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

TL;DR

This work uses interferometric observations of CS and SO toward AB Aur to estimate sulfur abundances and the C/O ratio in its protoplanetary disk. By combining NOEMA CS 3-2 data with ALMA SO measurements and applying Keplerian masking, the authors derive a CS/SO ratio of approximately $1.8$–$2.6$, which, when compared with NAUTILUS disk-chemistry models, favors a C/O ratio near unity and a depleted sulfur abundance, though no single model fits all species. The analysis reveals non-Keplerian motions in several tracers but shows CS to be a robust near-midplane indicator, highlighting the influence of disk structure and irradiation on sulfur chemistry. The results imply that planets forming in AB Aur's disk would exhibit elevated atmospheric C/O, and suggest that sulfur chemistry in disks remains incompletely understood, potentially requiring new formation or destruction pathways and consideration of turbulence.

Abstract

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.

Figures (13)

• Figure 1: From left to right: zeroth-, first, and second-moment map of CS 3-2 observed with NOEMA. The first- and second-moment maps were computed using a 3$\sigma$ threshold to avoid noisy channels. Contours in the zeroth- and second-moment maps are shown for 0.25, 0.5, 0.75, and 0.9 of the integrated intensity map peak. The white dashed ellipse in the zeroth-moment map shows the area used to compute the integrated spectrum shown in Fig. \ref{['Fig:CS_spectrum']}.
• Figure 2: CS spectrum extracted from a 2$\arcsec \times$1.8$\arcsec$ ellipse with PA=237 $^\circ$ centered on the CS emission disk (see Fig. \ref{['Fig:CS_moments']}, left panel). The blue curve shows a 2-Gaussian fit to the data, with the black dashed curves representing the individual Gaussians.
• Figure 3: Continuum map at 2 mm. Contours are shown for 0.25, 0.5, 0.75, and 0.9 of the map peak.
• Figure 4: Azimuthally averaged radial profile of the CS 3-2 velocity integrated intensity map (black line) and the continuum at 2 mm (blue line). The shaded areas account for the uncertainties in the profiles.
• Figure 5: Radial profiles of the second-moment maps of the different species surveyed thus far. The blue dashed line shows a power law fit to the data. The name of each species is shown in each panel.