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Discovery of sulfur oxides in the ejecta of a B[e] supergiant

Using ALMA observations, researchers detected sulfur oxides and other sulfur-bearing species in the ejecta of the B[e] supergiant HD 87643, revealing a rapidly evolving, oxygen-rich chemical environment where intense photochemistry drives mass-independent isotopic fractionation similar to processes on the early Earth.

Original authors: C. Bordiu, J. Ricardo Rizzo, D. Navarro-Almaida, A. Fuente, F. Bufano, G. Umana, S. Loru, A. C. Ruggeri, C. Buemi, F. Cavallaro, L. Cerrigone, A. Ingallinera, P. Leto, S. Riggi, C. Trigilio

Published 2026-07-03

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Original authors: C. Bordiu, J. Ricardo Rizzo, D. Navarro-Almaida, A. Fuente, F. Bufano, G. Umana, S. Loru, A. C. Ruggeri, C. Buemi, F. Cavallaro, L. Cerrigone, A. Ingallinera, P. Leto, S. Riggi, C. Trigilio

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 massive, aging star named HD 87643. It's a "B[e] supergiant," which is a fancy way of saying it's a brilliant, hot, and short-lived giant that is currently shedding its outer layers like a snake shedding skin. For a long time, astronomers thought the intense ultraviolet (UV) light from these stars was like a sterilizing ray gun: it was so powerful that it should have burned up any complex molecules in the surrounding gas, leaving nothing but simple atoms and dust.

The Big Discovery: Finding the "Forbidden" Molecules
Using a powerful telescope array in Chile called ALMA, the team of astronomers decided to take a closer look at the gas cloud swirling around HD 87643. They expected to find mostly simple stuff, but instead, they found a chemical treasure chest.

They discovered a rich soup of molecules, including sulfur oxides (SO and SO2). Think of this like finding a fully stocked kitchen in a house that everyone thought was on fire. This is the first time scientists have found these specific sulfur-based molecules around this type of hot, massive star. It proves that even in these harsh, high-radiation environments, complex chemistry can still happen.

The Mystery of the "Heavy" Sulfur
Here is where the story gets really weird. Sulfur comes in different "flavors" called isotopes. The most common flavor is like a standard apple, while a rare flavor (Sulfur-33) is like a slightly heavier, golden apple. In our solar system, for every 127 standard apples, you find one golden apple.

However, in the gas cloud around HD 87643, the astronomers found something bizarre: for every 15 standard apples, there was one golden apple. The ratio of the rare flavor was nearly ten times higher than it should be.

The Explanation: A Cosmic "Self-Shielding" Trick
How did this happen? The paper suggests it wasn't because the star created extra golden apples in its core (nuclear fusion). Instead, it's a trick of light and shadow called "mass-independent fractionation."

Imagine a crowd of people standing in front of a very bright spotlight.

The standard apples (Sulfur-32) are so numerous that they stand shoulder-to-shoulder, blocking the light from reaching the people behind them. They form a "shield" that protects themselves from the spotlight's destructive power.
The golden apples (Sulfur-33) are so rare that they are scattered far apart. The light slips right past the standard apples and hits the golden apples directly.

Because the golden apples are hit by the light more often, they break apart (photodissociation) and turn into the molecules the astronomers detected. The standard apples, protected by their own numbers, stay intact longer. This creates a situation where the "broken" golden pieces are surprisingly abundant compared to the standard ones.

Why This Matters
The paper compares this cosmic trick to what might have happened in the atmosphere of Earth billions of years ago (during the Archaean era) before life as we know it existed. Just like in this star system, ancient Earth might have had similar light-shielding effects that changed the chemical makeup of our atmosphere.

The Bottom Line
This star is acting like a unique laboratory. It shows us that:

Complex chemistry can survive even in the most hostile, UV-blasted environments.
The gas around the star is young and changing rapidly (evolving in just about 10,000 years), likely due to a recent burst of material being ejected from the star.
The strange mix of sulfur isotopes is a result of intense light interacting with gas, not necessarily a change in how the star was born.

In short, HD 87643 is a cosmic drama where light, shadow, and chemistry are playing out a complex dance that helps us understand how the ingredients for life might have been shuffled in the early universe.

Technical Summary: Discovery of Sulfur Oxides in the Ejecta of a B[e] Supergiant

Problem and Context
B[e] supergiants (sgB[e]) are rare, luminous, evolved massive stars characterized by dusty circumstellar disks and intense ultraviolet (UV) radiation fields. Historically, it was believed that these intense UV fields would sterilize the surrounding environment, preventing the survival of complex molecules beyond carbon monoxide (CO). Consequently, the chemical reservoirs of sgB[e] stars remained largely unexplored, leaving fundamental questions regarding their ability to sustain complex molecular chemistry unanswered. While CO has been detected in a few instances, sulfur-bearing species had never been confirmed in this class of stars, despite sulfur being a fundamental element for life and the tenth most abundant element in the Universe.

Methodology
The authors utilized the Atacama Compact Array (ACA) of the Atacama Large Millimeter/submillimeter Array (ALMA) to conduct the first millimeter survey of the sgB[e] star HD 87643 (V640 Car). Observations were part of the ALMA Cycle 10 program ULISSES, conducted in May 2024, yielding a total on-source integration time of 2.9 hours. The study employed a 90"×90" mosaic to cover the full extent of the Hα nebula surrounding the star.

Data reduction was performed using CASA and the ALMA pipeline, resulting in a continuum map and four spectral cubes with a synthesized beam of 7.1"×4.7". The analysis involved:

Spectral Identification: Identifying molecular transitions across four spectral windows (218–232 GHz).
Radiative Transfer Modeling: Using the MADCUBA software package to fit synthetic Local Thermodynamic Equilibrium (LTE) models to observed spectra. Excitation temperatures ( $T_{ex}$ ) were constrained by fitting the Spectral Energy Distribution (SED) from optical to radio wavelengths, yielding dust temperatures of 20–80 K.
Chemical Modeling: Employing the gas-grain chemical code NAUTILUS to reproduce the observed fractional abundances of OCS, SO, SO $_2$ , HNCO, and H $_2$ CO. The models varied gas densities ( $10^5$ – $10^8$ cm $^{-3}$ ) and chemical timescales to find the best fit for the observed abundances, assuming an oxygen-rich environment (C/O $\approx$ 0.4) and undepleted sulfur.

Key Results
The observations revealed a chemically rich molecular gas environment surrounding HD 87643, marking the first confirmed detection of sulfur-bearing species in an early-type evolved massive star.

Molecular Inventory: Ten distinct molecular species were detected: CO, $^{13}$ CO, OCS, H $_2$ CO, SO, $^{33}$ SO, SO $_2$ , HNCO, O $^{13}$ CS, and $^{13}$ CS.
Spatial Distribution: CO and $^{13}$ CO emission showed two components: dispersed clumps tracing the optical nebula and a central, resolved component at the stellar position. All other detected species were unresolved and coincident with the central stellar position, suggesting a circumbinary origin rather than a compact circumprimary disk.
Abundances: SO $_2$ was found to be the third most abundant species after CO and $^{13}$ CO. At an excitation temperature of 50 K, the fractional abundance of SO $_2$ relative to H $_2$ is $1.1 \times 10^{-5}$ , nearly matching the cosmic sulfur abundance. This implies that most available sulfur is locked in SO $_2$ , with no significant depletion into dust grains. The SO $_2$ /SO abundance ratio is exceptionally high (10–150), contrasting with typical cool supergiant envelopes where SO is usually more abundant.
Chemical Timescale: Chemical modeling indicates that the observed abundances, particularly the high SO $_2$ , are reproduced in a dense ( $0.3–1.6 \times 10^7$ cm $^{-3}$ ), oxygen-rich environment over a short chemical timescale of $\sim 10^4$ years. This suggests a rapidly evolving, non-equilibrium phase, likely linked to recent mass ejection or binary mass transfer.
Isotopic Anomaly: The study measured an anomalously low $^{32}$ SO/ $^{33}$ SO ratio of $\sim 15$ , implying a nearly order-of-magnitude enrichment of $^{33}$ S relative to Solar System values. Standard nucleosynthetic processes (weak s-process in massive stars) cannot account for this level of enrichment.

Significance and Interpretation
The paper posits that the anomalous $^{32}$ SO/ $^{33}$ SO ratio is not due to nucleosynthesis but rather mass-independent fractionation (MIF) driven by intense photochemistry. The authors propose a mechanism where the dominant parent molecule, $^{32}$ SO $_2$ , self-shields against UV radiation, while the less abundant $^{33}$ SO $_2$ remains optically thin and undergoes efficient photodissociation. This selective photolysis produces an excess of $^{33}$ SO daughter species.

This mechanism mirrors processes proposed to explain $^{33}$ S excesses in the atmosphere of the Archaean Earth and meteoritic sulfur anomalies. The findings suggest that B[e] supergiants serve as unique laboratories for studying sulfur chemistry under extreme radiation conditions. The detection of these molecules indicates that complex chemistry can persist in the ejecta of hot, evolved massive stars, challenging the notion that UV fields completely sterilize these environments. The results highlight a transient, out-of-equilibrium chemical state governed by the interplay of mass loss, UV radiation, and potentially shocks.

Conclusion
This work establishes a new chemical benchmark for early-type supergiants by confirming the presence of sulfur oxides in HD 87643. The observed high SO $_2$ abundance and anomalous isotopic ratios point to a dynamic, short-lived chemical phase. The authors conclude that future high-resolution, multi-transition ALMA observations are necessary to constrain the spatial distribution of the gas and fully trace the origin of these fractionation processes.

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