Astronomers have discovered a chemically rich collection of sulfur-bearing molecules surrounding the rare massive star HD 87643. The observations include the first detection of sulfur monoxide and sulfur dioxide around a B[e] supergiant. The paper was published in The Astrophysical Journal Letters on July 1.
Rare and old
B[e] supergiants, or sgB[e], are rare, evolved massive stars that have moved past their main hydrogen-burning phase. They are notable for distinctive emission lines—hence the “[e]” in their name—including rare “forbidden” lines produced by the surrounding gas and dust, along with an infrared glow far more intense than that of ordinary B-type stars.
As these stars approach the final stages of their lives, they shed enormous amounts of material, building up messy, complex surroundings: fast winds blasting outward from their poles and a slower, denser, dusty flow encircling their equator like a disk.
In this study, the researchers, led by Cristobal Bordiu of INAF Catania Astrophysical Observatory, used Atacama Large Millimeter/submillimeter Array’s Atacama Compact Array observations to conduct the first detailed millimeter-wavelength survey of HD 87643, a particularly extreme B[e] supergiant. Before this study, only one molecule (carbon monoxide) had been detected in its surroundings.
Their analysis revealed emissions from 10 different molecules: CO, 13CO, OCS, H2CO, SO, 33SO, SO2, HNCO, O13CS and 13CS. Several contain sulfur, making this the first detection of sulfur chemistry around any B[e] supergiant star.
A chemical benchmark
Sulfur is a relatively common element in the universe—the 10th most abundant overall—and one of just six elements essential to life as we know it. Astronomers have already found sulfur-based molecules in many cosmic environments, from our own solar system to planet-forming disks and star nurseries. But around hot, massive, dying stars specifically, sulfur chemistry has remained almost entirely unexplored. Before this study, only one sulfur-bearing molecule had been reported around a variable star. That makes the new detections in HD 87643 especially significant, opening the door to studying sulfur chemistry in these extreme environments.
“These detections substantially expand the molecular inventory of HD 87643, previously limited to CO,” the team writes in the paper. This establishes “a new chemical benchmark for early-type supergiants: this work marks the first detection of sulfur-bearing species toward an sgB[e] star.”
The researchers also uncovered a puzzle in this star’s chemistry. The ratio between two isotopes of sulfur monoxide is surprisingly low—around 15—compared with values found in other environments within our galaxy. This suggests the rarer, heavier isotope is present far more often near this star than expected anywhere else in the Milky Way.
Puzzling chemistry
Standard nuclear physics couldn’t explain the imbalance. So the team proposed an alternative explanation. The star’s intense ultraviolet light breaks apart sulfur dioxide molecules, turning them into sulfur monoxide. But not all isotopes of sulfur dioxide break down equally. The common, lighter isotope is abundant enough to shield itself against UV light, so only a small fraction gets broken apart into sulfur monoxide. The rarer, heavier isotope doesn’t have enough molecules to shield itself the same way, so nearly all of it gets exposed and destroyed.
As a result, a much larger share of the heavy isotope ends up being converted into sulfur monoxide—even though it was far less abundant to begin with. This process, known as mass-independent fractionation, has previously been documented only in ancient rock samples from Earth and in meteorites. If confirmed, HD 87643 would mark an extreme case of this chemical phenomenon.
Researchers note that future high-resolution ALMA observations would be useful in pinning down the complete picture of this star’s sulfur chemistry. Understanding how molecules form and survive in the harsh conditions around old stars holds clues to understanding the role of massive stars in the distribution of chemical elements needed for the formation of planets and the eventual emergence of life.
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Publication details
Cristobal Bordiu et al, Discovery of Sulfur Oxides in the Ejecta of a B[e] Supergiant, The Astrophysical Journal Letters (2026). DOI: 10.3847/2041-8213/ae7b2b
Key concepts
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ALMA uncovers sulfur for the first time around a rare class of supergiant star (2026, August 20)
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