A Submillimeter Survey Of CS Excitation In Protoplanetary Disks: Evidence Of X-ray-Driven Sulfur Chemistry
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 ≈10-40 K and column densities between 1012-1013 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.
Charles J. Law, Romane Le Gal, Karin I. Öberg, Ke Zhang, Yuri Aikawa, Sean M. Andrews, Jaehan Bae, Alice S. Booth, Gianni Cataldi, L. Ilsedore Cleeves, Feng Long, François Ménard, Chunhua Qi, Richard Teague, David J. Wilner
Comments: 24 pages, 12 figures, accepted for publication in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2511.09628 [astro-ph.EP] (or arXiv:2511.09628v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2511.09628
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Submission history
From: Charles Law
[v1] Wed, 12 Nov 2025 19:00:02 UTC (2,685 KB)
https://arxiv.org/abs/2511.09628
Astrobiology, Astronomy, Astrochemistry,
