🔭 astrophysics

Protostellar Outflows at the EarliesT Stages (POETS). IX. Magnetohydrodynamic disk winds traced by SO and SO $_2$ in luminous protostars

This study utilizes millimeter-wave observations of two massive young stellar objects to demonstrate that sulfur-bearing molecules (SO and SO $_2$ ) trace magnetohydrodynamic disk winds launched from scales of 100–1000 au, with their distinct spatial and kinematic properties supporting a radially extended wind model over a compact X-wind scenario.

Original authors: L. Moscadelli, H. Beuther, A. Sanna, M. T. Beltrán, C. Gieser, Th. Henning, P. D. Klaassen, R. Kuiper, S. Leurini, T. Möller, A. Palau, R. E. Pudritz, Á Sánchez-Monge, D. Semenov, J. S. Urquhart, H. Z

Published 2026-06-11

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CC BY 4.0

Original authors: L. Moscadelli, H. Beuther, A. Sanna, M. T. Beltrán, C. Gieser, Th. Henning, P. D. Klaassen, R. Kuiper, S. Leurini, T. Möller, A. Palau, R. E. Pudritz, Á Sánchez-Monge, D. Semenov, J. S. Urquhart, H. Zinnecker

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: How Stars Grow Up

Imagine a baby star (a protostar) forming in a cloud of gas and dust. To grow, it needs to swallow up material from a swirling disk surrounding it, like a child eating from a plate. But there's a problem: the spinning disk has too much "spin energy" (angular momentum). If the star tries to eat everything, the spinning gets so fast that the food flies off the plate instead of landing in the star's mouth.

To solve this, the star needs a way to slow down the spin and let the food fall in. Scientists believe the star uses a Magnetohydrodynamic (MHD) Disk Wind. Think of this as a cosmic vacuum cleaner or a powerful fan that blows material away from the disk. By blowing this wind, the star gets rid of the extra spin, allowing the rest of the material to fall inward and feed the star.

The Mystery: Is the Wind a "Jet" or a "Fountain"?

Scientists have been trying to figure out exactly how this wind works. There are two main theories:

The X-Wind (The Compact Jet): Imagine a tiny, high-pressure nozzle right at the very center of the star (like a garden hose nozzle). It shoots a narrow, fast beam of material. This theory suggests the wind only comes from the very inner edge of the disk.
The Disk Wind (The Wide Fountain): Imagine a wide sprinkler system that sprays water from the entire surface of the disk, not just the center. This wind is launched from a wide area of the disk and carries away spin from a much larger region.

The authors of this paper wanted to see which of these two "wind types" is actually happening in real, massive baby stars.

The Investigation: Two Different Stars

The researchers looked at two massive baby stars, which we'll call Star A and Star B.

Star A (IRAS 21078+5211): A slightly smaller massive star (about 5.6 times the mass of our Sun).
Star B (G035.02+0.35): A much heavier massive star (about 20 times the mass of our Sun).

They used powerful radio telescopes (like giant eyes in the sky) to look at these stars at a distance of 100 to 1,000 times the distance from the Earth to the Sun. They looked for specific chemical "smoke signals" (molecules like SO, SO2, and others) to trace the wind.

What They Found: The "Smoke" Tells the Story

1. The Spin is Real
In both stars, they found that the gas in the wind was spinning. It wasn't just flying straight out; it was rotating, like a spinning top. This confirmed that the wind is indeed removing the spin energy needed for the star to grow.

2. The Chemical Clues
The researchers looked at different chemicals to see how far they traveled from the star:

In Star A: They found a mix of chemicals. Some chemicals (like SO2) were hugging the center of the jet tightly, while others (like Methanol) were spread out in a wide, funnel shape.
- The Analogy: Imagine a fireworks display. The bright, fast sparks (SO2) are right in the center, while the wider, slower smoke trail (Methanol) spreads out further. This "nested" structure—where different chemicals live at different distances—looks exactly like a wide sprinkler (Disk Wind) rather than a single narrow hose.
In Star B: This star was different. They only found one chemical (SO) in the wind. All the other chemicals were stuck inside the disk and didn't fly out.
- The Analogy: Imagine a very hot, intense fire. The wind is so hot and energetic that it burns up (dissociates) all the complex chemicals, leaving only the simplest, toughest one (SO) to survive the trip. This suggests the wind is coming directly from the star's disk surface, which is being heated by shocks, rather than being a gentle breeze picking up dust from the surroundings.

The Verdict: It's a Wide Fountain, Not a Narrow Hose

The paper concludes that both stars are using the Disk Wind method (the wide fountain), not the X-Wind (the narrow hose).

Why? If it were a narrow hose (X-Wind), the wind would be picking up random dust and gas from the surroundings. We would expect to see a messy mix of chemicals everywhere. Instead, the chemicals are organized in specific patterns that match a wind launching directly from the disk itself.
The "SO" Super-Tracer: The paper highlights that the chemical SO (Sulfur Monoxide) is the best "smoke signal" to find these winds. It travels far out and shows the spinning motion clearly, acting like a glowing ribbon that traces the path of the wind.

Summary

The scientists used chemical fingerprints to prove that massive baby stars don't just shoot a single narrow beam of wind. Instead, they launch a wide, rotating wind from their entire disk surface. This "cosmic sprinkler" is the key mechanism that allows these stars to slow down their spin and grow up. The study confirms that this happens even in stars with very different masses, suggesting it's a universal rule for how stars form.

Technical Summary: Protostellar Outflows at the EarliesT Stages (POETS) IX

Problem and Context
The removal of angular momentum is a fundamental requirement for gas to accrete onto forming stars. While turbulent and magnetic stresses can drive this transport, Magnetohydrodynamic (MHD) disk winds (DWs) are increasingly favored as the primary mechanism for extracting angular momentum across extended disk radii, driving mass accretion, and setting conditions for planet formation. Previous work by the Protostellar Outflows at the EarliesT Stages (POETS) survey utilized 22 GHz water maser Very Long Baseline Interferometry (VLBI) to trace wind kinematics at scales of 10–100 au in luminous Young Stellar Objects (YSOs). These observations revealed two distinct kinematic configurations: fast, collimated jets and slower, less collimated winds tracing disk rotation. However, to fully validate the MHD DW hypothesis, these findings must be tested at larger scales (100–1000 au) using thermal molecular tracers to distinguish between compact X-winds (launched from the innermost disk) and extended disk winds.

Methodology
This study investigates two massive YSOs, IRAS 21078+5211 ( $\approx 5.6 M_\odot$ ) and G035.02+0.35 ( $\approx 20 M_\odot$ ), which represent the two characteristic maser kinematic configurations identified in the POETS survey. The authors employed high-resolution interferometric observations to analyze gas kinematics and physical conditions:

IRAS 21078+5211: Observed with the IRAM Northern Extended Millimeter Array (NOEMA) in the 1.3 mm band (A configuration, 2022–2023), achieving a linear resolution of $\approx 250$ au.
G035.02+0.35: Analyzed using archival Atacama Large Millimeter Array (ALMA) Cycle 4 Band 6 data, providing comparable linear and velocity resolutions.

The study utilized a suite of molecular tracers, specifically focusing on S-bearing species (SO, SO $_2$ ) and complex organics (CH $_3$ CN, CH $_3$ OH, HC $_3$ N). Position-velocity (PV) plots were constructed along the disk midplanes to analyze velocity gradients. Physical parameters (excitation temperature $T_{ex}$ , column density $N_{col}$ , velocity, and line width) were derived using the MADCUBA software suite, assuming Local Thermodynamic Equilibrium (LTE). The results were compared against shock chemistry models (e.g., van Gelder et al. 2021) to interpret the spatial distribution of molecular species.

Key Results

Kinematic Signatures of Rotating Winds:
- In IRAS 21078+5211, molecular emissions (SO, SO $_2$ , CH $_3$ CN, CH $_3$ OH) are elongated along the radio jet axis ( $PA \approx 50^\circ$ ) and exhibit a Local Standard of Rest (LSR) velocity gradient transversal to the jet. PV plots of SO reveal a Keplerian rotation profile consistent with a wind launched from the disk.
- In G035.02+0.35, molecular emissions (excluding SO) are distributed along the major axis of the rotating disk ( $PA \approx 150^\circ$ ), with velocity gradients tracing disk rotation. SO is the only species extending significantly outside the disk, also showing a Keplerian profile.
- In both sources, the SO emission displays the clearest Keplerian-like velocity profile and extends well beyond the disk midplane, while SO $_2$ is spatially compact and traces the highest velocities closest to the rotation axis.
Physical Conditions and Shock Chemistry:
- IRAS 21078+5211: The wind exhibits a nested morphology. SO $_2$ and HC $_3$ N are confined near the jet axis, SO extends to intermediate radii, and CH $_3$ CN/CH $_3$ OH show the widest radial extension. The authors attribute the irregular, blob-like distribution of CH $_3$ CN and CH $_3$ OH to external shocks interacting with dense ambient clumps, while the homogeneous SO/SO $_2$ emission arises from internal shocks within the wind.
- G035.02+0.35: Only SO traces the wind; other molecules are confined to the disk. The authors propose that the disk atmosphere in this source is shock-ionized and heated to temperatures ( $10^3$ – $10^4$ K) sufficient to dissociate complex molecules, leaving only SO (which can form efficiently in J-type shocks via dust desorption) as a tracer. The lack of UV flux from a potentially bloated star may prevent the formation of SO $_2$ at larger distances.
Differentiation of Wind Mechanisms:
- The radial distribution of molecules in IRAS 21078+5211 and the exclusive presence of SO in G035.02+0.35 are inconsistent with an X-wind scenario (where ambient gas is entrained in a fast, wide-angle wind), which would predict the presence of all observed molecular species in the outflow.
- Instead, the data support a Magneto-Centrifugal Disk Wind (MHD DW) model where gas is ejected directly from an extended portion of the disk. The varying molecular tracers are explained by the specific shock conditions (velocity and density) and the degree of molecular dissociation in the wind launch region.

Significance and Conclusions
This study provides compelling evidence that SO and SO $_2$ emissions can serve as robust tracers of MHD disk winds in luminous YSOs at scales of 100–1000 au. The findings confirm the MHD DW interpretation previously derived from water maser VLBI observations at 10–100 au scales. By demonstrating that the gas kinematics derived from thermal lines are consistent with those from masers, the paper validates the MHD DW as a common launching mechanism for protostellar winds across different YSO masses and environments. The results further suggest that the specific molecular content of the wind is a diagnostic tool for the physical conditions (shock strength and ionization state) of the wind launch region, distinguishing between compact X-winds and extended disk winds.