🔭 astrophysics

Probing the kinematics of FU Orionis objects through high-resolution near-infrared spectroscopy

This study utilizes high-resolution near-infrared spectroscopy of 15 FU Orionis objects to demonstrate that while some exhibit double-peaked Keplerian line profiles in the K-band, the characteristic signature is often obscured by molecular blending, disk winds, or infalling material, and the CO lines in the M-band likely originate from a different source than those in the K-band.

Original authors: Ellen Lee, Michael Connelley

Published 2026-05-05

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Ellen Lee, Michael Connelley

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 having a massive, decades-long tantrum. It suddenly swells in brightness, swallowing up its own disk of gas and dust. These stars are called FU Orionis objects (or FUors for short). Astronomers have long believed these stars are surrounded by a spinning, flat disk of material, much like a vinyl record spinning on a turntable. If you could "hear" this spinning disk, the physics of a spinning record suggests the sound should have a specific, double-humped shape (like a "W").

However, when astronomers look at these stars through powerful telescopes, they often don't see that perfect "W" shape. Instead, the lines look like simple hills or boxes. This paper by Ellen Lee and Michael Connelley is like a detective story where the team tries to figure out: Is the spinning disk really there, or is our "ear" (the telescope) being tricked?

Here is what they found, explained simply:

The Detective Work: Listening to the Stars

The team used a high-resolution camera on a telescope in Hawaii (the NASA Infrared Telescope Facility) to take "snapshots" of 15 of these outbursting stars. They looked at the stars in three different "colors" of light:

J-band: A bit like looking at the star through a slightly foggy window.
K-band: A clearer view, where the star's "voice" (specifically Carbon Monoxide gas) is loudest.
M-band: A very different view, where the star looks like it's wearing a different outfit entirely.

They tried to fit mathematical shapes (like smooth hills or double-peaked "W"s) to the data to see if the stars were spinning like a Keplerian disk (a disk where the inner parts spin faster than the outer parts).

The Big Findings

1. The "Double-Hump" is Hard to See
In five of the stars, they finally saw the clear "W" shape (the double-peaked profile) that proves the disk is spinning. But in the others, the shape was hidden. Why?

The "Crowded Room" Effect: The star's light is mixed with so many other chemical "voices" (molecular features) that the main signal gets blended together, making the double-hump look like a single, lumpy hill.
The "Wind" Effect: Sometimes, gas blowing away from the star (a disk wind) or falling onto it acts like a curtain, hiding one side of the "W" and making it look like a single peak.

2. The "Foggy Window" Problem (J-band vs. K-band)
The team expected that if they looked at the star in different colors, the "speed" of the spinning would look different (slower in redder light, faster in bluer light), just like how the edge of a spinning record looks different depending on where you stand.

The Result: They found that the speed measurements in the J-band and K-band were very similar. The linewidth didn't shrink as much as the simple "spinning record" model predicted.
The Takeaway: This doesn't mean the disk isn't there. It just means the simple model is too basic. The disk is complex, and the "fog" (other gases) makes it hard to see the expected changes.

3. The "Different Outfit" (M-band)
When they looked at the M-band (the longest wavelength), the story changed completely.

The Result: The lines in the M-band looked nothing like the lines in the J or K bands. They didn't show the spinning disk pattern at all.
The Takeaway: The gas glowing in the M-band isn't coming from the spinning disk. It's likely coming from a different place, perhaps hot dust closer to the star or a different layer of the atmosphere entirely. It's like the star is singing a completely different song in that color.

4. The "Speedy" Exceptions
They noticed that the stars spinning the fastest were the ones most likely to show the clear "W" shape. If a star is spinning slowly, the "W" gets squished together by turbulence (chaos in the gas) until it looks like a single hill. It's like trying to see the individual spokes of a bicycle wheel: if it's spinning slowly, you see them; if it's spinning super fast, they blur into a solid circle.

The Odd One Out: PR Ori B

One star, PR Ori B, was so weird that the team had to put it in a separate appendix. Its "voice" was so distorted that their models couldn't fit it at all. It spins so fast and looks so different that it might not even be a FU Orionis star, but a different type of young star entirely.

The Conclusion

The paper concludes that the spinning disk is likely still there, even if we can't always see the perfect "W" shape. The reason we don't see it is that the star's atmosphere is messy, crowded with other gases, and sometimes obscured by winds.

Think of it like trying to hear a specific instrument in a symphony orchestra. If the room is too loud, or if other instruments are playing the same notes, you might miss the unique sound of the violin. The violin (the spinning disk) is there, but you need to know how to filter out the noise to hear it clearly. The team's work helps us understand how to tune our ears to hear the true shape of these young, outbursting stars.

Technical Summary: Probing the kinematics of FU Orionis objects through high-resolution near-infrared spectroscopy

Problem Statement
FU Orionis (FUor) objects are rare, outbursting young stars characterized by a ~5 magnitude brightening lasting decades. While the standard theoretical model describes FUors as steady-state Keplerian accretion disks, a key observational signature of such disks—the characteristic double-peaked line profile—is not readily apparent in most near-infrared (NIR) spectra of these objects. Instead, NIR CO lines often appear Gaussian or box-shaped. Furthermore, sophisticated accretion disk models do not always fully reproduce the observed line structures in the NIR. This paper addresses the discrepancy between the expected Keplerian kinematics and the observed NIR line profiles of FUors, specifically investigating whether the lack of double-peaked features is due to the absence of a disk or other complicating factors such as blending, winds, or temperature gradients.

Methodology
The authors conducted a high-resolution spectroscopic survey of 15 FUors and FUor-like objects using the iSHELL spectrograph on the NASA Infrared Telescope Facility (IRTF). Observations were obtained between 2017 and 2024, covering the J-band (1.26–1.36 µm), K-band (2.09–2.38 µm), and M-band (4.52–5.25 µm) with a resolving power of $R \sim 50,000$ .

The analysis employed the following steps:

Template Selection: Intrinsic spectra were approximated using BT-Settl (CIFIST) cool atmosphere models ( $T_{\text{eff}} = 2000\text{--}3400$ K, $\log g = 4.5\text{--}5.5$ dex), justified by the spectral similarity between FUors and brown dwarfs at lower resolutions.
Convolution and Fitting: Observed spectra were modeled by convolving template spectra with various line profiles using the Markov Chain Monte Carlo (MCMC) technique (via the emcee package).
Line Profile Parameterization: The authors tested linear combinations of Gaussians (single, double, and triple peaks) and a "double-U" Keplerian profile (smoothed by a Gaussian to account for turbulence).
Model Averaging: To account for uncertainties in stellar parameters, Bayesian Model Averaging (BMA) was used to combine results from a grid of template spectra, weighted by their likelihoods ( $\chi^2$ ).
M-band Analysis: Due to the morphological differences in M-band spectra, the authors did not use convolution. Instead, they averaged ten uncontaminated $^{12}\text{CO}$ (1-0) lines and fitted them directly with a sum of Gaussians.

Key Results

Line Profile Morphology: Five of the 15 K-band targets exhibited double-peaked line profiles consistent with a Keplerian disk. The remaining objects were well-described by single-peaked profiles or required complex multi-component fits.
Correlation with $v \sin i$ : Double-peaked profiles were more frequently observed in objects with higher projected rotational velocities ( $v \sin i$ ). Objects with lower $v \sin i$ (typically $< 30\text{--}40$ km s $^{-1}$ ) showed blended peaks, likely due to turbulent broadening.
Wavelength Dependence: For the eight targets observed in both J- and K-bands, the line profiles were generally well-correlated. However, contrary to the prediction of a basic Keplerian disk model (which suggests narrower lines at longer wavelengths due to cooler, slower outer disk regions), the authors found no clear trend of decreasing linewidth with increasing wavelength.
M-band Discrepancy: CO lines in the M-band were morphologically distinct from their J- and K-band counterparts. They lacked the correlation seen in shorter wavelengths and did not exhibit Keplerian profiles, suggesting a different physical origin, possibly unrelated to the accretion disk.
Stability: Multi-epoch observations (spanning 3–4 years) indicated that the CO line structures are relatively stable, unlike the variable wind signatures seen in optical lines (e.g., H $\alpha$ ).
Outliers: PR Ori B was excluded from the main analysis as its spectrum could not be well-fit by the models, exhibiting an anomalously high $v \sin i$ and a bandhead structure more indicative of a Herbig Ae/Be star. V1057 Cyg was also excluded due to atypical spectral variability.

Significance and Claims
The paper concludes that the absence of a clearly visible double-peaked profile in NIR CO lines does not necessarily invalidate the Keplerian disk model for FUors. Instead, the authors propose that the line profiles are obscured or altered by several factors:

Blending: Extraneous molecular features and absorption from disk winds or infalling material can blend with CO lines, suppressing the double-peaked structure or creating asymmetries.
Turbulence: Significant Gaussian broadening ( $\sim 12\text{--}17$ km s $^{-1}$ ) is required to smooth Keplerian profiles, which can merge the peaks in objects with moderate $v \sin i$ .
Complex Origins: The lack of wavelength dependence in linewidth and the distinct morphology of M-band lines suggest that the NIR spectra are influenced by factors beyond a simple, self-luminous disk, such as circumstellar absorption or different emission regions.

The study emphasizes that while the basic Keplerian model provides a qualitative fit for some targets, a complete understanding of FUor kinematics requires models that incorporate additional components like disk winds, turbulence, and non-disk emission sources. The results align with previous optical studies noting subtle or absent double-peaked structures, reinforcing the complexity of modeling FUor spectra across the electromagnetic spectrum.

The Detective Work: Listening to the Stars

The Big Findings

The Odd One Out: PR Ori B

The Conclusion

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