https://arxiv.org/abs/2303.05753)  
**Date:** 2023-03-10  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2303.05753v1/

## Key points

- Spiral arms are observed in protoplanetary discs and can be triggered by companions on bound or unbound orbits.
- Researchers simulated an unbound perturber, or flyby, exciting spiral arms during a periastron passage.
- The team conducted three-dimensional hydrodynamical simulations of a parabolic flyby encountering a gaseous protoplanetary disc, with the perturber's mass ranging from 10 Jupiter masses to 1 solar mass.
- Higher mass perturbers produced more prominent spiral features.
- The initial pattern speed of the two arms was close to the angular velocity of the perturber at periastron but decreased over time.
- The pitch angle also decreased as the spiral wound up and disappeared after several local orbital times.
- An inclined prograde orbit flyby induced similar disc substructures as a coplanar flyby.
- A solar-mass flyby event caused increased eccentricity growth in the protoplanetary disc, leading to an eccentric disc structure that dampened over time.
- The morphology of spirals and disc eccentricity could potentially be used to search for potential unbound stars or planets around discs where a flyby is suspected.
- Future high-resolution observations and dedicated surveys will help constrain the frequency of such stellar encounters in nearby star-forming regions.

## Summary

Spiral arms are commonly observed in protoplanetary discs and can be triggered by companions on bound or unbound orbits. In a recent study, researchers simulated an unbound perturber, or flyby, exciting spiral arms during a periastron passage. The team conducted three-dimensional hydrodynamical simulations of a parabolic flyby encountering a gaseous protoplanetary disc, with the perturber's mass ranging from 10 Jupiter masses to 1 solar mass. The results showed that the perturber excited a two-armed spiral structure, with higher mass perturbers producing more prominent spiral features. Over time, the two arms evolved and eventually wound up, consistent with previous studies. The researchers focused on analyzing the pattern speed and pitch angle of these spirals throughout the process. They found that the initial pattern speed of the two arms was close to the angular velocity of the perturber at periastron but decreased over time. The pitch angle also decreased as the spiral wound up and disappeared after several local orbital times. Interestingly, an inclined prograde orbit flyby induced similar disc substructures as a coplanar flyby. A solar-mass flyby event caused increased eccentricity growth in the protoplanetary disc, leading to an eccentric disc structure that dampened over time. The morphology of spirals and disc eccentricity could potentially be used to search for potential unbound stars or planets around discs where a flyby is suspected. Future high-resolution observations and dedicated surveys will help constrain the frequency of such stellar encounters in nearby star-forming regions. Overall, this study sheds light on how unbound perturbers can excite spiral arms in protoplanetary discs and provides insights into their evolution over time.