A Submillimeter Survey of CS Excitation in Protoplanetary Disks: Evidence of X-ray-Driven Sulfur Chemistry

TL;DR

This study provides a comprehensive, multi-line CS analysis in 12 protoplanetary disks using SMA, ALMA, and NOEMA data to constrain disk-averaged CS excitation. Through LTE rotational diagrams and MCMC fitting, it finds $T_{rot}$ in the range $10$–$40$ K (median ≈ $21^{+6}_{-8}$ K) and $N_T$ between $10^{12}$ and $10^{13}$ cm$^{-2}$, with a wide dispersion across disks. A robust result is the positive correlation between stellar X-ray luminosity $L_X$ and CS column density $N_T$, suggesting that ion-neutral chemistry in the disk upper layers—driven by X-ray–heated $S^+$ and $C^+$—dominates CS production rather than disk mass or structure alone. The work highlights that CS, as a tracer of gas-phase sulfur, must be interpreted in the context of emitting region and X-ray irradiation, and calls for spatially resolved, larger-sample studies to map sulfur chemistry across diverse disk environments. It also situates CS within the broader disk chemistry by comparing trends with other tracers such as C$^{18}$O and HCO$^+$, noting potential chemical links while acknowledging caveats tied to line fluxes and optical depths.

Abstract

The sulfur chemistry in protoplanetary disks influences the properties of nascent planets, including potential habitability. Although the inventory of sulfur molecules in disks has gradually increased over the last decade, CS is still the most commonly-observed sulfur-bearing species and it is expected to be the dominant gas-phase sulfur carrier beyond the water snowline. Despite this, few dedicated multi-line observations exist, and thus the typical disk CS chemistry is not well constrained. Moreover, it is unclear how that chemistry - and in turn, the bulk volatile sulfur reservoir - varies with stellar and disk properties. Here, we present the largest survey of CS to date, combining both new and archival observations from ALMA, SMA, and NOEMA of 12 planet-forming disks, covering a range of stellar spectral types and dust morphologies. Using these data, we derived disk-integrated CS gas excitation conditions in each source. Overall, CS chemistry appears similar across our sample with rotational temperatures of ${\approx}$10-40 K and column densities between 10$^{12}$-10$^{13}$ cm$^{-2}$. CS column densities do not show strong trends with most source properties, which broadly suggests that CS chemistry is not highly sensitive to disk structure or stellar characteristics. We do, however, identify a positive correlation between stellar X-ray luminosity and CS column density, which indicates that the dominant CS formation pathway is likely via ion-neutral reactions in the upper disk layers, where X-ray-enhanced S$^+$ and C$^+$ drive abundant CS production. Thus, using CS as a tracer of gas-phase sulfur abundance requires a nuanced approach that accounts for its emitting region and dependence on X-ray luminosity.

Figures (12)

• Figure 1: Integrated intensity spectra of CS J=4--3 (top row), J=5--4 (middle row), and J=7--6 (bottom row) obtained from our SMA observations. Each column corresponds to one disk. Spectra are extracted using a Keplerian mask based on the known disk geometry and vertical errorbars show the 1$\sigma$ RMS. The narrow linewidth of J1604 is due to the nearly face-on geometry of this disk.
• Figure 2: CS J=5--4 integrated fluxes as a function of stellar mass (left), stellar bolometric luminosity (middle), and disk continuum flux at 265 GHz (right). Green, orange, and blue colors mark low-mass T Tauri (${<}1~M_{\odot}$), Solar-type ($1-1.3~M_{\odot}$), and Herbig (${>}1.3~M_{\odot}$) systems, respectively. The two gray square boxes correspond to V4046 Sgr and GG Tau, which are known binary/multiple systems whose masses and luminosities represents the system totals. The star symbols indicate disks (DR Tau, DG Tau, DO Tau) with known in-fall signatures and high accretion luminosities Manara14McClure19. Downward triangles indicate 3$\sigma$ flux upper limits. All fluxes have been scaled to a distance of 140 pc and continuum emission has been scaled to 265 GHz via F$_{\nu}\propto\nu^{2.2}$Andrews20 for flux F$_{\nu}$ and frequency $\nu$. Fluxes are compiled from Table \ref{['tab:obs-list']} and additional literature sources Pietu11LeGal19Long19Podio19Podio20_DGTauFacchini21PDSTeague22_CS54_fluxSmirnov22Pegues23Law25.
• Figure 3: Rotational diagrams of CS constructed using disk-integrated fluxes. Gray shaded regions show random draws from the fit posteriors. We included a 10% calibration uncertainty on all measured line fluxes.
• Figure 4: Derived CS rotational temperatures (top) and column densities (bottom) for our disk sample, sorted from left to right by column density. The median values are shown by the dashed orange lines and the shaded regions indicate the 16th-84th percentiles. Several points have uncertainties smaller than the markers. The values for the TW Hya and HD 163296 disks are taken from Teague18_TWHya and Law25, respectively.
• Figure 5: CS rotational temperatures (top row) and column densities (bottom row) versus stellar mass (left column), stellar bolometric luminosity (middle column), and disk continuum flux at 265 GHz normalized to 140 pc (right column). The two gray square boxes correspond to V4046 Sgr and GG Tau, which are known binary/multiple systems whose masses and luminosities represents the system totals. Colors and symbols are as in Figure \ref{['fig:CS_flux_comparison']}. Pearson's correlation coefficients and associated p-values are shown in the upper right corner of each panel.