AB Aur, a Rosetta stone for studies of planet formation

IV. C/O estimates from CS and SO interferometric observations

¹ Observatorio Astronómico Nacional (OAN,IGN), Calle Alfonso XII, 3, 28014 Madrid, Spain
² Centro de Astrobiología (CAB), INTA-CSIC, Carretera de Ajalvir Km. 4, Torrejón de Ardoz, 28850 Madrid, Spain
³ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d’Hères, France
⁴ Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), Université Grenoble Alpes, CNRS, 38000 Grenoble, France
⁵ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁶ Department of Chemistry, Ludwig-Maximilians-Universität, Butenandtstr. 5–13, 81377 München, Germany
⁷ Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 25 June 2025
Accepted: 3 February 2026

Abstract

Context. Protoplanetary disks are the birthplaces 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 toward 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 used the CS/SO ratio to study the C/O ratio. We compared our present and previous interferometric observations of AB Aur with a NAUTILUS disk model to gain an insight into the S elemental abundance and C/O ratio.

Results. We derived 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]=8 × 10⁻⁸, but we note that no single model can simultaneously fit all observed species.