🔭 astrophysics

The curved jet in the young star FN Tau

This paper reports the discovery of a curved microjet and four Herbig-Haro objects (HH 1267) near the young star FN Tau, attributing the jet's non-linear path to the precession of the inner protoplanetary disk and linking its onset to a stellar outburst approximately 60 years ago.

Original authors: M. A. Burlak, A. V. Dodin, A. V. Zharova, N. P. Ikonnikova, V. A. Kiryukhina, S. A. Lamzin, D. A. Lashin, B. S. Safonov

Published 2026-04-23

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Original authors: M. A. Burlak, A. V. Dodin, A. V. Zharova, N. P. Ikonnikova, V. A. Kiryukhina, S. A. Lamzin, D. A. Lashin, B. S. Safonov

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 named FN Tau as a toddler in a cosmic nursery. It's only about a million years old (which is basically a newborn in the universe's timeline) and is still surrounded by a giant, swirling disk of gas and dust—the "protoplanetary disk"—where future planets are being born.

Usually, these young stars are calm, but FN Tau is a bit of a drama queen. It has a history of throwing massive tantrums, suddenly brightening up like a lightbulb being turned on full blast.

Here is the story of what astronomers found when they looked closely at this star, explained simply:

1. The Cosmic Firehose (The Jet)

When young stars like FN Tau are growing, they don't just eat up gas from their disk; they also spit some of it back out. Think of it like a child who eats a lot of food but also has a very messy mouth.

In FN Tau's case, this "spitting" isn't just a messy dribble; it's a high-speed, laser-focused beam of gas shooting out into space. Astronomers call these jets. Usually, these jets shoot out in a straight line, like a laser pointer. But FN Tau is weird. Its jet isn't straight; it's curved.

2. The "Squiggly" Path

Imagine you are holding a garden hose and spraying water. If you stand perfectly still, the water goes straight. But if you start shaking your hand or spinning around, the water stream starts to wiggle or spiral.

That's exactly what is happening with FN Tau. The jet is curving because the "nozzle" (the inner part of the star's disk) is wobbling or spinning in a circle. This wobble is called precession. It's similar to how a spinning top wobbles as it slows down, tracing a circle with its tip.

3. The Cosmic Footprints (Herbig-Haro Objects)

As this curved jet blasts out, it slams into the cold, dark gas floating in space. When the fast jet hits the slow gas, it creates a shockwave, lighting up like a neon sign. These glowing knots of light are called Herbig-Haro (HH) objects.

Think of them as the "footprints" or "smoke rings" left behind by the jet. The astronomers found four of these footprints (named A, B, C, and D). By looking at where they are and how they are moving, they realized the jet had to be curving to hit them in that specific pattern.

4. The Mystery of the Missing Twin

Usually, these jets come in pairs, shooting out in opposite directions (like a double-sided firehose). One side is the "jet," and the other is the "counter-jet."

However, with FN Tau, the astronomers could only see the main jet clearly. The other side was hidden. Why? Because we are looking at the star almost straight on, like looking down the barrel of a gun. The star's disk is tilted so flat relative to us that it's blocking our view of the other side, like a pizza crust hiding the toppings underneath.

5. The 60-Year-Old Tantrum

The paper connects the dots between the star's past and its present. About 60 years ago, FN Tau had a massive outburst (a tantrum) that lasted a few months. The astronomers believe this is when the "microjet" (the very tip of the jet we see now) was launched.

It's like a volcano that erupted 60 years ago. The lava (the jet) is still flowing, and we are just now seeing the smoke rings (the HH objects) it created as it traveled through space.

6. Is There a Secret Partner?

Why is the jet wobbling in the first place? The leading theory is that FN Tau might have a secret partner—a brown dwarf (a "failed star") or a giant planet orbiting it.

Imagine a figure skater spinning. If they hold a heavy weight in one hand, their spin gets wobbly. If a giant planet or brown dwarf is orbiting FN Tau, its gravity could be tugging on the inner disk, causing the disk to wobble and the jet to curve.

The astronomers tried to find this partner using powerful telescopes and special cameras, but they couldn't see it yet. It's likely too small, too dim, or too close to the bright star to be spotted. But the "wobble" in the jet is strong evidence that something is there.

The Big Picture

This paper is a detective story. By looking at the curved path of a gas jet and the glowing footprints it left behind, astronomers figured out:

FN Tau is shooting out a curved jet.
The curve is caused by the star's inner disk wobbling (precessing).
This wobble was likely triggered by a hidden companion (a planet or brown dwarf).
The jet started about 60 years ago, right after the star had a massive brightening event.

It's a beautiful example of how looking at the "scars" and "footprints" in space can tell us the history of a star's life, even when we can't see the star's family members directly.

1. Problem Statement

The study focuses on the young Classical T Tauri star (CTTS) FN Tau, a low-mass star ( $M \approx 0.25 M_{\odot}$ ) located in the Taurus molecular cloud at a distance of $\approx 130$ pc. While previous studies had identified spectral signatures suggesting a high-velocity outflow (jet) in FN Tau, the spatial position of this emission coincided with the star itself within observational uncertainties, preventing direct confirmation of a collimated jet. Furthermore, the nature of the outflow's morphology (specifically whether it is straight or curved) and its connection to the star's historical photometric variability remained unresolved. The authors aimed to:

Directly detect and characterize the jet and counter-jet of FN Tau.
Identify associated Herbig–Haro (HH) objects.
Determine the cause of the jet's non-rectilinear (curved) trajectory.
Investigate the link between the jet's onset and the star's historical outbursts.
Assess the possibility of a binary companion driving the observed dynamics.

2. Methodology

The authors employed a multi-faceted observational approach combining historical data analysis with modern high-resolution imaging and spectroscopy:

Historical Photometry: Constructed a light curve spanning from 1951 to 2025 using $\sim$ 400 photographic plates from the Sternberg Astronomical Institute (SAI MSU) and modern CCD observations (UBV $_R$ I $_C$ ) from the Caucasian Mountain Observatory (CMO).
Imaging: Obtained narrowband images using the 2.5-m telescope at CMO with a 4K $\times$ 4K CCD. Filters included H $\alpha$ (656 nm) and a continuum filter (643 nm) to isolate emission nebulae.
Spectroscopy: Utilized the Transient Double-beam Spectrograph (TDS) on the 2.5-m telescope. Crucially, they acquired spectra with varying slit position angles (PA) to spatially resolve the high-velocity emission components relative to the stellar position.
Polarimetry & Interferometry: Conducted speckle polarimetry and speckle interferometry in the $R_c$ and $I_c$ bands to search for close companions and measure polarization degrees.
Kinematic Modeling: Developed a ballistic model for a precessing jet to fit the observed positions and radial velocities of the HH objects and the microjet. The model accounted for jet deceleration due to interaction with the interstellar medium.

3. Key Contributions and Results

A. Discovery of HH 1267 and the Microjet

HH Objects: The authors identified four Herbig–Haro objects (labeled A, B, C, D) surrounding FN Tau. Based on their positions and radial velocities, they were designated as HH 1267.
- Object A: Located on the counter-jet side (receding, $V_r \approx +98$ km s $^{-1}$ ).
- Objects B, C, D: Located on the jet side (approaching, $V_r \approx -101$ to $-24$ km s $^{-1}$ ).
Microjet Detection: By analyzing the spatial offset of the [S II] $\lambda$ $λ$ 6731 emission centroid relative to the star as a function of slit angle, the authors detected a microjet.
- Offset: The centroid is displaced by $\approx 0.22''$ ( $\sim 30$ au) from the star.
- Velocity: The microjet has a radial velocity of $V_r \approx -91$ km s $^{-1}$ .
- Confirmation: This provides the first direct spatial confirmation of the jet previously only inferred from spectral line profiles.

B. Jet Morphology and Precession

Curvature: The chain of HH objects (B, C, D) does not form a straight line; the position angle decreases with distance from the star, indicating a clockwise curvature.
Precession Model: The authors ruled out simple interaction with ambient density inhomogeneities as the primary cause. Instead, they fitted a precession model where the inner accretion disk wobbles.
- Best-fit parameters: Disk inclination $i \approx 14^\circ$ , precession cone half-angle $\beta \approx 28^\circ$ , initial jet velocity $V^0_j \approx 110$ km s $^{-1}$ .
- Timescale: The dynamical age of the microjet is estimated at $\sim 10$ years, while the HH objects range from $10^3$ to $10^4$ years. The precession period is constrained to be $> 5 \times 10^4$ years.

C. Connection to Photometric Variability

Outburst History: The historical light curve reveals significant brightening events in the 1960s, with amplitude $\Delta m_{pg} \sim 2^m$ lasting several months.
Correlation: The dynamical age of the microjet ( $\sim 10$ years) combined with the travel time suggests the jet was launched during the outburst activity observed approximately 60 years ago. This supports the hypothesis that accretion rate enhancements (outbursts) trigger jet ejection.

D. Search for a Binary Companion

Motivation: Jet precession is often induced by a companion on an eccentric orbit.
Search Results:
- Speckle Interferometry: No companions detected down to contrast limits at separations of $0.08'' - 1.5''$ .
- Radial Velocity: Analysis of 25 high-resolution spectra showed a scatter of only $\sigma_V \approx 0.2$ km s $^{-1}$ , consistent with stellar spots rather than a companion.
- Astrometry: The Gaia RUWE value (1.32) is low, suggesting no significant astrometric wobble.
Conclusion: If a companion exists, it must be a low-mass object (Brown Dwarf or Giant Planet) that is currently undetectable by current methods.

4. Significance

Direct Confirmation: The paper provides the first spatially resolved detection of the jet in FN Tau, confirming the link between spectral high-velocity components and physical outflows.
Jet Precession Mechanism: It offers strong observational evidence that the curved morphology of young stellar jets can be explained by the precession of the inner accretion disk, even in systems viewed nearly pole-on ( $i < 20^\circ$ ).
Accretion-Jet Link: The study reinforces the causal link between large-amplitude photometric outbursts (accretion events) and the launching of microjets in CTTSs.
Asymmetry: It highlights the common asymmetry in bipolar outflows (three HH objects on the jet side vs. one on the counter-jet side), likely due to the obscuration of the counter-jet by the nearly face-on protoplanetary disk.
Binary Constraints: It places stringent limits on the presence of a massive companion, suggesting that if precession is driven by a companion, it is likely a substellar object, which has implications for planet formation theories in young disks.

In summary, this work characterizes FN Tau as a dynamic system where a precessing inner disk drives a curved jet, triggered by episodic accretion, while ruling out a stellar-mass binary companion as the driver.