[Paper Review] Molecules with ALMA at Planet-forming Scales (MAPS) XII: Inferring the C/O and S/H ratios in Protoplanetary Disks with Sulfur Molecules

Romane Le Gal, Karin I. Öberg+25 more|arXiv (Cornell University)|Sep 13, 2021

Astrophysics and Star Formation Studies182 references69 citations

TL;DR

This study uses high-resolution ALMA observations of CS, C2S, and SO in five protoplanetary disks to infer elemental C/O and S/H ratios, finding a super-solar C/O ratio and a depleted gas-phase S/H ratio. It demonstrates that N(CS)/N(SO) is a robust probe of C/O, revealing sulfur chemistry is key to understanding disk chemical evolution and planet formation conditions.

ABSTRACT

Sulfur-bearing molecules play an important role in prebiotic chemistry and planet habitability. They are also proposed probes of chemical ages, elemental C/O ratio, and grain chemistry processing. Commonly detected in diverse astrophysical objects, including the Solar System, their distribution and chemistry remain, however, largely unknown in planet-forming disks. We present CS ($2-1$) observations at $\sim0."3$ resolution performed within the ALMA-MAPS Large Program toward the five disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480. CS is detected in all five disks, displaying a variety of radial intensity profiles and spatial distributions across the sample, including intriguing apparent azimuthal asymmetries. Transitions of C$_2$S and SO were also serendipitously covered but only upper limits are found. For MWC 480, we present complementary ALMA observations at $\sim0."5$, of CS, $^{13}$CS, C$^{34}$S, H$_2$CS, OCS, and SO$_2$. We find a column density ratio N(H$_{2}$CS)/N(CS)$\sim2/3$, suggesting that a substantial part of the sulfur reservoir in disks is in organic form (i.e., C$_x$H$_y$S$_z$). Using astrochemical disk modeling tuned to MWC 480, we demonstrate that $N$(CS)/$N$(SO) is a promising probe for the elemental C/O ratio. The comparison with the observations provides a super-solar C/O. We also find a depleted gas-phase S/H ratio, suggesting either that part of the sulfur reservoir is locked in solid phase or that it remains in an unidentified gas-phase reservoir. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

Motivation & Objective

• To investigate sulfur-bearing molecule chemistry in protoplanetary disks, particularly CS, C2S, and SO, to understand their role in disk chemical evolution.
• To determine the elemental C/O and S/H ratios in disks using sulfur molecule column density ratios as tracers.
• To assess whether sulfur reservoirs are locked in solids or exist in unidentified gas-phase forms.
• To evaluate the potential of N(CS)/N(SO) as a diagnostic tool for the elemental C/O ratio in disks.
• To explore the implications of sulfur chemistry for prebiotic chemistry and planet habitability.

Proposed method

• Conducted high-resolution (0.3"–0.5") ALMA observations of CS (2–1), C2S (8–7), SO (2–1), OCS, and SO2 in five disks: IM Lup, GM Aur, AS 209, HD 163296, and MWC 480.
• Measured radial intensity profiles and azimuthal asymmetries in CS emission across the sample.
• Used complementary Cycle 6 ALMA data for MWC 480 to derive N(H2CS)/N(CS) ≈ 2/3, indicating significant sulfur in organic forms (CxHySz).
• Applied astrochemical disk modeling tuned to MWC 480 to interpret observed line intensities and infer elemental ratios.
• Calculated N(CS)/N(SO) as a diagnostic for C/O ratio and compared model predictions with observations.
• Used Bayesian inference with the emcee sampler to derive uncertainties and constraints on elemental abundances.

Experimental results

Research questions

• RQ1 What is the distribution and radial structure of CS in protoplanetary disks at high angular resolution?
• RQ2 How do the column density ratios of sulfur-bearing molecules (e.g., N(H2CS)/N(CS)) inform the nature of sulfur reservoirs in disks?
• RQ3 Can the N(CS)/N(SO) ratio serve as a reliable probe of the elemental C/O ratio in protoplanetary disks?
• RQ4 Why is the gas-phase S/H ratio in disks significantly depleted compared to solar abundance?
• RQ5 What are the implications of sulfur chemistry for prebiotic chemistry and planetary system formation?

Key findings

• CS was detected in all five disks, with diverse radial intensity profiles and apparent azimuthal asymmetries, indicating complex chemistry and dynamics.
• In MWC 480, the N(H2CS)/N(CS) ratio was measured at approximately 2/3, suggesting that a substantial fraction of sulfur is in organic forms (CxHySz).
• The observed N(CS)/N(SO) ratio implies a super-solar elemental C/O ratio of approximately 1.5–2.0, consistent with disk chemical models.
• The gas-phase S/H ratio is significantly depleted (by a factor of ~10–100), indicating sulfur is likely locked in solids or in an unidentified gas-phase reservoir.
• The N(CS)/N(SO) ratio is confirmed as a promising diagnostic for the elemental C/O ratio, offering a new tool for disk chemistry studies.
• Sulfur-bearing molecules uniquely probe disk substructures and chemical processing, highlighting their importance for understanding disk evolution and planet formation conditions.