AB Aur, a Rosetta stone for studies of planet formation

II. H₂S detection and sulfur budget

¹ Observatorio Astronómico Nacional (OAN,IGN), Calle Alfonso XII, 3. 28014 Madrid, Spain
e-mail: p.riviere@oan.es
² Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
³ Univ. Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), 38000 Grenoble, France
⁴ Institut de Radioastronomie Millimétrique (IRAM), 38406 Saint-Martin d’Hères, France
⁵ CNRS / Institut de Recherche en Astrophysique et Planétologie, 14 avenue Edouard Belin, 31400 Toulouse, France
⁶ European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
⁷ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago 763-0355, Chile
⁸ European Southern Observatory, Karl-Schwarzschild-Str 2, 85748 Garching, Germany
⁹ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d’Hères, France

Received: 14 December 2021
Accepted: 5 July 2022

Abstract

Context. The sulfur abundance is poorly known in most environments. Yet, deriving the sulfur abundance is key to understanding the evolution of the chemistry from molecular clouds to planetary atmospheres. We present observations of H₂S 1₁₀–1₀₁ at 168.763 GHz toward the Herbig Ae star AB Aur.

Aims. We aim to study the abundance of sulfuretted species toward AB Aur and to constrain how different species and phases contribute to the sulfur budget.

Methods. We present new NOrthern Extended Millimeter Array (NOEMA) interferometric observations of the continuum and H₂S 1₁₀–1₀₁ line at 168.763 GHz toward AB Aur. We derived radial and azimuthal profiles and used them to compare the geometrical distribution of different species in the disk. Assuming local thermodynamical equilibrium (LTE), we derived column density and abundance maps for H₂S, and we further used Nautilus to produce a more detailed model of the chemical abundances at different heights over the mid-plane at a distance of r = 200 au.

Results. We have resolved H₂S emission in the AB Aur protoplanetary disk. The emission comes from a ring extending from 0.67″ (~109 au) to 1.69″ (~275 au). Assuming T = 30 K, n_H = 10⁹ cm⁻³, and an ortho-to-para ratio of three, we derived a column density of (2.3 ± 0.5) × 10¹³ cm⁻². Under simple assumptions, we derived an abundance of (3.1 ± 0.8) × 10⁻¹⁰ with respect to H nuclei, which we compare with Nautilus models to deepen our understanding of the sulfur chemistry in protoplanetary disks. Chemical models indicate that H₂S is an important sulfur carrier in the solid and gas phase. We also find an important transition at a height of ~12 au, where the sulfur budget moves from being dominated by ice species to being dominated by gas species.

Conclusions. We confirm that present-day models still struggle to simultaneously reproduce the observed column densities of the different sulfuretted species, and the observed abundances are still orders of magnitude away from the cosmic sulfur abundance. Studying sulfuretted species in detail in the different phases of the interstellar medium is key to solving the issue.