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The Structure and Kinematics of Three Class 0 Protostellar Jets from JWST

Using JWST's NIRSpec and MIRI instruments, this study characterizes the morphologies, kinematics, and shock structures of three Class 0 protostellar jets (B335, HOPS 153, and HOPS 370) within 2000 au, revealing their wiggling motions, velocity ranges, and complex interactions between circumstellar disks and magnetic fields.

Original authors: Samuel A. Federman, S. Thomas Megeath, Alessio Caratti o Garatti, Mayank Narang, Himanshu Tyagi, Neal J. Evans, Carolin N. Kimmig, Lukasz Tychoniec, Henrik Beuther, Amelia Stutz, P. Manoj, Robert Gute

Published 2026-01-15

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CC BY 4.0

Original authors: Samuel A. Federman, S. Thomas Megeath, Alessio Caratti o Garatti, Mayank Narang, Himanshu Tyagi, Neal J. Evans, Carolin N. Kimmig, Lukasz Tychoniec, Henrik Beuther, Amelia Stutz, P. Manoj, Robert Gutermuth, Tyler L. Bourke, Joel Green, Lee Hartmann, Pamela Klaassen, Rolf Kuiper, Leslie W. Looney, Pooneh Nazari, Thomas Stanke, Dan M. Watson, Yao-Lun Yang, Wafa Zakri

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 the universe as a giant construction site where new stars are being born. These baby stars, called protostars, are messy, chaotic, and deeply buried inside thick clouds of gas and dust. To grow, they have to swallow up material from their surroundings, but as they spin and eat, they need to get rid of the extra "stuff" (angular momentum) to keep from spinning apart. Nature's solution? They shoot out powerful, high-speed jets of gas from their poles, like a firehose spraying water in opposite directions.

This paper is a detailed report card on three of these baby stars: B335, HOPS 153, and HOPS 370. The authors used the James Webb Space Telescope (JWST), which acts like a super-powered pair of night-vision goggles, to peer through the dusty clouds and see these jets in incredible detail for the first time.

Here is what they found, explained simply:

1. The "Flashlight" Effect: Seeing the Invisible

Normally, these jets are hidden. But the JWST looked for specific glowing "signposts" made of ionized iron and other elements. Think of these glowing knots of gas as flares or fireflies lighting up the dark tunnel of the jet.

The Finding: The telescope saw these flares stretching out for hundreds of miles (astronomical units) from the baby stars. The iron flares showed the full path of the jet, while other elements only lit up specific, bumpy knots along the way.

2. The Jets Aren't Straight; They Wiggle!

If you've ever tried to spray a garden hose while standing on a spinning merry-go-round, the water doesn't go in a straight line; it makes a wavy, snake-like pattern.

The Finding: These cosmic jets are doing the same thing. They wiggle and bend as they shoot out.
The Twist: The wiggles aren't symmetrical. For some stars, the "blue" jet (coming toward us) wiggles, but the "red" jet (going away) is straight. For others, it's the opposite. This suggests that the "steering wheel" of the jet (the magnetic fields and the spinning disk around the star) is being twisted or warped, perhaps by an invisible companion star or a magnetic tug-of-war.

3. The "Traffic Jam" of Speed

The scientists measured how fast the gas is moving.

The Finding: For two of the stars (HOPS 153 and HOPS 370), the jets are like a train on a straight track: they move at a steady, constant speed.
The Surprise: For the smallest star, B335, the jet is acting like a car slowing down or speeding up as it travels. The gas near the star is moving at a different speed than the gas further out.
The Mystery: This star is also a bit of a trickster. When the scientists tried to calculate its speed using the angle of its surrounding dust cloud, the math gave them an impossible speed (faster than physics should allow). However, by tracking a specific "firefly" knot over an 8-month gap between observations, they realized the jet is actually pointing in a different direction than the dust cloud. It's like a lighthouse beam that has started to spin, pointing in a new direction than the tower it sits on.

4. The "Burst" Connection

The star B335 recently had a "growth spurt" or outburst, where it suddenly got much brighter.

The Finding: The scientists noticed that during this bright burst, the jet seemed to shoot out more of those glowing "firefly" knots. It's as if the star got a sudden energy boost, and the jet responded by firing off more bullets of gas. While they can't say for sure that the burst caused the knots, the timing suggests a strong link.

5. The Shape of the Stream

Jets usually start narrow and get wider as they go, like a cone.

The Finding: These jets are doing something weird. They don't just get wider smoothly. They get wider, then narrower, then wider again. It's like a garden hose that suddenly kinks and then straightens out. This non-smooth widening suggests the jets are interacting with the messy environment around them, or that the way they are being launched is changing over time.

The Big Picture

In short, this paper tells us that baby stars are not quiet, orderly factories. They are dynamic, messy, and constantly changing. Their jets are not just straight beams of light; they are wiggling, bending, speeding up, and slowing down, likely because the magnetic fields and spinning disks around them are complex and unstable. The JWST gave us the first clear look at this cosmic dance, revealing that even the smallest baby stars have complex personalities.

Technical Summary: The Structure and Kinematics of Three Class 0 Protostellar Jets from JWST

Problem and Motivation
High-velocity collimated jets are a fundamental consequence of magnetically mediated accretion in rotating protostellar systems, serving to transport angular momentum and expel mass. While shock-ionized [Fe II] emission has been identified as a ubiquitous tracer of these jets in Class 0 protostars, detailed morphological and kinematic analyses of the inner jet regions (within 2000 au) have been limited by spatial resolution and sensitivity. Previous studies often relied on lower-resolution data or focused on specific mass ranges. This paper addresses the need for a rigorous examination of jet structure and kinematics across the protostellar mass spectrum using the unprecedented capabilities of the James Webb Space Telescope (JWST). Specifically, the authors investigate whether jet morphology and kinematics vary systematically with protostellar mass and explore the complex interactions between circumstellar disks, magnetic fields, and the surrounding environment.

Methodology
The study utilizes data from the "Investigating Protostellar Accretion and Outflow Across the Mass Spectrum" (IPA) JWST Cycle 1 GO program (PID: 1802). The sample consists of three deeply embedded protostars: the low-mass Class 0 sources B335 ( $0.25 M_\odot$ ) and HOPS 153 ( $0.6 M_\odot$ ), and the intermediate-mass source HOPS 370 ( $2.5 M_\odot$ ).

Observations were conducted using the JWST NIRSpec and MIRI Integral Field Units (IFUs), covering wavelengths from 2.9 to 27.9 $\mu$ m. The analysis focuses on shock-ionized fine-structure lines, primarily [Fe II] (4.115, 5.340, 17.936, 24.519, and 25.988 $\mu$ m), as well as [Ni II], [Ar II], and [Ne II].

Morphology: The authors generated continuum-subtracted line maps to identify shocked knots and measure jet widths. Gaussian fitting was applied to cross-sections of the jets to determine centroids and Full Width at Half Maximum (FWHM), with corrections made for the Point Spread Function (PSF).
Kinematics: Position-velocity (P-V) diagrams were constructed to analyze radial velocities ( $V_{rad}$ ). Due to systematic uncertainties in absolute wavelength calibration, the authors assumed symmetric velocities between blue- and red-shifted lobes to derive jet velocities, a technique validated against previous work on IRAS 16253.
Proper Motion: For B335, an 8-month gap between NIRSpec and MIRI observations allowed for the measurement of the tangential velocity ( $V_{tan}$ ) of a specific shocked knot (B335-C), enabling a direct calculation of the total 3D jet velocity and inclination.

Key Results

Jet Morphology and Knots:
- [Fe II] emission traces the full extent of the inner 1000–2000 au of the jets, while other ions mostly trace isolated shocked knots.
- Knot sizes are consistent at $\sim$ 100 au across all three sources, independent of distance or protostellar mass.
- Asymmetry: Significant asymmetries exist between blue- and red-shifted lobes. B335 shows a deviation of $11.5^\circ \pm 0.3^\circ$ from $180^\circ$ bipolar symmetry in its launch angle. HOPS 153 and HOPS 370 show no such significant deviation.
- Wiggles and Bends: All three jets exhibit wiggles (angular deflections of $2.3^\circ$ to $13.8^\circ$ ) and larger bends ( $>40$ au amplitude). These features are asymmetric; for instance, B335's blue-shifted lobe shows sinusoidal wiggles while its red-shifted lobe does not.
Jet Widths and Opening Angles:
- Jet widths vary non-monotonically with distance from the protostar, often narrowing at the location of shocked knots and widening in the spaces between them.
- Opening angles for the inner jets (within 1500 au) range from $2.1^\circ$ to $<10.1^\circ$ . The red-shifted lobes of B335 and HOPS 370 show a clear trend of widening with distance, whereas other lobes are constrained only by upper limits.
Kinematics and Velocities:
- HOPS 153 and HOPS 370: These sources exhibit relatively constant velocities along their jet lengths. Corrected total velocities are $186 \pm 75$ km s $^{-1}$ for HOPS 153 and $150 \pm 26$ km s $^{-1}$ for HOPS 370.
- B335: The P-V diagram reveals a resolved slope in radial velocity with distance. Correcting for the disk inclination derived from scattered light modeling ( $83.5^\circ$ ) yields an unrealistically high total velocity ( $\sim$ 800 km s $^{-1}$ ). However, combining the measured $V_{rad}$ with the proper motion-derived $V_{tan}$ ( $140 \pm 28$ km s $^{-1}$ ) yields a realistic total velocity of $166 \pm 16$ km s $^{-1}$ and a jet inclination of $57^\circ \pm 2^\circ$ . This discrepancy suggests the jet inclination differs from the large-scale outflow cavity inclination, likely due to precession along the line of sight.
Variability and Accretion:
- B335 recently underwent a 10-year outburst. While the dynamical timescales of shocked knots do not strictly correlate with the outburst start/end dates, the frequency of knot ejections appears higher during the burst phase.

Significance and Claims
The paper demonstrates that JWST can resolve the detailed structure of protostellar jets within the inner 2000 au, revealing complex morphologies that challenge simple ballistic or symmetric models. The authors claim that the observed asymmetries in wiggles, bends, and launch angles strongly suggest complex interactions between circumstellar disks and magnetic fields, potentially involving disk warping or precession.

Specifically, the study highlights that:

Jet kinematics and morphology do not show obvious trends with protostellar mass within this small sample, likely due to the stochastic nature of accretion and environmental factors.
The discrepancy between the jet inclination and the outflow cavity inclination in B335 provides evidence for jet precession along the line of sight, a phenomenon that can be quantified through multi-epoch proper motion measurements.
The non-monotonic variation in jet width and the presence of asymmetric wiggles indicate that jet collimation and direction are dynamically evolving, possibly driven by magnetic torques or asymmetric infall from the envelope.

The authors conclude that while shock modeling of these ionic lines is required to fully determine mass, momentum, and energy flows (to be addressed in a companion paper), the current structural and kinematic data provide critical context for understanding the launch mechanisms of protostellar jets.