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

Context. Protoplanetary disks are the birthplace of planets. As such, they set the initial chemical abundances available for planetary atmosphere formation. Thus, studying elemental abundances, molecular compositions, and abundance ratios in protoplanetary disks is key to linking planetary atmospheres to their formation sites. Aims. We aim to derive the sulfur abundance and the C/O ratio in the AB Aur disk using interferometric observations of CS and SO. Methods. New NOEMA observations of CS 3-2 towards AB Aur are presented. We used velocity-integrated intensity maps to determine the inclination and position angles. Keplerian masks were constructed for all observed species to assess the presence of non-Keplerian motions. We use the CS/SO ratio to study the C/O ratio. We compare our present and previous interferometric observations of AB Aur with a NAUTILUS disk model to gain insight into the S elemental abundance and C/O ratio. Results. We derive an observational CS/SO ratio ranging from 1.8 to 2.6. Only NAUTILUS models with C/O > 1 can reproduce such ratios. The comparison with models points to strong sulfur depletion, with [S/H]=8e-8, but we note that no single model can simultaneously fit all observed species.

💡 Research Summary

This paper presents a detailed interferometric study of the protoplanetary disk around the Herbig Ae star AB Aur, focusing on the sulfur chemistry and the carbon‑to‑oxygen (C/O) elemental ratio. New NOEMA observations of the CS 3‑2 transition were obtained with the full 12‑antenna array in the AB configuration, achieving a synthesized beam of 0.79″ × 0.48″ and a line sensitivity sufficient to map the CS emission across the disk. Complementary ALMA data of the SO 5₆‑4₅ line were also used. The CS moment‑zero map reveals an elliptical ring extending from ~0.9″ to 2″ (≈140–310 au), with a peak at ~1.3″ (≈212 au). The first‑moment map displays the classic Keplerian velocity pattern, and the second‑moment map shows low velocity dispersion in the same regions. A double‑peaked CS spectrum is centered at 5.86 km s⁻¹, with blue and red peaks at 5.0 and 6.7 km s⁻¹, respectively. Continuum imaging at 2 mm shows a compact central source and a surrounding dust ring peaking at ~0.96″ (≈156 au).

Using a suite of previously observed molecular lines (¹³CO, C¹⁸O, H₂CO, H₂S, HCO⁺, HCN, etc.), the authors performed MCMC fitting of the velocity fields (with the eddy package) to derive a consistent disk geometry: inclination i = 22.0° ± 0.5° and position angle PA = 237.0° ± 0.7°, in agreement with earlier studies.

To isolate Keplerian emission, a Keplerian mask was generated (Teague & Loomis 2020) employing the derived geometry, a stellar mass of 2.4 M⊙, and a systemic velocity of 5.8 km s⁻¹. The radial dependence of the line width was measured, yielding a reference width W₀ ≈ 404 m s⁻¹ at 1″ and a power‑law index q ≈ ‑0.65. The mask was applied at a height of z/r ≈ 0.075 (≈15 au at 200 au radius), consistent with previous work indicating CS originates slightly above the mid‑plane. After masking, CS emission remains largely Keplerian, with only three localized residual spots above the 3σ level. Similar analysis of SO shows no significant non‑Keplerian features, while CO isotopologues exhibit pronounced deviations, reflecting their origin in the upper, more turbulent layers of the disk.

The integrated intensity ratio CS/SO is measured to be between 1.8 and 2.6 across the disk. To interpret this ratio, the authors employed the NAUTILUS astrochemical disk model, varying the elemental C/O ratio and the sulfur elemental abundance