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The accretion-ejection connection in the asymmetric Th 28 jet revealed by MUSE-NFM

Using high-resolution VLT/MUSE observations, this study reveals that the Classical T Tauri star Th 28 exhibits frequent episodic jet ejections linked to a rising mass accretion rate, maintaining a constant mass outflow-to-accretion ratio and supporting momentum conservation across its asymmetric jet lobes.

Original authors: A. Murphy, E. T. Whelan, F. Bacciotti, A. Kirwan, D. Coffey, M. Birney, J. Eislöffel, H. Takami

Published 2026-03-25

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CC BY 4.0

Original authors: A. Murphy, E. T. Whelan, F. Bacciotti, A. Kirwan, D. Coffey, M. Birney, J. Eislöffel, H. Takami

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 young star, Th 28, as a chaotic toddler in the cosmic nursery. It's surrounded by a swirling disk of gas and dust (its "protoplanetary disk"), and like many toddlers, it has a lot of energy to burn. Sometimes, it swallows a mouthful of this cosmic soup (accretion), and sometimes, it spits it back out in powerful, high-speed jets (ejection).

For a long time, astronomers knew these two actions—eating and spitting—were connected, but they didn't know exactly how they were linked, especially when the "spit" looked different on the left side compared to the right side.

This paper is like upgrading from a blurry, old security camera to a high-definition, super-zoom lens to watch Th 28 in real-time. Here is the story of what they found, explained simply:

1. The New "Super-Telescope" Glasses

Previously, astronomers looked at Th 28 with a telescope that was a bit like looking through a foggy window. They could see the general shape of the jets, but the details were blurry. The red jet (pointing one way) looked like a string of tight knots, while the blue jet (pointing the other way) looked like a diffuse, messy cloud.

The researchers used a new mode on the Very Large Telescope called MUSE-NFM, combined with Adaptive Optics (which is like a pair of glasses that corrects for the twinkling of the Earth's atmosphere). This gave them a view four times sharper than before. Suddenly, the "messy cloud" on the blue side resolved into distinct structures, and they could see the inner workings of the jets right next to the star.

2. The "Popcorn" Effect: Frequent Ejections

One of the biggest surprises was how often the star was "spitting."

Old View: Astronomers thought Th 28 ejected a knot of gas every 10 to 15 years. It was like a slow, rhythmic heartbeat.
New View: With the sharper lens, they saw a whole string of new knots that had been ejected in just the last 3 to 6 years.

The Analogy: Imagine you thought a popcorn machine popped one kernel every 15 minutes. But when you got a closer look, you realized it was actually popping a kernel every 3 minutes. The star is much more active and "snappy" than we thought. This suggests the star is having frequent, small bursts of eating (accretion) that trigger these rapid spit-outs.

3. The Asymmetry Mystery: Why is one side different?

Th 28's jets are famously lopsided:

The Red Jet: Slow, compact, and looks like a string of tight beads.
The Blue Jet: Fast, wide, and looks like a broad, expanding fan.

The Analogy: Think of two garden hoses spraying water.

The Red Hose is narrow and steady, shooting a tight stream.
The Blue Hose is wide open, spraying a misty, fast fan.

For years, scientists wondered: Is the star actually shooting out less water on the blue side, or is it just that the water is spreading out faster?

By measuring the density and speed of the gas, the team found that the amount of mass being ejected is actually roughly the same on both sides. The blue side just shoots the gas much faster and spreads it out wider (like a mist), while the red side shoots it slower and keeps it tight (like a solid stream).

This is a crucial discovery because it suggests that momentum is conserved. The star is balancing the "kick" it gets from spitting out gas. If it shoots a little bit of gas very fast, it balances out shooting a lot of gas slowly.

4. The Connection: Eating and Spitting

The most important finding is the link between how much the star eats and how much it spits.

Between 2014 and 2023, the star's "eating rate" (accretion) doubled.
Coincidentally, the "spitting rate" (outflow) in the red jet also doubled.

The Analogy: It's like a person who eats a double portion of lunch and immediately feels the need to run a double-distance marathon. The star's "digestion" (accretion) and its "exercise" (ejection) are perfectly synchronized. When the star gets a bigger meal, it immediately pushes out a bigger jet to balance the energy.

5. The "Time Travel" Problem

There is a slight delay between when the star eats and when the knot appears in the jet. The knot has to travel from the star's surface out into space.

The team calculated that a knot launched from the inner disk would take a few years to reach the spot where they see the new knots.
This timing matches up perfectly with the increase in the star's eating rate. It's like seeing a wave hit the shore a few seconds after you see a rock thrown into the water.

The Bottom Line

This paper tells us that Th 28 is a highly active, "fidgety" star that is constantly adjusting its balance.

It eats and spits in sync: When it eats more, it spits more.
It's faster than we thought: It's spitting out knots every few years, not every decade.
It balances the books: Even though the two jets look totally different (one fast and wide, one slow and tight), they are actually ejecting the same amount of "stuff," just in different ways to keep the star's momentum balanced.

This discovery makes Th 28 a perfect "laboratory" for astronomers to study how stars grow and how they interact with the disks that will eventually become new solar systems. By watching it closely, we can understand the fundamental rules of how stars and planets are born.

1. Problem and Scientific Context

Young Stellar Objects (YSOs), specifically Classical T Tauri Stars (CTTS), launch bipolar jets closely linked to mass accretion from their protoplanetary disks. While Magnetohydrodynamic (MHD) models suggest jets are launched from the inner disk (1–3 au), the precise connection between episodic mass accretion and mass ejection remains poorly understood. Key open questions include:

Temporal Correlation: Do knot ejections in jets directly trace variable accretion bursts?
Asymmetry: Many jets exhibit significant asymmetries in morphology, velocity, and gas properties between the red-shifted and blue-shifted lobes. It is unclear if these asymmetries are intrinsic to the launching mechanism or extrinsic (caused by interaction with the circumstellar medium).
Mass Balance: Is the mass outflow rate ( $\dot{M}_{out}$ ) symmetric, or does the faster lobe carry less mass to conserve linear momentum?

The target, Th 28 (Sz 102), is a CTTS in the Lupus III cloud driving a highly asymmetric bipolar jet (HH 228). Previous observations (e.g., VLT/MUSE Wide Field Mode) were limited by seeing ( $\sim0.9''$ ), preventing detailed study of the inner jet base ( $<1''$ ) where contamination between lobes obscured the blue-shifted side and unresolved knots prevented accurate kinematic analysis.

2. Methodology

The authors utilized new VLT/MUSE Narrow Field Mode (NFM) observations combined with Adaptive Optics (AO) to achieve high spatial and spectral resolution.

Observations: Data were collected over three nights (June 2023, January 2024) covering two fields of view (FoV A and B) centered on the red- and blue-shifted lobes, respectively.
Resolution: The AO correction and subsequent image deconvolution (using a modified Richardson-Lucy algorithm) achieved an effective angular resolution of $0.12''$ (approx. 20 au at 160 pc), a factor of $>4$ improvement over previous seeing-limited data.
Data Analysis:
- Morphology: Deconvolved spectro-images in H $\alpha$ and forbidden emission lines (FELs) were used to identify knot structures and measure proper motions.
- Diagnostics: The Bacciotti & Eislöffel (BE) method was applied to optical FEL ratios ([S II], [N II], [O I]) to derive electron density ( $n_e$ ), ionization fraction ( $x_e$ ), electron temperature ( $T_e$ ), and shock velocities ( $v_{shock}$ ).
- Extinction: Extinction ( $A_V$ ) was estimated using the H $\alpha$ /H $\beta$ ratio, accounting for shock velocity effects, with a conservative value of $A_V \approx 1.26$ mag applied to the source region.
- Rates: Mass accretion rates ( $\dot{M}_{acc}$ ) were derived from accretion tracer lines (corrected for obscuration), and mass outflow rates ( $\dot{M}_{out}$ ) were calculated using three independent methods: (1) density/velocity/area, (2) FEL luminosities, and (3) shock front modeling.

3. Key Contributions

Unprecedented Resolution: First high-resolution ( $0.12''$ ) view of the inner jet base ( $<200$ au) of Th 28, resolving the blue-shifted lobe previously contaminated by the brighter red-shifted lobe.
Discovery of New Knots: Identification of multiple new knots ejected within the last decade, revealing a much more frequent ejection cycle than previously thought.
Quantifying the Accretion-Ejection Link: Direct comparison of $\dot{M}_{acc}$ and $\dot{M}_{out}$ across two epochs (2014 vs. 2023) to test the conservation of mass and momentum in an asymmetric system.
Asymmetry Analysis: Detailed characterization of the physical conditions (density, temperature, ionization) in both lobes to determine if the asymmetry is intrinsic or extrinsic.

4. Key Results

A. Morphology and Kinematics

Knot Ejection Frequency: The study identified new knots in both lobes, including a knot at $\sim -1''$ (blue) and several at $+1.2''$ to $+3.5''$ (red). The separation suggests an ejection timescale of 3–6 years, significantly more frequent than the previously estimated 10–15 years.
Proper Motions: The red-shifted jet moves at $\sim 0.34''$ yr $^{-1}$ , while the blue-shifted jet is faster at $\sim 0.47''$ yr $^{-1}$ .
Jet Width: The red-shifted jet has a stable opening angle of $\sim 20^\circ$ . The blue-shifted jet shows a discrepancy: $\sim 25^\circ$ in H $\alpha$ (tracing broad bow shocks) but only $6-8^\circ$ in [S II] (tracing the core), suggesting the asymmetry is driven by broad shock structures rather than intrinsic collimation differences.

B. Physical Conditions (Gas Diagnostics)

Red-Shifted Jet: Lower shock velocities ( $v_{shock} \approx 40-70$ km s $^{-1}$ ), lower electron temperature ( $T_e \approx 1.9 \times 10^4$ K), and moderate density.
Blue-Shifted Jet: Significantly higher excitation. Shock velocities exceed 90 km s $^{-1}$ (up to $\sim 400$ km s $^{-1}$ based on line profile widths). It exhibits higher $T_e$ ( $\sim 2.7 \times 10^4$ K) and higher electron density ( $n_e \approx 1.5 \times 10^4$ cm $^{-3}$ ).
Intrinsic Asymmetry: The high excitation in the blue lobe is intrinsic, not caused by extinction or interaction with a dense medium, as confirmed by the lack of increased extinction in [Fe II] lines.

C. Accretion and Outflow Rates

Mass Accretion ( $\dot{M}_{acc}$ ): The accretion rate has doubled between 2014 and 2023, rising from $\sim 1.3 \times 10^{-7}$ to $2.11 \times 10^{-7} M_\odot$ yr $^{-1}$ .
Mass Outflow ( $\dot{M}_{out}$ ):
- Close to the jet base ( $<1''$ ), $\dot{M}_{out}$ in both lobes converges to $1-3 \times 10^{-8} M_\odot$ yr $^{-1}$ .
- The ratio $\dot{M}_{out} / \dot{M}_{acc}$ is approximately 0.1, consistent with MHD jet launching models.
- The increase in $\dot{M}_{out}$ in the red-shifted lobe matches the factor-of-2 increase in $\dot{M}_{acc}$ .
Momentum Conservation: While the mass outflow rates are similar near the base, the faster blue-shifted lobe carries less mass. The ratio of mass outflow rates aligns with the ratio of jet velocities ( $v_{red}/v_{blue}$ ), suggesting that linear momentum is conserved between the two lobes rather than mass.

5. Significance and Conclusions

Validation of the Accretion-Ejection Connection: The simultaneous doubling of accretion and outflow rates confirms that episodic accretion drives episodic ejection. The time delay between knot launching and accretion bursts is consistent with MHD wind models launching from $\sim 0.1-1$ au.
Nature of Asymmetry: The Th 28 jet demonstrates that strong morphological and kinematic asymmetries can exist even when the underlying mass ejection is symmetric. The blue-shifted lobe's "faintness" and broad appearance are due to high-velocity, broad bow shocks spreading energy over a larger area, not a lack of mass ejection.
Future Prospects: Th 28 is identified as a prime target for monitoring. The frequent (3–6 year) ejection cycle allows for rapid testing of the accretion-ejection link. Future work requires high-resolution spectroscopy of high-ionization lines (e.g., [Ne III]) to better constrain the density and ionization fraction in the highly excited blue-shifted lobe.

In summary, this study provides the most detailed view to date of the inner workings of an asymmetric T Tauri jet, confirming that mass loss is a constant fraction of mass accretion and that momentum, rather than mass, is the conserved quantity across asymmetric jet lobes.