🔭 astrophysics

JOYS $+$ : A JWST/MIRI survey of the evolution of H $_2$ winds and jets from low-mass protostars

Using JWST/MIRI observations of 33 protostars, the JOYS $+$ survey reveals that H $_2$ outflows evolve from narrow, high-velocity jets in Class 0 sources to wide-angle, low-velocity winds in Class I sources with declining mass-loss rates, a trend consistent with magnetohydrodynamic disk wind models.

Original authors: L. Francis, Ł. Tychoniec, E. F. van Dishoeck, A. D. Sellek, A. Caratti o Garatti, V. J. M. Le Gouellec, C. Gieser, H. Beuther, J. M. Vorster, M. E. Ressler, P. Nazari, B. Tabone, K. Assani, R. Devaraj

Published 2026-04-16

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: L. Francis, Ł. Tychoniec, E. F. van Dishoeck, A. D. Sellek, A. Caratti o Garatti, V. J. M. Le Gouellec, C. Gieser, H. Beuther, J. M. Vorster, M. E. Ressler, P. Nazari, B. Tabone, K. Assani, R. Devaraj, J. J. Tobin, Maria Gabriela Navarro, P. C. Cortés, J. M. Girart, M. Güdel, Th. Henning, G. Östlin, G. Wright, T. Ray

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

Imagine a newborn baby star. It's not just sitting there; it's actively growing, pulling in gas and dust from its surroundings like a giant vacuum cleaner. But this process is messy. As the star eats, it has to get rid of the extra "weight" (angular momentum) to keep spinning without flying apart. To do this, it shoots out powerful streams of material in opposite directions, like a garden hose spraying water.

This paper is a deep dive into those streams, specifically looking at the very youngest stars (called Class 0 and Class I protostars) using the James Webb Space Telescope (JWST). The team, led by L. Francis, wanted to understand how these streams change as the baby star grows up.

Here is the story of their discovery, broken down into simple concepts:

1. The Two Types of "Streams"

Think of the outflow from a baby star as having two distinct parts, like a firehose and a gentle breeze:

The Jet (The Firehose): A narrow, super-fast, high-velocity stream shooting straight out. In the youngest stars, this is often made of molecules like carbon monoxide (CO) and silicon monoxide (SiO).
The Wind (The Breeze): A wide, slower, fan-shaped flow surrounding the jet. This is mostly made of molecular hydrogen ( $H_2$ ), the most common gas in the universe.

The Big Question: How does this "breeze" change as the star gets older? Does it get wider? Does it slow down? Does it carry less stuff?

2. The New "Super-Microscope"

Before JWST, looking at these baby stars was like trying to see a firefly in a foggy forest at night. The gas clouds surrounding the stars were too thick for our old telescopes to see the base of the streams.

JWST is like a high-powered night-vision camera that can see through the fog. The team used JWST's MIRI instrument to look at the heat (infrared light) coming from the hydrogen gas ( $H_2$ ) right at the base of these streams, within a few hundred "astronomical units" (distances roughly the size of our solar system).

3. What They Found: The "Growing Up" Story

A. The Wind Gets Wider (The Umbrella Analogy)

Imagine holding a garden hose. When you first turn it on, the water shoots out in a tight, narrow stream. As you get older and the pressure changes, maybe you open the nozzle, and the water sprays out in a wide cone.

The team found that the wind gets wider as the star ages.

Class 0 (Newborn): The wind is tight, like a narrow cone (about 20 degrees wide).
Class I (Toddler): The wind has opened up like a giant umbrella (about 90 degrees wide).
This suggests that as the star grows, it clears out more of the surrounding "fog" (the envelope), allowing the wind to spread out more.

B. The Temperature is Surprisingly Stable

The team expected the gas to get hotter or cooler as the star evolved. Instead, they found the gas stays at a surprisingly steady temperature.

There is a "Warm" component (about 600°C or 1100°F) and a "Hot" component (about 1500–3000°C).
The Analogy: It's like a campfire. Even as the fire changes size, the temperature of the glowing embers stays roughly the same. This suggests the heat isn't coming from the star directly, but from shocks—like when a fast car slams into a slow car, creating a burst of heat. The gas is being heated by collisions as the wind crashes into the surrounding material.

C. The "Mass" Mystery (The Invisible Ghost)

This is the most surprising part. The team calculated how much mass (stuff) is being blown away by the wind.

They found that the warm hydrogen wind is losing mass, but the amount drops drastically as the star gets older (from Class 0 to Class I).
The Problem: When they compared their JWST data to older radio telescope data (which sees cold gas), there was a huge mismatch. The radio telescopes saw 10 to 1,000 times more mass than JWST saw.
The Explanation: JWST is great at seeing "warm" gas, but it can't see "cold" gas very well. The team believes there is a massive amount of cold, invisible hydrogen that JWST is missing. It's like trying to count the snowflakes in a blizzard by only looking at the ones that are glowing red-hot. The cold snowflakes are there, but they are invisible to this specific camera.

4. The Theory Check: Magnetic Launchers

Scientists have two main theories for how these stars launch their winds:

X-Wind: Launched from a tiny spot very close to the star.
MHD Disk Wind: Launched from a wide area of the spinning disk around the star, like water spinning off a wet tire.

The Verdict: The observations (the wind getting wider, the lack of high-speed jets in older stars) fit the MHD Disk Wind theory much better. It's as if the star is spinning a wet towel; the water flies off in a wide arc, not just a single needle-point stream.

Summary: The Takeaway

This paper tells us that baby stars are messy, dynamic factories.

They start tight: Young stars shoot out narrow, fast winds.
They grow wide: As they age, the winds spread out like an opening umbrella.
They hide mass: A huge amount of the material being ejected is so cold and invisible that we need new tools to see it.
They are magnetic: The winds are likely launched by magnetic fields spinning the disk around the star, not just a tiny explosion near the surface.

JWST has given us the first clear, high-definition look at the "nursery" of these stars, proving that while the winds change shape, the physics of how they are launched remains consistent with magnetic theories. It's a bit like finally getting a clear photo of a baby's first steps, realizing they are walking a path we predicted, but with a few surprises hidden in the shadows.

1. Problem and Scientific Context

The formation of low-mass stars involves the accretion of material onto a protostar, a process mediated by the ejection of mass via bipolar outflows. These outflows consist of two distinct components:

Collimated, high-velocity jets: Often traced by atomic lines or high-velocity molecular lines (CO, SiO).
Wide-angle, low-velocity winds: Thought to be magnetocentrifugally launched from the protoplanetary disk to remove angular momentum.

While the evolution of outflows from Class 0 (youngest, deeply embedded) to Class I and Class II stages is known to involve changes in morphology (opening angles) and composition (molecular to atomic), the specific properties of the molecular hydrogen (H2) component near the launching point (within a few hundred AU) have been difficult to observe due to high extinction and the lack of sensitive mid-infrared instruments.

Key Questions:

How does the H2 wind morphology, mass-loss rate, and velocity structure evolve from Class 0 to Class I?
What are the temperature and density structures of these winds?
Do observations support Magnetohydrodynamic (MHD) disk wind models or X-wind models?

2. Methodology

The study utilizes data from the JOYS+ (JWST Origins, Exoplanets, and Star formation) survey, specifically targeting a sample of 33 low-mass protostars (13 single and 20 multiple systems) in the Class 0 and Class I evolutionary stages.

Observations:
- JWST/MIRI/MRS: Used to observe pure rotational H2 lines (S(1) to S(8)) in the mid-infrared (5–28 µm). This provides unprecedented spatial resolution (down to 0.3 arcseconds, or ~40–125 AU) and sensitivity to probe the base of the outflows.
- ALMA: Archival sub-mm CO (2-1 or 3-2) data were used to trace the entrained, cold gas component of the outflows for comparison.
Data Analysis:
- Mapping: Continuum-subtracted maps of H2 S(1) and S(7) line flux and velocity centroids were constructed.
- Morphology: Opening angles of the wide-angle wind were measured using the extent of the faint H2 S(1) emission in the blue-shifted lobes.
- Spectroscopy: Spectra were extracted from apertures (typically 300 AU radius) in the blue-shifted outflow lobes.
- Rotation Diagrams: A two-component temperature model (warm and hot) was fitted to the H2 rotational lines to derive column densities, excitation temperatures, and ortho-to-para ratios (OPR), while simultaneously estimating extinction.
- Mass Loss Rates: Calculated using the formula $\dot{M} = 2m_H N_{H2} L_{\perp} v_{\perp}$ , combining column density, outflow width, and inclination-corrected velocity.

3. Key Contributions

Largest Sample to Date: This is the first study to systematically analyze H2 wind and jet properties across a large, homogeneous sample of Class 0 and I protostars using JWST.
Resolution of the Launching Region: The study probes the outflow base (within ~300 AU), a region previously inaccessible to ground-based telescopes due to extinction.
Decoupling Components: The authors successfully distinguish between the wide-angle "wind" (traced by low-J H2) and the collimated "jet" (traced by high-J H2 and atomic lines).
Quantification of Mass Loss: Provides the first robust measurements of warm H2 mass-loss rates and their evolution, comparing them directly to cold CO outflow rates.

4. Key Results

A. Morphology and Evolution

Opening Angles: The opening angle of the H2 wind (traced by S(1)) broadens significantly from ~20° in Class 0 sources to ~90° in Class I sources. This trend mirrors the evolution seen in cold CO outflows.
Jet Presence: High-velocity, collimated molecular jets (traced by high-J H2 lines) are prominent in Class 0 sources but absent in Class I sources. Class I sources instead show conical atomic jets (e.g., [Fe II]) surrounded by a conical H2 component.
Velocity Stratification: In Class 0 sources, higher velocities are observed along the jet axis, while the wide-angle wind is slower (0–20 km/s).

B. Temperature and Excitation

Two Components: The H2 emission is well-described by two temperature components:
- Warm Component: ~600 K (dominant mass).
- Hot Component: 1500–3000 K (associated with shocks/knots).
Lack of Evolution: Surprisingly, the excitation temperatures of both components remain relatively constant across the Class 0 to Class I transition. This contradicts some steady-state MHD wind models that predict temperature increases with evolutionary stage.

C. Mass Loss and Momentum

Mass Dominance: The warm component contains two orders of magnitude more mass than the hot component and dominates the total H2 mass-loss rate.
Evolutionary Decline: The warm H2 mass-loss rate declines by two orders of magnitude from Class 0 ( $\sim 3 \times 10^{-7} M_\odot \text{yr}^{-1}$ ) to Class I ( $\sim 3 \times 10^{-8} M_\odot \text{yr}^{-1}$ ).
Correlation: The mass-loss rate correlates with the bolometric luminosity ( $L_{bol}$ ) of the driving source.
The "Missing" Mass: There is a 10–1000x mismatch between the mass/momentum flux derived from warm H2 (JWST) and cold CO (ALMA). This implies the presence of a significant amount of cold (<150 K) molecular H2 that is entrained from the envelope but undetectable by JWST/MIRI (as the S(1) line is too faint at these temperatures).

D. Ortho-to-Para Ratio (OPR)

The observed OPR is typically 2–3 (close to the LTE value of 3), suggesting the warm H2 near the base is launched from the disk rather than being purely entrained cloud material (which would have a low OPR). However, a colder entrained component cannot be ruled out.

5. Significance and Conclusions

Support for MHD Disk Winds: The observed broadening of opening angles, the presence of wide-angle molecular winds in Class 0, and the transition to atomic jets in Class I are consistent with MHD disk wind models. These models predict that as the envelope dissipates, the wind launching region moves outward, and inner streamlines become too hot for molecules to survive.
Challenge to Steady-State Models: The lack of temperature evolution in the H2 components suggests that the observed temperatures may reflect shock conditions (C-shocks) in a time-variable outflow rather than the thermal structure of a steady wind.
Role of Entrainment: The large discrepancy between warm H2 and cold CO mass fluxes highlights that a significant portion of the outflow mass is cold, entrained gas that is invisible to current mid-infrared instruments.
Future Prospects: The paper emphasizes the need for future facilities (like ELT/METIS or PRIMA/FIRESS) to detect the H2 S(0) line to probe the cold gas component and complete the mass budget of protostellar outflows.

In summary, this paper establishes that JWST/MIRI can resolve the complex structure of protostellar outflows, revealing that while the molecular wind dominates the mass loss in early stages and evolves in geometry, a significant cold, entrained component remains hidden, and the thermal structure of the outflow is likely governed by shock physics rather than simple steady-state wind thermodynamics.