# Molecules with ALMA at Planet-forming Scales (MAPS). A Circumplanetary Disk Candidate in Molecular Line Emission in the AS 209 Disk

**Authors:** Jaehan Bae, Richard Teague, Sean M. Andrews, Myriam Benisty, Stefano Facchini, Maria Galloway-Sprietsma, Ryan A. Loomis, Yuri Aikawa, Felipe Alarcon, Edwin Bergin, Jennifer B. Bergner, Alice S. Booth, Gianni Cataldi, L. Ilsedore Cleeves, Ian Czekala, Viviana V. Guzman, Jane Huang, John D. Ilee, Nicolas T. Kurtovic, Charles J. Law  
**arXiv:** [2207.05923](https://arxiv.org/abs/2207.05923)  
**Date:** 2022-07-13  
**Summary page:** https://www.summarizepaper.com/en/paper/2207.05923v1/

## Key points

- Discovery of a circumplanetary disk (CPD) candidate in the circumstellar disk of T Tauri star AS 209 at 200 au
- CPD isolated via $^{13}$CO $J=2-1$ emission, first detection through gaseous emission
- CPD appears as spatially unresolved point source, estimated diameter less than 14 au
- CPD situated within annular gap previously identified using $^{12}$CO and near-infrared scattered light observations, associated with localized velocity perturbations in $^{12}$CO
- CPD gas temperature above 35 K, higher than surrounding circumstellar disk temperature at that radial location
- Estimated gas mass of CPD approximately 0.095 Jupiter masses or around 30 Earth masses based on standard $^{13}$CO abundance
- No millimeter continuum emission detected at CPD location, indicating low dust mass of less than 0.027 Earth masses or approximately 2.2 lunar masses
- Formation mechanism discussed in context of gravitational instability and pebble accretion processes, suggesting planet may have entered runaway accretion phase within last million years
- Future observational directions include additional ALMA observations to detect chemical asymmetries induced by planetary/CPD heating and infrared observations using telescopes like James Webb Space Telescope to probe thermal emissions from planet and constrain its mass further

## Summary

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We report the discovery of a circumplanetary disk (CPD) candidate embedded in the circumstellar disk of the T Tauri star AS 209 at a radial distance of about 200 au. The CPD was isolated via $^{13}$CO $J=2-1$ emission, marking the first instance of CPD detection through gaseous emission and allowing for the tracing of its overall mass. It appears as a spatially unresolved point source in $^{13}$CO, with an estimated diameter of less than 14 au. The CPD is situated within an annular gap previously identified using $^{12}$CO and near-infrared scattered light observations and is associated with localized velocity perturbations in $^{12}$CO, indicating a common origin with an embedded giant planet. Further analysis reveals that the CPD gas temperature is above 35 K, higher than the surrounding circumstellar disk temperature at that radial location. This suggests the presence of localized heating sources within the CPD. The estimated gas mass of the CPD is approximately 0.095 Jupiter masses or around 30 Earth masses based on a standard $^{13}$CO abundance. Additionally, no millimeter continuum emission was detected at the location of the CPD, indicating a dust mass of less than 0.027 Earth masses or approximately 2.2 lunar masses, resulting in a low dust-to-gas mass ratio. The formation mechanism of the CPD-hosting giant planet is discussed within the framework of gravitational instability and pebble accretion processes. The long core accretion timescale suggests that the planet may have entered a runaway accretion phase within the last million years due to slower radial drift or particle trapping enhancing pebble accretion efficiency. Future observational directions include additional ALMA observations to detect chemical asymmetries induced by planetary/CPD heating and infrared observations using telescopes like James Webb Space Telescope to probe thermal emissions from the planet and constrain its mass further. Monitoring for accretion signatures such as Hα line emission could provide valuable insights into the evolutionary stage and current position of the planet. In conclusion, this discovery sheds light on the complex interplay between giant planets, circumplanetary disks, and their host stars, offering new avenues for studying planetary formation processes on wide orbits in young stellar systems like AS 209.