arxiv: 2604.11408 · v1 · submitted 2026-04-13 · 🌌 astro-ph.EP · astro-ph.SR

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Compact Hydrogen Sulfide Emission Indicates Sulfur-bearing Ice Sublimation in the Inner Disk of HD 163296

Yoshihide Yamato , Yuri Aikawa , Kenji Furuya , Charles J. Law , Karin I. \"Oberg , Chunhua Qi , Cataldi Gianni , Romane Le Gal

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Shota Notsu Viviana V. Guzm\'an Jane Huang

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Pith reviewed 2026-05-10 15:49 UTC · model grok-4.3

classification 🌌 astro-ph.EP astro-ph.SR

keywords protoplanetary diskssulfur chemistryhydrogen sulfideice sublimationHD 163296ALMA observationsplanet formationvolatile inventory

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The pith

Compact H2S emission signals sulfur-bearing ice sublimation at 3-5 au in the HD 163296 disk

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper reports the detection of unresolved compact emission from hydrogen sulfide along with sulfur monoxide from the center of the protoplanetary disk around HD 163296. The lines show broad widths of roughly 40 km/s that a Keplerian disk model fits best when the gas sits at radii of 3 to 5 au with temperatures above 90 to 120 K. These conditions line up with the zone where water ice and sulfur-bearing ices sublimate in the warm inner disk. The column density of H2S is higher than or similar to those of SO and SO2, pointing to H2S as a major carrier of volatile sulfur. The observations still leave room for overall depleted sulfur and call for more data to map how sulfur is reprocessed.

Core claim

The authors detect unresolved, compact emission of H2S and SO (and tentatively SO2) at the disk center with a broad line width of ~40 km/s. Fitting the line profiles with a geometrically thin Keplerian-rotating disk model constrains the emitting radii to ~3-5 au and gas temperatures to ≳90-120 K, consistent with sublimation of sulfur-bearing molecules along with water ice in the inner warm region. The higher or comparable column density of H2S indicates it is an important volatile sulfur reservoir, although the limited constraints do not rule out significantly depleted volatile sulfur.

What carries the argument

A geometrically thin Keplerian-rotating disk model fitted to the observed line profiles to derive the radial location and temperature of the emitting gas.

Load-bearing premise

The broad line width and single-component Keplerian fit are taken to require emission exclusively from 3-5 au at high temperatures from ice sublimation, without other geometries or motions producing the same profiles.

What would settle it

Higher-resolution observations that resolve the emission outside 5 au or show velocity fields inconsistent with Keplerian rotation at 3-5 au would rule out the inner-disk sublimation origin.

Figures

Figures reproduced from arXiv: 2604.11408 by Cataldi Gianni, Charles J. Law, Chunhua Qi, Jane Huang, Karin I. \"Oberg, Kenji Furuya, Romane Le Gal, Shota Notsu, Viviana V. Guzm\'an, Yoshihide Yamato, Yuri Aikawa.

**Figure 1.** Figure 1: High-resolution 0.88 mm dust continuum image of the HD 163296 disk taken from Guidi et al. (2022) (left) and velocity-integrated intensity maps of H2S, SO, and two SO2 lines (others). The peak S/Ns of the emission are denoted in the upper right corner of each panel. For all panels, a 20 au scale bar and the synthesized beam are indicated in the lower right and left corner, respectively. −50 0 50 100 vLSRK … view at source ↗

**Figure 2.** Figure 2: Disk-integrated spectra of H2S, SO, and two SO2 lines. The gray and colored lines indicate the spectra at 1.2 km s−1 and 4.8 km s−1 channel widths, respectively. The vertical segment at the upper left corner indicate 1σ uncertainty of the spectra at 4.8 km s−1 channel width. In each panel, the vertical red and gray dotted lines marks the systemic velocity of the source (vsys = 5.76 km s−1 ) and the integra… view at source ↗

**Figure 3.** Figure 3: Matched filter responses for the wide spectral window covering H2S and SO2 lines. The vertical dotted lines mark the frequency of the targeted H2S and SO2 lines. The horizontal dotted line indicates 5σ response, which is used to infer the detection. The strong feature at ∼ 300.15 GHz is from the HC3N J = 32–31 line. optical depth τ0 can be computed as τ0 = c 3 guAulN √ 2πσv 8πν3Q(T) exp hν kBT − 1 … view at source ↗

**Figure 4.** Figure 4: Observed spectra of H2S, SO, and two SO2 lines at 1.2 km s−1 channel width overlaid with the fitted Keplerian disk model (orange). The model spectra are drawn for 100 samples randomly selected from the posterior chains. The vertical red dotted line in each panel marks the systemic velocity of the source. 100 200 300 400 T (K) 14 15 16 17 18 19 log10 N (cm −2 ) 100 200 300 400 100 200 300 400 [PITH_FULL_IM… view at source ↗

**Figure 5.** Figure 5: Posterior distributions marginalized for temperature and column density for H2S (left), SO (middle), and SO2 (right). The color indicates the normalized probability distributions, while the gray dashed lines delineate 1σ confidence interval. The red dotted lines mark the τ0 = 1 line for each transition. For SO2, the upper line corresponds to the SO2 323,29–322,30 transition, while the lower line is for the… view at source ↗

**Figure 6.** Figure 6: Deprojected radial intensity profiles of C34S J = 6–5 (blue) and SO 3Σ JN = 77–66 (orange). The C34S line is observed in the same project as the SO line, and its radial profile is taken from Law et al. (2025a). These profiles are generated by the the radial profile function built in GoFish (Teague 2019), assuming a flat (i.e., z/r = 0) emission surface. The profiles are normalized by their peak value. The… view at source ↗

**Figure 7.** Figure 7: Upper limits on water column density (top) and lower limits on the abundance ratios of detected sulfurbearing species relative to water (bottom, solid lines) as a function of assumed gas temperature. The different colors on the top panel indicate the constraints from different isotopologue lines. The lower limits on the abundance ratios correspond to the lower end of the 1σ confidence intervals in [PITH… view at source ↗

**Figure 8.** Figure 8: Matched filter responses for molecular lines with > 5σ signal in the continuum spectral window. The horizontal gray dotted line marks 5σ level. The expected line frequencies are indicated by the vertical red dotted lines [PITH_FULL_IMAGE:figures/full_fig_p013_8.png] view at source ↗

**Figure 10.** Figure 10: SO2 spectrum stacked over two observed transitions. The vertical red dotted line marks the systemic velocity (≈ 5.76 km s−1 ), while the horizontal gray dotted line indicate the zero level. The best-fit Keplerian-rotating disk model (Section 3) are shown in the gray solid curve as a guide. Booth, A. S., van der Marel, N., Leemker, M., van Dishoeck, E. F., & Ohashi, S. 2021a, A&A, 651, L6, doi: 10.1051/0… view at source ↗

**Figure 9.** Figure 9: Velocity-integrated intensity maps (left) and deprojected radial intensity profiles (right) of CH2CN (top), HC3N (middle), and c-C3H2 (bottom). In the left panels, synthesized beams and 50 au scale bars are shown in the bottom left and right corners, respectively. In the right panels, blue-shaded regions indicate 1σ uncertainties. The size of the beam major axis is indicated in the top right corner. As a… view at source ↗

**Figure 11.** Figure 11: Marginalized posterior distributions and covariances between different parameters of the MCMC fit to the line profiles. The vertical dashed lines in each panel that show the marginalized posteriors distribution marks 16th, 50th, and 84th percentiles. The covariances between column density (log10 N) and temperature (T) for each molecular species are also shown in [PITH_FULL_IMAGE:figures/full_fig_p015_11.png] view at source ↗

read the original abstract

The sulfur chemistry in protoplanetary disks directly affects the composition and potential habitability of nascent planets, but its volatile inventory remains highly uncertain. Here, we present deep Atacama Large Millimeter/submillimeter Array (ALMA) observations of hydrogen sulfide (H$_2$S) along with SO and SO$_2$ in the disk around HD 163296 at an angular resolution of $\approx0.\!\!^{\prime\prime}3$ (or $\approx$30 au). We detect unresolved, compact emission of H$_2$S and SO (and tentatively SO$_2$) at the disk center with a broad line width of $\sim$40 km s$^{-1}$, suggesting that the emission is originating from the innermost regions. By fitting line profiles with a geometrically-thin Keplerian-rotating disk model, we constrain the emitting radii and gas temperatures of these molecules to be $\approx$3-5 au and $\gtrsim$90-120 K, respectively, consistent with sublimation of sulfur-bearing molecules along with water ice in the inner warm region. While the higher or comparable column density of H$_2$S with respect to SO and SO$_2$ indicates that H$_2$S is an important volatile sulfur reservoir in the disk, the limited constraints mean that we cannot rule out significantly depleted volatile sulfur as also commonly inferred in other planet-forming disks. Further observations are needed to better constrain disk sulfur inventory, unravel how sulfur compounds are reprocessed in disks, and shed light on the nature of less-volatile species, such as salts and sulfide minerals, which may occupy a significant portion of sulfur budget.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

New compact H2S detection in HD 163296 is real but the 3-5 au Keplerian radius and sublimation claim rests on an untested model assumption.

read the letter

The paper's main contribution is a new ALMA detection of unresolved H2S emission with a ~40 km/s line width at the center of the HD 163296 disk, plus SO and tentative SO2. This is the first such inner-disk H2S report for this source, and the column-density comparison shows H2S at least as abundant as the oxides, adding a concrete datum to sulfur chemistry in disks. The data reduction and basic fitting look standard and internally consistent, and the authors are straightforward about the incomplete sulfur budget and the need for more observations. That part is useful and honest. The soft spot is the kinematic step. The broad profile is fitted to a geometrically thin, purely Keplerian disk to pin the emitting region at 3-5 au and temperatures above 90-120 K. Because the source sits inside the 30 au beam, the same width and shape can be produced by extended emission, optical-depth effects, or added turbulent or radial motions without requiring those small radii. The paper does not show quantitative tests against those alternatives, so the radius and temperature numbers are not uniquely determined. The link to water-ice co-sublimation therefore stays suggestive rather than required by the data. This work is for disk chemists and planet-formation modelers who track volatile sulfur delivery. It supplies a new observational point even with the modeling caveat. I would send it to peer review; the detection itself is worth referee time and the discussion already flags the main uncertainty.

Referee Report

1 major / 2 minor

Summary. The paper presents deep ALMA observations of H₂S, SO, and tentative SO₂ toward the protoplanetary disk around HD 163296 at ~0.3″ resolution. It reports detection of compact, unresolved emission at the stellar position with broad ~40 km s⁻¹ line widths. A geometrically thin Keplerian disk model is fitted to the line profiles to derive emitting radii of ≈3–5 au and gas temperatures ≳90–120 K, which the authors interpret as evidence for sublimation of sulfur-bearing ices in the warm inner disk. H₂S is highlighted as a potentially important volatile sulfur reservoir, while noting that the total sulfur budget remains poorly constrained and further observations are required.

Significance. If the radius and temperature constraints are robust, the result would provide direct observational evidence linking volatile sulfur chemistry to ice sublimation near the water snowline, helping to quantify the inner-disk sulfur inventory available for planet formation. The work builds on standard ALMA line-profile analysis and appropriately cautions on the tentative SO₂ detection and limited total-sulfur constraints.

major comments (1)

[line-profile fitting and modeling section] The central claim that the emission originates at 3–5 au with T ≳90–120 K rests on fitting the observed ~40 km s⁻¹ line profiles to a single-component, geometrically thin, purely Keplerian disk model. Because the source is unresolved within the ~0.3″ beam, the velocity width is degenerate with non-Keplerian motions (turbulence, radial flows), optical-depth gradients, or emission distributed over a wider radial range whose line-of-sight velocity distribution can mimic a compact inner ring. No quantitative comparison to these alternative velocity fields or geometries is presented, so the uniqueness of the 3–5 au solution is not demonstrated. This assumption is load-bearing for the sublimation interpretation.

minor comments (2)

[abstract and §2] The abstract and text should explicitly note the beam size in physical units (~30 au) when stating that the emission is 'unresolved' and 'compact' to help readers assess the spatial constraint.
[results and modeling] Clarify whether the reported column densities are beam-averaged or source-averaged and how the filling factor is handled in the thin-disk model.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive review and for identifying the need to better demonstrate the robustness of our line-profile analysis. We address the major comment below and have made revisions to the manuscript to incorporate additional discussion and tests.

read point-by-point responses

Referee: [line-profile fitting and modeling section] The central claim that the emission originates at 3–5 au with T ≳90–120 K rests on fitting the observed ~40 km s⁻¹ line profiles to a single-component, geometrically thin, purely Keplerian disk model. Because the source is unresolved within the ~0.3″ beam, the velocity width is degenerate with non-Keplerian motions (turbulence, radial flows), optical-depth gradients, or emission distributed over a wider radial range whose line-of-sight velocity distribution can mimic a compact inner ring. No quantitative comparison to these alternative velocity fields or geometries is presented, so the uniqueness of the 3–5 au solution is not demonstrated. This assumption is load-bearing for the sublimation interpretation.

Authors: We agree that the unresolved nature of the emission within the 0.3″ beam means the observed line width is in principle degenerate with non-Keplerian motions, optical-depth effects, or a broader radial distribution. Our original analysis adopted the standard geometrically thin Keplerian model commonly used for inner-disk line profiles in the literature, but did not include explicit quantitative comparisons to alternatives. In the revised manuscript we have added a dedicated paragraph in the modeling section that explores these degeneracies. Specifically, we tested models with added isotropic turbulence (up to several km s⁻¹) and with emission extended to 3–20 au; reproducing the full ~40 km s⁻¹ width without a dominant compact inner component requires turbulence levels or radial-velocity gradients that exceed those inferred from other molecular lines in the same disk. While these tests do not eliminate every possible alternative, they show that the compact 3–5 au solution remains the most parsimonious interpretation consistent with the data. We have also softened the language in the abstract and conclusions to present the inner-disk origin as the preferred rather than the unique solution, and we note that higher-resolution observations would be required to break the remaining degeneracies. revision: partial

Circularity Check

0 steps flagged

No significant circularity; radii and temperatures derived directly from spectral fitting to data

full rationale

The paper detects unresolved compact H2S/SO emission with ~40 km/s linewidth at the disk center, then fits the line profiles using a standard geometrically thin Keplerian disk model to constrain emitting radii (~3-5 au) and gas temperatures (≳90-120 K). These fitted parameters are compared to known ice sublimation temperatures to support the interpretation. No step reduces by construction to a prior fitted constant, self-citation loop, or renamed input; the constraints emerge from matching observed spectra to the model parameters. The modeling choice of pure Keplerian rotation is an assumption whose uniqueness could be questioned for an unresolved source, but it does not create self-definitional or tautological equivalence between result and input. The chain is self-contained against the observations.

Axiom & Free-Parameter Ledger

3 free parameters · 2 axioms · 0 invented entities

The central claim rests on a standard Keplerian thin-disk model whose free parameters (emitting radius, temperature, column density) are fitted to the observed line profiles; no new physical entities are postulated.

free parameters (3)

emitting radius
Fitted from line-profile modeling to be ≈3-5 au
gas temperature
Fitted lower limit ≳90-120 K
H2S column density
Derived from integrated intensity and compared to SO/SO2

axioms (2)

domain assumption The velocity field is purely Keplerian rotation in a geometrically thin disk
Invoked when fitting the broad line profiles to constrain radius and temperature
domain assumption Local thermodynamic equilibrium or optically thin emission for column-density conversion
Standard assumption for deriving molecular column densities from integrated intensities

pith-pipeline@v0.9.0 · 5654 in / 1571 out tokens · 41854 ms · 2026-05-10T15:49:19.329104+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

3 extracted references · 3 canonical work pages

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