Investigating the Sulfur Mystery in Protoplanetary Disks Through Chemical Modeling

TL;DR

This study updates a 2D time-dependent disk chemical framework to include an expanded sulfur reaction network, enabling a detailed exploration of the main sulfur reservoirs in protoplanetary disks. By varying volatile sulfur abundance, C/O ratio, initial sulfur forms, and cosmic-ray ionization, the work shows that a high C/O ratio drives CS-rich, SO-poor gas in the warm upper layers, aligning with disk observations that favor CS detections but little SO. The results reveal that ice abundances in the lower disk are strongly set by the initial sulfur form, with significant implications for comet formation and future JWST ice observations, while allotropes are not a major sulfur reservoir under most conditions. The work highlights the importance of sulfur chemistry in linking disk composition to planetary and cometary inventories and outlines key uncertainties, particularly binding energies and X-ray/CR processing, that future experiments and observations should address.

Abstract

Sulfur is a critical element to life on Earth, and with detections of sulfur-bearing molecules in exoplanets and comets, questions arise as to how sulfur is incorporated into planets in the first place. In order to understand sulfur's journey from molecular clouds to planets, we need to understand the molecular forms that sulfur takes in protoplanetary disks, where the rotational emission from sulfur-bearing molecules in the gas phase indicates a very low abundance. To address this question, we have updated the 2D time-dependent disk chemical modeling framework of Fogel et al. (2011) to incorporate several new sulfur species and hundreds of new sulfur reactions from the literature. Specifically, we investigate the main molecular forms that sulfur takes in a disk orbiting a solar mass young T Tauri star. We explore the effects of different volatile (reactive) sulfur abundances, C/O ratios, initial sulfur molecular forms, and cosmic-ray ionization rates. We find that a high C/O ratio can explain both the prevalence of CS observed in disks and the lack of SO detections, consistent with previous results. Additionally, initial sulfur form greatly affects the ice abundances in the lower layers of the disk, which has implications for comet formation and future observations with JWST.

Figures (8)

• Figure 1: Physical environment of the 2D model, showing gas density, gas temperature, ultraviolet radiation, and X-ray radiation.
• Figure 2: Abundances of 15 sulfur-bearing species in a generic disk for the standard case (mid1). The total sulfur abundance is S/H$_2$$=2\times10^{-8}$, C/O = 0.36, and the initial sulfur form is gas-phase CS and SO.
• Figure 3: Variations in the abundance of CS and SO for different volatile sulfur abundances. The first two plots compare mid1 to low1 (S/H = $10^{-8}$ to $10^{-9}$), and the second two plots compare high1 to mid1 (S/H = $10^{-7}$ to $10^{-8}$). We only plot regions where the abundance of the shown molecule is greater than $10^{-15}$ relative to H$_2$.
• Figure 4: Log of the CS/SO ratio as C/O ratio increases from 0.36 to 1.40 (see Table \ref{['run_names']}). The light gray contour lines show where the CS/SO ratio equals the C/O ratio for that run. For each plot, we only plot regions where the abundance of at least one molecule is greater than $10^{-15}$ relative to H$_2$.
• Figure 5: Abundances of four ice species, H$_2$S, SO$_2$, OCS, and SO, for the four models where we varied the initial sulfur form (mid1: CS and SO, mid7: H$_2$S, mid8: S$_8$ ice, and mid9: S). For more specifics on the model runs, refer to Table \ref{['run_names']}.