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Kinematics of young stellar objects in NGC 2024 based on infrared proper motions

Using multi-epoch near-infrared observations from ESO surveys, this study measured proper motions for thousands of sources in NGC 2024 to reveal that the cluster formed via rapid collapse with age-dependent kinematic segregation, showing that young embedded YSOs are beginning to decouple from their parent gas cloud.

Original authors: Alena Rottensteiner, Monika G. Petr-Gotzens, Stefan Meingast, João Alves, Emmanuel Bertin, Hervé Bouy, Martin Piecka, Sebastian Ratzenböck, Andrea Socci

Published 2026-02-18

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Original authors: Alena Rottensteiner, Monika G. Petr-Gotzens, Stefan Meingast, João Alves, Emmanuel Bertin, Hervé Bouy, Martin Piecka, Sebastian Ratzenböck, Andrea Socci

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

The Big Picture: Taking a "Time-Lapse" of Baby Stars

Imagine you are trying to understand how a family grows up. You have a photo album, but it's missing the most important part: the pictures of the babies. You have photos of the teenagers and adults, but the infants are hidden inside a thick, dark fog (dust) that your camera can't see through.

This is exactly the problem astronomers face with NGC 2024, a famous "stellar nursery" (a cloud where stars are born) in the Orion constellation. Most of the newest stars here are wrapped in so much dust that they are invisible to optical telescopes like the famous Gaia satellite. Gaia is like a super-powerful camera that sees the whole universe, but it only sees visible light. It misses the "babies" hiding in the dust.

The Solution: The team behind this paper decided to use infrared light (which is like heat vision). Infrared can pass through the dust fog, revealing the hidden baby stars. They combined data from three different ground-based surveys (like taking photos at different times over 10 years) to create a "time-lapse movie" of these stars. By seeing how they moved between photos, they could calculate their speed and direction.

The Main Characters: The "Classes" of Stars

To understand the story, you need to know the "ages" of the stars. The astronomers group them into classes, kind of like school grades:

Class I & Flat: The newborns. They are still wrapped in their birth cocoons (dust disks).
Class II: The toddlers. They have cleared some dust but still have a disk around them.
Class III: The teenagers/adults. They have cleared away most of the dust and look like normal stars.

The Detective Work: What Did They Find?

The team measured the movement of 6,769 stars, including 362 baby stars (YSOs) that had never had their movements measured before. Here is what the "time-lapse" revealed:

1. The "Inside-Out" Growing Up

Previously, astronomers noticed that the youngest stars were in the very center of the cluster, while older stars were further out.

The Old Theory: Maybe the center is a factory that keeps pumping out new stars, and the older ones just stay put.
The New Discovery: The movement data suggests the opposite. The older stars are actually moving outward, drifting away from the center. It's like a crowded party where the older guests are slowly drifting toward the exit, while the new arrivals are still huddled near the dance floor.

2. The "Monolithic" Collapse vs. The "Puzzle Piece" Theory

There was a big debate about how star clusters form:

Theory A (Hierarchical): Imagine a puzzle. Small groups of stars form in different corners, then slowly crash into each other and merge to make one big cluster. This takes a long time (millions of years).
Theory B (Monolithic): Imagine a giant balloon collapsing all at once. The whole cloud falls inward quickly, forming a tight cluster in less than a million years.

The Verdict: The data strongly supports Theory B.
The stars are moving in a way that suggests the whole cluster collapsed very quickly (in under 1 million years) and then "bounced" back out. There was no evidence of small puzzle pieces slowly merging. If there had been, we would see distinct groups of stars moving in different directions, but everyone is moving together in a chaotic mix.

3. The "Bouncy" Babies

Here is a surprising twist. Usually, you expect older stars to be moving faster and more chaotically because they've had more time to bump into each other.

The Finding: The youngest stars (Class I) were actually moving slightly faster and more wildly than the "toddlers" (Class flat).
The Analogy: Imagine a trampoline. If you jump on it, the first bounce is the highest and wildest. As you settle, you bounce less. The astronomers think the cluster just finished its "big collapse" and is currently in the "rebound" phase. The youngest stars were born right as the cluster bounced back, so they got a little extra "kick" of speed.

4. The Gas Connection: Are They Still Holding Hands?

Stars are born inside gas clouds. Eventually, they break free and drift away.

The Test: The team compared the speed of the stars to the speed of the gas cloud they were born in.
The Result: The youngest stars are still "holding hands" with the gas; they are moving at the same speed. The older stars (Class II and III) are starting to let go and drift away on their own. This tells us that the "kinematic decoupling" (breaking away from the gas) happens relatively quickly, within a few hundred thousand years.

Why Does This Matter?

This paper is a big deal because it's the first time we've been able to track the "babies" of a star cluster with the same precision as the "adults."

Before: We were guessing how clusters formed because we couldn't see the youngest members.
Now: We have a clear picture. NGC 2024 didn't grow slowly like a puzzle; it collapsed quickly like a balloon, bounced, and is now slowly expanding as the older stars drift away.

In a nutshell: The astronomers used "heat vision" to take a time-lapse of hidden baby stars. They discovered that this star cluster formed in a rapid, dramatic collapse rather than a slow, messy merger, and that the youngest stars are currently experiencing a "rebound" effect that makes them move a bit faster than their slightly older siblings.

1. Problem Statement

The Gap in Astrometry: While the Gaia mission has revolutionized the understanding of stellar kinematics, it is an optical instrument limited by dust extinction. Consequently, it cannot probe the crucial first few million years of star formation where Young Stellar Objects (YSOs) are deeply embedded in dense molecular clouds.
The Specific Challenge: NGC 2024 (the Flame Nebula) is a massive, active star-forming region (~420 pc away) with a high density of embedded YSOs. Previous attempts to determine cluster membership and kinematics using Gaia data were limited to only a fraction (30–60 stars) of the estimated cluster census because the embedded population is optically invisible.
Scientific Question: The formation history of star clusters (hierarchical merging vs. monolithic collapse) and the timescales for the kinematic decoupling of stars from their natal gas remain debated. These questions require precise, homogeneous kinematic data for the youngest, embedded populations, which has been lacking.

2. Methodology

The authors developed a ground-based, multi-epoch infrared astrometric pipeline to measure proper motions (PMs) with quality comparable to Gaia.

Data Sources: The study utilizes 11 epochs of near-infrared (NIR) observations from three ESO public surveys using the VISTA/VIRCAM instrument:
- SV (Science Verification): 1 epoch (Oct 2009) in $J, H, K_s$ .
- VHS (Vista Hemisphere Survey): 1 epoch (Nov 2014) in $J, K_s$ .
- VISIONS: 6 epochs (2017–2020) in $H$ -band.
- Baseline: A total temporal baseline of $\approx 10.3$ years.
Astrometric Reduction Workflow:
1. Pipeline Processing: Used vircampype (superior to CASU for VISIONS data) to calibrate images and extract source catalogs.
2. De-nebulization: Applied a custom algorithm to subtract the extended nebulosity of the Flame Nebula to improve source detection in crowded regions.
3. Hybrid Detection: To address crowding in the cluster core where vircampype failed, a custom PSF-fitting pipeline (IRAF/daophot) was applied to the central regions.
4. Catalog Merging: Sources were matched across epochs using STILTS. A "9-plus epoch" catalog was created for PM calculation to ensure robust sampling.
5. Proper Motion Calculation: Used position-error-weighted linear regression. Uncertainties were estimated using the jackknife technique (iteratively fitting $N-1$ points) rather than bootstrapping, as it provided more stable error estimates for limited datasets.
Target Sample:
- Total Sources: 6,769 stars.
- YSO Candidates: 362 bona fide YSOs (Class I to Class III) identified via cross-matching with the Roquette et al. (2025) catalog and infrared excess indices.
- Validation: Cross-validated against Gaia DR3 using 899 optically visible shared sources.

3. Key Contributions

First Homogeneous NIR PMs for NGC 2024: Provided the first high-quality, homogeneous proper motion measurements for the embedded population of NGC 2024.
Significant Sample Expansion:
- Achieved PMs for >56% of all known YSO candidates in the region.
- Provided a >13-fold increase in kinematic data for the youngest classes (Class I and flat-spectrum sources), which were previously almost entirely missing from kinematic studies.
Methodological Advancement: Demonstrated that ground-based NIR surveys can achieve astrometric precision ( $\sim 0.22$ mas yr $^{-1}$ ) comparable to Gaia for embedded populations, bridging a critical observational gap.

4. Key Results

Astrometric Quality:
- Mean PM uncertainty: 0.22 mas yr $^{-1}$ ( $\approx 0.44$ km s $^{-1}$ at 420 pc).
- Excellent agreement with Gaia for shared sources (mean offset near zero, FWHM $\approx 1.5$ mas yr $^{-1}$ ), confirming the reliability of the ground-based measurements.
Spatial Distribution & Age Segregation:
- Confirmed an "inside-out" age gradient: Younger sources (Class I/flat) are tightly concentrated in the cluster center, while older sources (Class II/III) are more spatially dispersed toward the outskirts.
- The spatial distribution of Class II/flat sources follows the N-S elongation of the Flame Nebula's dark lane, suggesting coupling to the gas structure, whereas Class III objects are more isotropic.
Kinematic Properties:
- Velocity Dispersion: Found a general trend of increasing velocity dispersion with evolutionary age (Class flat $\to$ Class III).
- Anomaly in Class I: Class I sources exhibited higher 1D and 2D velocity dispersions ( $\sim 3.55$ km s $^{-1}$ ) compared to Class flat sources ( $\sim 2.36$ km s $^{-1}$ ), contrary to the expectation that younger stars should be colder.
- Statistical Uniformity: Despite visual substructures, statistical tests (KS, permutation, energy-distance) revealed that all YSO classes share a common velocity distribution within $\le 2$ km s $^{-1}$ . No significant kinematic subclusters were detected.
Star-Gas Coupling:
- Radial velocities of YSOs generally align with gas velocities traced by $^{12}$ CO, HNC, and HCN.
- Class flat and younger objects show tight correlation with gas.
- Some Class II and III objects show increased scatter, indicating the onset of kinematic decoupling from the gas.
- Weak correlation with N $_2$ H $^+$ , consistent with rapid dispersal of dense gas by feedback from massive stars.

5. Significance and Conclusions

Cluster Formation Scenario: The results strongly favor a rapid collapse scenario (< 1 Myr) over prolonged hierarchical assembly.
- The lack of kinematic substructure and the common velocity distribution suggest that any initial subclusters merged and dynamically mixed very quickly.
- The observed age gradient is likely driven by the outward movement of older stars (via N-body interactions/dynamical heating) rather than the infall of new stars.
Dynamical State: The higher velocity dispersion of Class I objects suggests they may have formed during a "rebound" phase immediately following the initial cluster collapse, placing them in a supervirial state.
Future Implications: The study validates that NIR proper motions can effectively trace the 3D kinematics of young clusters. It highlights the need for more radial velocity measurements for Class I objects to fully constrain the third velocity dimension and understand the early dynamical evolution of star-forming regions.

In summary, this paper provides a breakthrough in observing the "dark" phase of star formation, using infrared astrometry to reveal that NGC 2024 formed rapidly and is currently undergoing dynamical relaxation, with the youngest stars retaining a "memory" of the initial collapse dynamics.