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AB Aur, a Rosetta stone for studies of planet formation (IV): C/O estimates from CS and SO interferometric observations

Using new NOEMA interferometric observations of CS and SO in the AB Aur protoplanetary disk, the study derives a high CS/SO ratio that implies a carbon-to-oxygen ratio greater than unity and significant sulfur depletion, although current chemical models struggle to simultaneously reproduce all observed molecular species.

Original authors: Pablo Rivière-Marichalar, Asunción Fuente, Romane le Gal, Roberto Neri, Gisela Esplugues, Dmitry Semenov, Richard Teague, Alejandro Santamaría Miranda

Published 2026-02-09

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CC BY 4.0

Original authors: Pablo Rivière-Marichalar, Asunción Fuente, Romane le Gal, Roberto Neri, Gisela Esplugues, Dmitry Semenov, Richard Teague, Alejandro Santamaría Miranda

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a protoplanetary disk as a giant, swirling cosmic kitchen where planets are being baked. Just like a baker needs to know exactly what ingredients are in the dough to understand what the final cake will taste like, astronomers need to know the chemical ingredients of these disks to understand what kind of planets will form.

This paper is a detailed "ingredient check" of a specific cosmic kitchen called AB Aur, a young star system that is still in the process of forming planets. The researchers, led by P. Rivi`ere-Marichalar, used a powerful radio telescope (NOEMA) to sniff out two specific chemical "scents" in the disk: CS (carbon monosulfide) and SO (sulfur monoxide).

Here is what they found, explained through simple analogies:

1. The Map of the Kitchen

First, the team had to figure out the shape and orientation of the disk. Imagine looking at a dinner plate from an angle; it looks like an oval. By tracking how the gas moves (like cars on a racetrack), they determined the disk is tilted about 22 degrees relative to us and is rotated at a specific angle. This allowed them to create a clear, 3D map of where the gas is located.

2. The "Ghost" Motions

The researchers looked for gas that wasn't moving in a perfect circle (Keplerian motion). In a calm disk, everything spins smoothly like a record player. However, they found that some gases (like CO) were moving chaotically, like cars swerving in a traffic jam.

The Analogy: They found that the chaotic "traffic jams" (non-Keplerian motions) were caused by a giant stream of gas crashing into the disk, creating spirals and turbulence.
The Discovery: Interestingly, the CS gas they were studying was calm. It didn't show these traffic jams. This suggests that CS lives in the "basement" of the disk (closer to the middle), where it is shielded from the chaotic surface winds, making it a perfect tracer for the quiet, planet-forming zone.

3. The Great Sulfur Mystery

Sulfur is a key ingredient in life (part of the CHONPS elements), but in space, it often seems to disappear. Astronomers expected to find a lot of sulfur-bearing molecules, but they are usually missing.

The Finding: In AB Aur, the sulfur they did find was surprisingly scarce. The team calculated that the sulfur abundance is extremely low—about 80 times less than what we see in our own Sun.
The Analogy: It's like baking a cake where the recipe calls for a cup of sugar, but you only find a single grain of sugar in the bowl. The sulfur is likely "locked up" in frozen ice on dust grains, hiding from the telescope.

4. The Carbon-to-Oxygen Ratio (The Flavor Profile)

The most important result of this paper is the C/O ratio (Carbon to Oxygen). This ratio is like the "flavor profile" of the planets that will form.

The Method: The team compared the amount of CS to the amount of SO. In the universe, if there is more Carbon than Oxygen, the CS/SO ratio goes up.
The Result: They found a CS/SO ratio between 1.8 and 2.6.
The Conclusion: This high ratio is a smoking gun. It tells us that the Carbon-to-Oxygen ratio is greater than 1.
- What this means: Planets forming in this disk will be carbon-rich. Instead of being like Earth (which is oxygen-rich and rocky), these future planets will likely be made of carbon-heavy materials, perhaps resembling "carbon planets" or diamond-rich worlds.

5. The "Rosetta Stone" Problem

The paper calls AB Aur a "Rosetta stone" because it helps us decode the language of planet formation. However, the team hit a wall: No single computer model could explain all the data at once.

They could explain the CS, but not the SO.
They could explain the sulfur depletion, but not the carbon levels perfectly.
The Takeaway: This suggests our "recipe books" (astrochemical models) are missing some steps. We don't fully understand how sulfur gets locked away or how it moves around in these disks yet.

Summary

In short, this paper tells us that the AB Aur disk is a carbon-rich, sulfur-poor environment. The gas is mostly calm in the deep layers where planets are born, but the surface is turbulent. If a planet forms there (like the candidate planet AB Aur b), it will likely be a strange, carbon-heavy world, very different from our own Solar System. The study also highlights that while we are getting better at reading these cosmic recipes, we still need to figure out where the missing sulfur ingredients have gone.

Technical Summary: AB Aur, a Rosetta stone for studies of planet formation (IV): C/O estimates from CS and SO interferometric observations

Problem and Context
Protoplanetary disks serve as the birthplaces of planets, setting the initial chemical abundances that determine planetary atmospheric compositions. A critical parameter in this context is the Carbon-to-Oxygen (C/O) ratio, which influences planetary mineralogy and atmospheric chemistry. While sulfur is the tenth most abundant element in the universe, its chemistry in the interstellar medium (ISM) and protoplanetary disks remains poorly understood, often showing significant depletion in dense gas compared to cosmic values. Furthermore, while planets form in the disk mid-plane, most molecular tracers originate in the upper "warm molecular layer," complicating the study of the planet-forming region. This study addresses the need to constrain the sulfur abundance and the C/O ratio in the AB Aur protoplanetary disk by utilizing the CS/SO abundance ratio, a method previously limited by the scarcity of SO detections.

Methodology
The authors present new interferometric observations of the CS 3-2 transition toward AB Aur obtained with the IRAM NOEMA array (Project W21BA) in early 2022. These data were combined with existing ALMA observations of SO (Project 2021.1.00690) and previous NOEMA observations of other molecular species (HCN, HCO+, CO isotopologues, H2CO, H2S).

The analysis followed a rigorous multi-step process:

Kinematic Characterization: Velocity-integrated intensity maps (moment 0), first-moment maps (intensity-weighted velocity), and second-moment maps were generated. The inclination ( $i$ ) and position angle (PA) were derived by fitting first-moment maps of multiple species using the eddy code (Teague 2019) and bettermoments.
Keplerian Masking: A Keplerian mask was constructed using the derived disk parameters ( $i \approx 22^\circ$ , $PA \approx 237^\circ$ , $M_* = 2.4 M_\odot$ , $d = 155.9$ pc) to isolate Keplerian emission from non-Keplerian motions (e.g., spirals, streamers). This allowed for the creation of clean column density maps and the identification of residual non-Keplerian features.
Column Density Estimation: Assuming Local Thermodynamic Equilibrium (LTE) and optically thin emission at a gas temperature of 30 K, column densities for CS and SO were calculated.
Chemical Modeling: The observational data were compared against a suite of NAUTILUS astrochemical models (Rivière-Marichalar et al. 2020) varying three key parameters: the gas-to-dust mass ratio, the sulfur abundance ([S/H]), and the C/O ratio. The models were adjusted to include a cavity ( $R_{in}=100$ au, $R_{out}=300$ au) and convolved to match the observational beam sizes.

Key Results

Disk Geometry: The analysis confirms a disk inclination of $22.0^\circ \pm 0.5^\circ$ and a position angle of $237.0^\circ \pm 0.7^\circ$ , consistent across all observed molecular species.
Spatial Distribution: The CS 3-2 emission forms a ring peaking at $\sim 1.3''$ ( $\sim 212$ au), situated between the peaks of SO ( $\sim 1.4''$ ) and H2S ( $\sim 1.2''$ ). The CS ring width (FWHM) is approximately $0.86''$ ( $\sim 141$ au).
Kinematics: CS, H2S, and SO exhibit predominantly Keplerian motion with no significant non-Keplerian residuals at the $3\sigma$ level. In contrast, CO isotopologues show strong non-Keplerian residuals tracing large-scale spiral arms and streamers, while HCO+ shows residuals associated with a misaligned inner disk and the candidate planet AB Aur b. This suggests CS and H2S are better tracers of the quiescent mid-plane than CO.
Chemical Segregation: No correlation was found between the radial emission peaks of the surveyed species and their binding energies, indicating that thermal desorption is not the primary driver of chemical segregation in this disk.
C/O Ratio and Sulfur Abundance:
- The observational CS/SO column density ratio ranges from 1.8 to 2.6, with local variations reaching $>2.0$ near the dust trap.
- Comparison with NAUTILUS models indicates that only models with a C/O ratio $\gtrsim 1$ can reproduce the observed high CS/SO ratios.
- The data point to strong sulfur depletion, with a derived sulfur abundance of [S/H] $\approx 8 \times 10^{-8}$ , significantly lower than the cosmic value ( $\sim 1.5 \times 10^{-5}$ ).
- The best-fitting model configuration includes a C/O ratio of 1.0, a gas-to-dust mass ratio of 40, and the depleted sulfur abundance. However, the authors note that no single model successfully reproduces the radial profiles of all observed species simultaneously (specifically failing to fit H2CO and HCO+).

Significance and Claims
The paper positions AB Aur as a critical case study ("Rosetta stone") for understanding planet formation chemistry. The primary significance lies in the derivation of a C/O ratio $\gtrsim 1$ for the AB Aur disk, based on the high CS/SO ratio. This implies that planets forming in the remaining disk, including the candidate AB Aur b at $\sim 93$ au, will likely possess carbon-rich atmospheres and mineralogies distinct from Solar System planets.

The authors modestly conclude that while the CS/SO ratio provides a strong constraint favoring C/O $\approx 1$ and sub-solar sulfur abundance, the inability of current astrochemical networks to simultaneously reproduce the abundances of CS, SO, and H2S suggests that important formation and destruction routes for sulfur-bearing species are missing from current models. Additionally, the study highlights that turbulence and non-Keplerian dynamics (such as streamers impacting the disk) may significantly alter molecular abundances, complicating the interpretation of quiescent disk models.