# Detection of exocometary CO within the 440 Myr-old Fomalhaut belt: a similar CO+CO$_2$ ice abundance in exocomets and Solar System comets

## TL;DR

This study reports the first detection of exocometary CO gas in the Fomalhaut belt, revealing similar ice compositions to Solar System comets and providing insights into volatile delivery and cometary origins in planetary systems.

## Contribution

It presents the first detection of CO in the Fomalhaut belt and estimates exocometary ice composition, highlighting similarities with Solar System comets and implications for planetary system formation.

## Key findings

- Detected CO gas with 5.4σ significance in Fomalhaut belt.
- Estimated exocometary CO+CO₂ mass fraction between 4.6% and 76%.
- Found potential azimuthal asymmetry in CO distribution near belt pericentre.

## Abstract

Recent ALMA observations present mounting evidence for the presence of exocometary gas released within Kuiper belt analogues around nearby main sequence stars. This represents a unique opportunity to study their ice reservoir at the younger ages when volatile delivery to planets is most likely to occur. We here present the detection of CO J=2-1 emission co-located with dust emission from the cometary belt in the 440 Myr-old Fomalhaut system. Through spectro-spatial filtering, we achieve a 5.4$\sigma$ detection and determine that the ring's sky-projected rotation axis matches that of the star. The CO mass derived ($0.65-42 \times10^{-7}$ M$_{\oplus}$) is the lowest of any circumstellar disk detected to date, and must be of exocometary origin. Using a steady state model, we estimate the CO+CO$_2$ mass fraction of exocomets around Fomalhaut to be between 4.6-76%, consistent with Solar System comets and the two other belts known to host exocometary gas. This is the first indication of a similarity in cometary compositions across planetary systems that may be linked to their formation scenario and is consistent with direct ISM inheritance. In addition, we find tentative evidence that $(49\pm 27)$% of the detected flux originates from a region near the eccentric belt's pericentre. If confirmed, the latter may be explained through a recent impact event or CO pericentre glow due to exocometary release within a steady state collisional cascade. In the latter scenario, we show how the azimuthal dependence of the CO release rate leads to asymmetries in gas observations of eccentric exocometary belts.