# Vertical evolution of exocometary gas: I. How vertical diffusion shortens the CO lifetime

**Authors:** S. Marino, G. Cataldi, M. R. Jankovic, L. Matrà, M. C. Wyatt  
**arXiv:** [2206.11071](https://arxiv.org/abs/2206.11071)  
**Date:** 2022-06-22  
**Summary page:** https://www.summarizepaper.com/en/paper/2206.11071v2/

## Key points

- Exocometary gas study is important for understanding planetary system formation and evolution.
- Bright debris discs contain large amounts of CO gas, which was initially thought to be a protoplanetary remnant.
- Recent research shows that CO could be released in collisions of volatile-rich solids.
- Interstellar UV radiation photodissociates CO producing CI, which can shield CO allowing a large CO mass to accumulate.
- CI is inefficient at shielding if CO and CI are vertically mixed.
- The vertical evolution of gas was studied for the first time to determine how vertical mixing affects the efficiency of shielding by CI.
- A 1D model was presented that accounts for gas release, photodissociation, ionisation, viscous evolution, and vertical mixing due to turbulent diffusion.
- If the gas surface density is high and the vertical diffusion weak ($\alpha_{\rm v}/\alpha<[H/r]^2$), CO photodissociates high above the midplane forming an optically thick CI layer that shields the CO underneath. Conversely, if diffusion is strong ($\alpha_{\rm v}/\alpha>[H/r]^2$), CI and CO become well mixed shortening the lifetime of CO.
- Diffusion could also limit dust settling.
- High-resolution ALMA observations could resolve the vertical distribution of CO and CI and thus constrain vertical mixing and the efficiency of CI shielding.
- The researchers found that the CO and CI scale heights may not be good probes of the mean molecular weight or composition of gas.
- Strong enough mixing might shorten the lifetime of CO so it cannot spread interior to where gas is produced in planetesimal belts.
- This study provides crucial insights into exocometary gas's vertical evolution and its effect on shielding efficiency by CI with significant implications for our understanding of planetary system formation and evolution.

## Summary

The study of exocometary gas is essential in understanding the formation and evolution of planetary systems. Bright debris discs have been found to contain large amounts of CO gas, which was initially thought to be a protoplanetary remnant. However, recent research has shown that it could be released in collisions of volatile-rich solids. As CO is released, interstellar UV radiation photodissociates CO producing CI, which can shield CO allowing a large CO mass to accumulate. However, this picture was challenged because CI is inefficient at shielding if CO and CI are vertically mixed. In this study, conducted by S. Marino et al., the vertical evolution of gas was studied for the first time to determine how vertical mixing affects the efficiency of shielding by CI. The researchers presented a 1D model that accounts for gas release, photodissociation, ionisation, viscous evolution, and vertical mixing due to turbulent diffusion. They found that if the gas surface density is high and the vertical diffusion weak ($\alpha_{\rm v}/\alpha<[H/r]^2$), CO photodissociates high above the midplane forming an optically thick CI layer that shields the CO underneath. Conversely, if diffusion is strong ($\alpha_{\rm v}/\alpha>[H/r]^2$), CI and CO become well mixed shortening the lifetime of CO. Moreover, diffusion could also limit the amount of dust settling. High-resolution ALMA observations could resolve the vertical distribution of CO and CI and thus constrain vertical mixing and the efficiency of CI shielding. The researchers also found that the CO and CI scale heights may not be good probes of the mean molecular weight or composition of gas. Finally, they showed that if mixing is strong enough then the lifetime of CO might not be long enough for it to spread interior to where gas is produced in planetesimal belts. This study provides crucial insights into the vertical evolution of exocometary gas and how it affects the shielding efficiency of CI. The findings could have significant implications for our understanding of planetary system formation and evolution.