🔭 astrophysics

Southern massive stars at high angular resolution: WR 25 is a massive hierarchical triple system

This study presents the first VLTI/PIONIER interferometric resolution of WR 25 as a hierarchical triple system, confirming the presence of an O7 tertiary companion and providing precise dynamical masses for all three components to serve as a benchmark for stellar evolution models.

Original authors: K. Deshmukh, L. Mahy, H. Sana, A. J. Frost, E. Gosset, C. Lanthermann, J. -B. LeBouquin, D. Pauli, T. Shenar

Published 2026-07-15

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Original authors: K. Deshmukh, L. Mahy, H. Sana, A. J. Frost, E. Gosset, C. Lanthermann, J. -B. LeBouquin, D. Pauli, T. Shenar

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 cosmic dance floor in the Carina nebula, where the most massive, energetic stars in our galaxy are cutting a rug. For years, astronomers knew about a famous pair of dancers here, a system called WR 25. They knew it was a "binary" system—a massive Wolf-Rayet star (the WN6ha) and a hot O-type companion (the O5) locked in a tight, 208-day waltz. But there was a rumor, a whisper in the spectroscopic data, that a third dancer was crashing the party, hiding in the shadows.

In this new study, researchers finally turned on the high-powered spotlights to see the whole scene. Using the VLTI/PIONIER instrument, which acts like a giant, super-sharp camera made of four telescopes working together, they didn't just guess; they actually saw all three stars for the first time.

The Big Reveal: A Triple Act
The team confirmed that WR 25 isn't just a couple; it's a hierarchical triple system. Think of it like a tiny solar system within a star cluster.

The Inner Pair: The two main stars are incredibly close, separated by a projected distance of 1.68 ± 0.02 milliarcseconds (mas). To put that in perspective, if the Earth were a marble, this separation is like the width of a human hair seen from a few kilometers away. Because the observation happened at a perfect moment in their orbit—when they were crossing the "line of nodes" (imagine them crossing the stage exactly side-by-side from our view)—the astronomers could calculate their true physical separation: 3.90 ± 0.21 au (astronomical units).
The Intruder: Then, there's the third star, the "tertiary." It's hanging out much further away, at a distance of 27.69 ± 0.02 mas from the main pair. The team calculated that the chance of this star just happening to line up by accident is less than 10⁻⁴ (less than 0.01%). It's almost certainly a real family member, gravitationally bound to the inner duo.

Weighing the Stars
Before this, astronomers had to guess the weights of these giants based on how they moved in their orbits, but those guesses were fuzzy. Because the team caught the inner pair at that special "side-by-side" moment, they could finally weigh them with much better precision.

The total weight of the inner binary is 93 ± 18 M⊙ (solar masses).
The primary star (the WN6ha) weighs in at 62 ± 13 M⊙.
The secondary star (the O5) is 31 ± 7 M⊙.
The tertiary star (the O7 intruder) has an estimated evolutionary mass of 25.6 +2.8 / -2.3 M⊙.

These aren't just guesses; they are dynamical measurements, meaning they are based on the actual physics of gravity and motion, not just models.

The Spectral Detective Work
Seeing the stars was only half the battle. To understand what they are made of, the team had to untangle their light. Since the stars are so bright and their light mixes together, it's like trying to hear three different singers in a choir when they are all shouting at once. Using a technique called "spectral disentangling," they mathematically separated the voices.

The primary is a WN6ha star, rich in nitrogen and helium, with a surface temperature of 42.2 ± 1.0 kK.
The secondary is an O5 star, slightly cooler at 41.4 ± 1.7 kK.
The tertiary is an O7 star, a bit cooler still at 36.3 ± 2.6 kK.

The analysis suggests all three stars were born around the same time, roughly 2.0 ± 0.2 Myr (million years) ago, making them a true family.

What About the Future?
The team also ran simulations to guess how the third star moves. They estimate its orbit takes between 19 and 82 years to complete one lap around the inner pair. It's moving slowly across the sky, about 1.3 to 3.2 mas per year. This means if we look at it again in a few years with the same powerful telescopes, we should see it have shifted position, confirming the orbit for sure.

What's Still a Mystery?
While the paper is a huge step forward, it doesn't have all the answers yet.

The exact 3D shape of the inner binary's orbit is still a bit fuzzy because they only have one snapshot of its position. They need more observations over time to pin down the tilt of the orbit (the inclination) and break the remaining uncertainties.
The tertiary star's surface gravity and wind properties are still "highly uncertain" because its light is so mixed with the primary's that it's hard to get a clean reading.
The paper explicitly states that while the evolutionary masses (calculated from models) and the new dynamical masses (calculated from gravity) are consistent, the evolutionary masses seem slightly higher. This suggests our models of how massive stars grow and lose mass might need a little tweaking, but the paper doesn't claim to have solved that puzzle yet.

In short, WR 25 has been upgraded from a mysterious couple to a confirmed triple system. It's now a "benchmark" for astronomers—a real-life laboratory where they can test their theories about how the universe's biggest stars are born, live, and interact. The show is just getting started, and with more observations, the full choreography of this cosmic triple will soon be revealed.

Technical Summary: WR 25 is a Massive Hierarchical Triple System

Problem Statement
WR 25 is a massive, eccentric (208-day) colliding-wind binary system located in the Carina nebula, comprising a WN6ha primary and an O5 companion. Previous spectroscopic analyses (e.g., Gosset et al. 2026) estimated the total binary mass to approach 100 $M_\odot$ with a mass ratio $q \approx 2$ , suggesting an extremely massive primary. Furthermore, spectroscopic signatures hinted at a third "intruder" component, likely an O-type star, but its physical association with the binary remained unconfirmed due to the lack of spatial resolution. While massive eclipsing binaries allow for precise mass measurements, WR 25's long period precludes eclipses, and wind modeling introduces significant uncertainties. Consequently, accurate dynamical masses for the components and confirmation of the system's triple nature were lacking.

Methodology
The authors utilized long-baseline interferometry and spectral disentangling to resolve the system's components and derive physical parameters:

Interferometric Observations: The team obtained H-band (1.52–1.76 $\mu$ m) observations of WR 25 using the VLTI/PIONIER instrument with four 1.8-m Auxiliary Telescopes. The observation occurred on January 16, 2019.
Geometric Modeling: Using the PMOIRED software, the authors modeled the squared visibilities ( $V^2$ ) and closure phases ( $T3_{PHI}$ ). They tested single-disk, binary, and triple-component models to determine the best fit for the spatial structure.
Chance Alignment Probability: To verify the physical association of the third component, the authors calculated the probability of a chance alignment ( $P_{spur}$ ) using 2MASS catalog data within the PIONIER field of view.
Spectral Disentangling: Leveraging the flux ratios derived from interferometry, the authors applied a three-component Fourier spectral disentangling code to archival FEROS spectroscopic data (3700–9200 Å) and an IUE UV spectrum. This allowed for the isolation of individual spectra for the primary, secondary, and tertiary.
Atmospheric Modeling: The disentangled spectra were analyzed using the non-LTE radiative transfer code CMFGEN to derive effective temperatures, surface gravities, wind parameters, and chemical abundances.
Dynamical and Evolutionary Analysis: The authors combined the interferometric astrometry with the spectroscopic orbital solution (specifically noting the observation occurred near the line of nodes) to calculate the total dynamical mass. They further employed the BONNSAI Bayesian tool to estimate evolutionary masses and ages, comparing them against the dynamical constraints.

Key Contributions and Results

Spatial Resolution of a Triple System: The study presents the first spatial resolution of all three components of WR 25. The inner binary (WN6ha + O5) is resolved with a projected angular separation of $1.68 \pm 0.02$ mas. A third component (tertiary) is detected at a separation of $27.69 \pm 0.02$ mas.
Confirmation of Physical Association: The probability of the tertiary being a chance alignment is calculated to be $< 10^{-4}$ , strongly supporting WR 25 as a hierarchical triple system.
Dynamical Masses: Due to the fortuitous timing of the observation (near the line of nodes), the authors derived the total dynamical mass of the inner binary without relying on the poorly constrained radial velocity of the secondary. The results are:
- Total binary mass: $M_{tot} = 93 \pm 18 M_\odot$ .
- Primary mass ( $M_1$ ): $62 \pm 13 M_\odot$ .
- Secondary mass ( $M_2$ ): $31 \pm 7 M_\odot$ .
- Orbital semi-major axis: $a = 3.11 \pm 0.20$ au.
Stellar Parameters:
- Primary (WN6ha): $T_{eff} = 42.2 \pm 1.0$ kK, $\log(L/L_\odot) = 6.21 \pm 0.06$ , with a mass-loss rate of $\log \dot{M} = -5.15 \pm 0.10 M_\odot \text{yr}^{-1}$ .
- Secondary (O5): $T_{eff} = 41.4 \pm 1.7$ kK, $\log(L/L_\odot) = 5.48 \pm 0.07$ .
- Tertiary (O7): Identified as an O7 star with $T_{eff} \approx 36.3$ kK and an evolutionary mass of $25.6^{+2.8}_{-2.3} M_\odot$ .
Orbital Constraints: The tertiary's orbital period is estimated to be between 19 and 82 years, with a sky-plane angular velocity of 1.3–3.2 mas yr $^{-1}$ .

Significance
The paper establishes WR 25 as a rare, spatially resolved massive hierarchical triple system. Its significance lies in providing a benchmark for:

Mass Calibration: Offering accurate, model-independent dynamical masses for a WN6ha + O5 binary, which are crucial for calibrating stellar evolution and atmosphere models for very massive stars.
Triple System Dynamics: Enabling the study of the formation and dynamical stability of massive hierarchical triples, a potential channel for merging compact objects detected by gravitational wave observatories.
Stellar Evolution: The components appear broadly coeval (ages $\sim 2$ Myr), allowing for constraints on internal mixing and wind mass-loss rates when comparing components of different masses within the same system.

The authors conclude that while the current single-epoch interferometric data provides a robust mass estimate, continued monitoring is necessary to fully constrain the 3D orbits of both the inner binary and the tertiary, and to further refine the dynamical masses.

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