arxiv: 2604.19366 · v1 · submitted 2026-04-21 · 🌌 astro-ph.GA

Recognition: unknown

HOTDISK. Finding Massive Protostellar Disks with Water and Refractory Molecular Species

Kai Yang , Yichen Zhang , Kei E. I. Tanaka , Tie Liu , Nami Sakai , Ziwei E. Zhang , Gyuho Lee , Kee-Tae Kim

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Adam Ginsburg Lile Wang Yao Wang Yongzhi Tang Yu Cheng Hongli Liu Wenyu Jiao Fengwei Xu Xunchuan Liu Xiaofeng Mai Dongting Yang

Authors on Pith no claims yet

Pith reviewed 2026-05-10 02:26 UTC · model grok-4.3

classification 🌌 astro-ph.GA

keywords massive protostarsprotostellar disksALMA observationsmolecular tracersvibrationally excited waterNaClSiShot cores

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The pith

Vibrationally excited water, NaCl, and SiS trace compact rotating disks around massive protostars on ~100 au scales

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

The paper uses high-angular-resolution ALMA Band 6 observations to search for hot-disk chemical signatures in ten massive protostars pre-selected for strong CH3CN and central SiO emission. Vibrationally excited water is detected in seven sources, always appearing as compact blueshifted and redshifted components on opposite sides of the 1.3 mm continuum peak with velocity gradients perpendicular to the outflow axes. NaCl and SiS show matching kinematics in five of those seven sources. Standard hot-core tracers such as CH3CN and SO2 instead map larger envelope gas. The high detection rate indicates that hot-disk chemical patterns, and therefore compact rotating disks, are common at least among sources that already possess well-developed rotating envelopes.

Core claim

High-resolution observations detect vibrationally excited water toward seven of ten targets. In every detection the blueshifted and redshifted emission is compact, lies on opposite sides of the continuum peak, and exhibits velocity gradients approximately perpendicular to the outflow axes, consistent with rotation on ~100 au disk scales. NaCl and SiS are detected in five of the seven sources and display the same kinematic pattern. In contrast, commonly used hot-core tracers primarily probe larger-scale envelope gas. These results establish vibrationally excited water, NaCl, and SiS as effective tracers of disk structures when observed at sufficient angular resolution and sensitivity.

What carries the argument

Vibrationally excited water (and the refractory species NaCl and SiS) as selective tracers whose emission is compact, kinematically distinct from the envelope, and aligned with expected disk rotation.

Load-bearing premise

That blueshifted and redshifted compact emission with velocity gradients perpendicular to the outflow axes represents rotation in disk structures on ~100 au scales rather than other kinematic features or projection effects.

What would settle it

Higher-resolution observations or additional kinematic tracers that show the velocity gradients are aligned with the outflow, absent, or inconsistent with any plausible disk rotation curve at the same location as the continuum peak.

Figures

Figures reproduced from arXiv: 2604.19366 by Adam Ginsburg, Dongting Yang, Fengwei Xu, Gyuho Lee, Hongli Liu, Kai Yang, Kee-Tae Kim, Kei E. I. Tanaka, Lile Wang, Nami Sakai, Tie Liu, Wenyu Jiao, Xiaofeng Mai, Xunchuan Liu, Yao Wang, Yichen Zhang, Yongzhi Tang, Yu Cheng, Ziwei E. Zhang.

**Figure 1.** Figure 1: ALMA 1.3 mm continuum images of the ten selected sources at low resolution (TM2) and high resolution (TM1+TM2). Contours are drawn at levels of 10σ × 2 n (n = 0, 1, 2, 3, . . .), where the rms noise levels σ are listed in Table 3. The synthesized beam is shown in the bottom-left corner of each panel, and the scale bar is indicated in the bottom-right corner. The crosses mark the continuum sources, includi… view at source ↗

**Figure 2.** Figure 2: Moment 0 and moment 1 maps of CH3CN (124 − 114), SO2 (283,25 − 282,26), and SiO (5 − 4) toward our sample. Contours show the high-resolution 1.3 mm continuum emission, with the same levels as in [PITH_FULL_IMAGE:figures/full_fig_p018_2.png] view at source ↗

**Figure 2.** Figure 2: (Continued.) [PITH_FULL_IMAGE:figures/full_fig_p019_2.png] view at source ↗

**Figure 3.** Figure 3: Integrated blueshifted and redshifted emission maps and moment 1 maps of H2O overlaid on the 1.3 mm continuum emission. Red and blue arrows indicate the outflow directions. The integrated vlsr ranges are labeled in each moment 0 panel. Blue and red contours are plotted at levels of 3σarea × n (n = 1, 2, 3, . . .), where σarea = rms √ ∆Vint δVchan. Black contours show the 1.3 mm continuum emission at levels… view at source ↗

**Figure 4.** Figure 4: Integrated blueshifted and redshifted emission maps of selected hot-disk molecular lines toward I16484−4603, overlaid on the 1.3 mm continuum emission (grayscale and black contours). The molecule names, transitions, upper-state energies, and integrated vlsr ranges are labeled in each panel. Crosses mark the continuum peaks. Blue and red contours are plotted at levels of 3σarea × n (n = 1, 2, 3, . . .), whe… view at source ↗

**Figure 5.** Figure 5: Same as [PITH_FULL_IMAGE:figures/full_fig_p022_5.png] view at source ↗

**Figure 6.** Figure 6: Same as [PITH_FULL_IMAGE:figures/full_fig_p023_6.png] view at source ↗

**Figure 7.** Figure 7: Same as [PITH_FULL_IMAGE:figures/full_fig_p023_7.png] view at source ↗

**Figure 8.** Figure 8: Same as [PITH_FULL_IMAGE:figures/full_fig_p024_8.png] view at source ↗

**Figure 9.** Figure 9: Position–velocity (PV) diagrams of H2O emission (black contours) overlaid on SiO emission (color scale) toward the hot-disk sources. The PV cuts are taken along the velocity-gradient direction of each source. Dashed horizontal and vertical lines mark the systemic velocity and the central position, respectively. Dashed curves show the Keplerian rotation curves corresponding to different central masses assum… view at source ↗

**Figure 10.** Figure 10: Position–velocity (PV) diagrams of H2O emission (black contours) overlaid on NaCl (18−17) (left), CH3CN (124−114) (middle), and SO2 (283,25−282,26) (right) emission (color scale) toward I16484−4603 (upper panels) and I17008−4040 (lower panels). The PV cuts are taken along the velocity-gradient direction of each source. Conventional hot-core tracers CH3CN, SO2, etc. ~1000au, ≳100K Hot-disk tracers H2O (vib… view at source ↗

**Figure 11.** Figure 11: Schematic illustration of the hot inner regions of massive protostellar systems, highlighting the hierarchical structure and associated chemical tracers. Conventional hot-core tracers (e.g., CH3CN and SO2) primarily probe rotation on envelope scales (∼1000 au), whereas vibrationally excited H2O and refractory species (e.g., NaCl, SiO, and SiS) trace compact, disk-scale rotation (∼100 au) closer to the cen… view at source ↗

**Figure 12.** Figure 12: Rotational and vibrational diagrams derived from NaCl lines toward I16484−4603 and I17008−4040. Column densities are measured within the central beam-sized region. Rotational and vibrational temperatures calculated using different sets of lines are shown in each panel [PITH_FULL_IMAGE:figures/full_fig_p027_12.png] view at source ↗

**Figure 13.** Figure 13: Averaged spectra of of H2O (v2 = 1; 55,0 −64,3), NaCl (v = 0; 18−17), SiS (v = 0; 12−11) and CH3CN (124−114) transitions toward the hot-disk sources. The H2O, NaCl, and SiS spectra are extracted from regions with integrated emission above 3σ. The CH3CN spectra are extracted from the central 0.3′′ region (about one beam size of the TM2 observation). The vertical dashed lines represent the vsys [PITH_FULL_… view at source ↗

**Figure 14.** Figure 14: Spectral energy distribution (SED) fitting results for the ten sources. Observed data points are shown as black symbols with error bars. Model SEDs are color-coded by their χ 2 values. Only models satisfying Rcore < 2Raperture and with χ 2 between χ 2 min and max(2, 2χ 2 min) (good models) are shown [PITH_FULL_IMAGE:figures/full_fig_p029_14.png] view at source ↗

read the original abstract

We present high-angular-resolution ($\sim0.05^{\prime\prime}$) ALMA Band~6 observations from the HOTDISK project (Hot-Origin Tracer survey of DISKs of massive protostars) aimed at investigating the "hot-disk" chemical pattern traced by vibrationally excited water, NaCl, SiS, and SiO in the innermost regions around massive protostars. Ten targets were selected based on strong CH$_3$CN emission exhibiting clear rotational signatures and centrally concentrated SiO emission from lower-resolution observations. We detect vibrationally excited water emission toward 7 of the 10 sources. In all detections, the blueshifted and redshifted components are compact and located on opposite sides of the 1.3 mm continuum peak, with velocity gradients approximately perpendicular to the outflow axes, consistent with rotation on disk scales. Emission from NaCl and SiS is detected toward 5 of these 7 sources and exhibits similar kinematics, further supporting the presence of compact rotating structures. In contrast, commonly used hot-core tracers (e.g., CH$_3$CN and SO$_2$) primarily probe larger-scale envelope gas. These results demonstrate that vibrationally excited water, NaCl, and SiS are powerful tracers of disk structures on $\sim$100 au scales, when observed at sufficient angular resolution and sensitivity. The high detection rate suggests that hot-disk chemical patterns - and thus compact rotating disks - are common in massive star-forming regions, at least among sources with well-developed rotating envelopes.

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

HOTDISK reports v-H2O, NaCl, and SiS tracing compact velocity gradients in 7/10 massive protostars, but the disk interpretation lacks the modeling needed to separate it from envelope or projection effects.

read the letter

The main thing to know is that this paper finds vibrationally excited water in seven of ten massive protostars, with NaCl and SiS in five of those, all showing compact blueshifted and redshifted emission on opposite sides of the continuum with gradients roughly perpendicular to the outflows. That pattern is presented as evidence for hot-disk chemistry on ~100 au scales being fairly common in sources that already have rotating envelopes. The high detection rate in this sample is the clearest new result, extending earlier single-source work to a larger set observed at 0.05 arcsec resolution with ALMA Band 6. The contrast with more extended tracers like CH3CN and SO2 is also useful and directly shown in the data. The observations themselves appear clean for what they set out to do: new high-resolution detections that highlight these particular lines as practical inner-region probes. The softer spot is the dynamical side. The abstract calls the kinematics consistent with rotation in distinct compact disks, yet there is no position-velocity diagram analysis, Keplerian fitting, or radiative-transfer modeling to test against alternatives such as projected larger-scale envelope motions, shocks, or unresolved non-circular flows. The targets were pre-selected for CH3CN rotation signatures, which adds a bias toward sources with organized motion already. Without those extra checks the conclusion that hot disks are common stays suggestive rather than fully secured. This is the sort of paper that people working on massive star formation and inner-disk chemistry would want to read for the tracer suggestions and occurrence rates. It deserves peer review because the sample and resolution are reasonable for the field, even if the authors will need to strengthen the interpretation or add caveats during revision.

Referee Report

2 major / 2 minor

Summary. The manuscript presents high-angular-resolution (~0.05 arcsec) ALMA Band 6 observations from the HOTDISK survey targeting 10 massive protostars pre-selected for strong CH3CN emission with rotational signatures and centrally concentrated SiO. Vibrationally excited water is detected toward 7 sources, showing compact blueshifted and redshifted components on opposite sides of the 1.3 mm continuum peak with velocity gradients approximately perpendicular to outflow axes, interpreted as tracing rotation in disk structures on ~100 au scales. NaCl and SiS emission, detected in 5 of these 7 sources, exhibits similar kinematics, while standard hot-core tracers like CH3CN and SO2 primarily probe larger-scale envelope gas. The authors conclude that v-H2O, NaCl, and SiS are powerful tracers of hot-disk chemistry and that such compact rotating disks are common in massive star-forming regions with well-developed rotating envelopes.

Significance. If the kinematic interpretation is confirmed, the results would establish vibrationally excited water, NaCl, and SiS as effective tracers for identifying compact ~100 au disks around massive protostars at high angular resolution, distinct from larger envelope structures. The high detection rate (7/10 sources) and the use of new ALMA data to reveal these patterns represent a clear strength, providing observational evidence that hot-disk chemical signatures may be prevalent among sources with rotating envelopes. This advances the field by offering a practical method to probe the innermost regions of massive star formation.

major comments (2)

[Results] The central claim that the observed compact emission with velocity gradients traces rotation in distinct ~100 au disks (rather than envelope infall, shocks, or projection effects) is not supported by position-velocity diagrams, Keplerian rotation curve fits, or radiative-transfer modeling. This interpretive step is load-bearing for the conclusion that these species trace disk structures on ~100 au scales and that hot-disk chemistry is common.
[Discussion] The inference of ~100 au disk scales and the distinction from envelope gas relies on qualitative descriptions of compactness and orientation; no quantitative comparison of observed velocities to expected Keplerian speeds (based on protostellar masses) or spatial scale measurements with error bars is provided to substantiate the scale and rule out larger-scale contributions.

minor comments (2)

The abstract states velocity gradients are 'approximately perpendicular' to outflows; including measured position angles and their uncertainties would improve precision and allow readers to assess the consistency claim directly.
[Figure captions] Figure captions should explicitly note the synthesized beam size, position angle, and contour levels for all moment maps and continuum images to facilitate reproducibility.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their positive evaluation of the significance of our results and for the constructive major comments, which help clarify the interpretive basis of the kinematic analysis. We address each point below and will incorporate revisions to strengthen the supporting evidence.

read point-by-point responses

Referee: [Results] The central claim that the observed compact emission with velocity gradients traces rotation in distinct ~100 au disks (rather than envelope infall, shocks, or projection effects) is not supported by position-velocity diagrams, Keplerian rotation curve fits, or radiative-transfer modeling. This interpretive step is load-bearing for the conclusion that these species trace disk structures on ~100 au scales and that hot-disk chemistry is common.

Authors: We agree that the kinematic interpretation would be more robust with explicit position-velocity diagrams and quantitative analysis. The current support rests on the observed compactness of the blueshifted and redshifted components (spatially coincident with the ~0.05 arcsec beam, implying ~100 au scales at the source distances), their placement on opposite sides of the 1.3 mm continuum peak, the approximate perpendicularity of the velocity gradients to the outflow axes, and the consistency across multiple species (v-H2O, NaCl, SiS) while contrasting with larger-scale tracers like CH3CN. These features are difficult to reconcile with pure infall or shocks. To address the concern directly, we will add position-velocity diagrams extracted along the inferred major axes in the revised manuscript, along with a discussion of how the observed patterns favor rotation over alternatives. revision: yes
Referee: [Discussion] The inference of ~100 au disk scales and the distinction from envelope gas relies on qualitative descriptions of compactness and orientation; no quantitative comparison of observed velocities to expected Keplerian speeds (based on protostellar masses) or spatial scale measurements with error bars is provided to substantiate the scale and rule out larger-scale contributions.

Authors: We acknowledge that quantitative measurements with uncertainties and direct velocity comparisons would better substantiate the ~100 au scales and the distinction from envelope gas. The scale estimate follows from the angular resolution and source distances, with emission appearing compact relative to the envelope tracers. In the revision, we will include measured spatial extents (or upper limits) of the v-H2O, NaCl, and SiS emission with error bars, a comparison of observed velocity gradients to approximate Keplerian expectations using available protostellar mass estimates from the literature, and explicit size contrasts with CH3CN to quantify the difference from envelope scales. Limitations in mass constraints for individual sources will be noted explicitly. revision: yes

Circularity Check

0 steps flagged

Purely observational ALMA study with no equations, fits, or derivations

full rationale

The paper reports new high-resolution ALMA Band 6 observations of 10 massive protostars, presenting detection statistics for vibrationally excited water, NaCl, and SiS, along with direct descriptions of their spatial and kinematic properties (compact blueshifted/redshifted emission with gradients perpendicular to outflows). No equations, model fittings, parameter derivations, or predictive calculations are present that could reduce to inputs by construction. Target selection from prior lower-resolution data is a sample criterion only and does not enter any derivation chain. The claims rest on the new data and straightforward kinematic interpretation, making the work self-contained with no circularity.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard astrophysical interpretations of molecular line kinematics and chemical excitation conditions in star-forming regions; no free parameters or new entities are introduced in the abstract.

axioms (1)

domain assumption Compact emission with velocity gradients perpendicular to outflow axes indicates rotating disk structures on ~100 au scales.
This kinematic interpretation directly supports linking the detected molecules to disks.

pith-pipeline@v0.9.0 · 5650 in / 1330 out tokens · 69622 ms · 2026-05-10T02:26:16.941284+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

14 extracted references · 11 canonical work pages

[1]

T., & de Wit, W

Beltr´ an, M. T., & de Wit, W. J. 2016, A&A Rv, 24, 6, doi: 10.1007/s00159-015-0089-z Bosman, A. D., Bergin, E. A., Loomis, R. A., et al. 2021, ApJS, 257, 15, doi: 10.3847/1538-4365/ac1433 CASA Team, Bean, B., Bhatnagar, S., et al. 2022, PASP, 134, 114501, doi: 10.1088/1538-3873/ac9642 Cesaroni, R., Galli, D., Lodato, G., Walmsley, C. M., & Zhang, Q. 2007...

work page doi:10.1007/s00159-015-0089-z 2016
[2]

′′042,76 ◦ 15 I18134−1942 0.5

′′061×0. ′′042,76 ◦ 15 I18134−1942 0.5

1942
[3]

′′073,−65 ◦ 24 0.6 a Typical noise levels for the spectral windows per channel, as the noise level slightly changes with the frequency

′′105×0. ′′073,−65 ◦ 24 0.6 a Typical noise levels for the spectral windows per channel, as the noise level slightly changes with the frequency. See the image captions of the presented lines for more information. b I18117−1753 was imaged with robust−0.5 to achieve higher resolution. Hot Disk15 T able 4.Parameters of the spectral lines. Molecule Transition...

work page arXiv
[4]

232686.7000 3461.9 4.8×10 −6 NaCl 18−17 (v=

work page arXiv
[5]

234251.8300 106.8 5.9×10 −3 NaCl 18−17 (v=

work page arXiv
[6]

232509.9500 625.7 5.8×10 −3 NaCl 17−16 (v=

work page arXiv
[7]

219614.9220 614.5 4.9×10 −3 NaCl 17−16 (v=

work page arXiv
[8]

217980.2149 1128.4 4.9×10 −3 SiS 12−11 (v=

work page arXiv
[9]

217817.6630 68.0 1.7×10 −4 SiS 13−12 (v=

work page arXiv
[10]

234812.9678 1150.0 2.2×10 −4 PN 5−4 (v=

work page arXiv
[11]

234935.6630 33.8 5.2×10 −4 PN 5−4 (v=

work page arXiv
[12]

233271.8000 1937.3 5.0×10 −4 16Yang et al. T able

work page arXiv 1937
[13]

45"50"55

(K) (K) (K) (K) (K) (K) (K) (K) (K) I08303−4303 18.6 – – – – – – – 24.3 I16484−4603 37.1 26.1 20.9 24.1 17.5 30.9 20.3 42.3 185 I17008−4040 124 50.0 54.5 46.2 51.7 59.7 52.2 – 596 I18117−1753 21.3 11.9 8.75 13.8 6.65 11.1 – 39.1 113 I18434−0242 45.6 51.0 33.7 51.8 16.6 59.8 43.0 – 166 I18507+0121 35.4 – – – – – – – 146 I18517+0437 22.7 18.8 – – – 25.3 8.8...

1942
[14]

26.5"27.0

The scale bar shows the velocity range of the moment-1 maps for the three lines in each source (km s −1). Hot Disk19 18h16m22.20s 22.15s 22.10s 22.05s 19°41'26.0" 26.5"27.0"27.5"28.0" I18134 1942 [4, 16] km s 1 + 500 AU + 500 AU [5, 22] km s 1 + 500 AU + 500 AU [4, 16] km s 1 + 500 AU + 500 AU 6 8 10 12 14 16 18h46m03.80s 03.75s 03.70s 2°39'21.5" 22.0"22....

1942