Massive stars end their lives through short but violently active evolutionary phases. One of them is the B[e] supergiant stage, when a star sheds material at a high rate and builds a cool, dense disk around itself, rich in molecules and dust. These disks act like archives, recording the star's entire mass-loss history and shaping what happens next.

LHA 115-S 18 is one such object, located in the Small Magellanic Cloud, a dwarf galaxy orbiting the Milky Way. A team of astronomers obtained new high-resolution near-infrared spectra of the star. The result exceeded expectations: alongside well-known molecular and hydrogen features, the researchers found something never before seen in the atmosphere of any B[e] supergiant.

A carbon monoxide ring and a hydrogen-driven wind

The first thing that stands out in the spectra of LHA 115-S 18 is emission from carbon monoxide (CO) molecules, broadened by rotation. The shape of these lines matches a Keplerian molecular ring — a structure orbiting under gravity around a central mass, much like planets circling a star.

Alongside this, both the H and K bands show strong hydrogen lines typical of a stellar wind. The researchers also counted numerous metallic emission lines. All of this fits the expected picture of a B[e] supergiant: a hot star surrounded by a complex, layered structure of gas and dust.

Hot water vapor where none was expected

The most surprising result was the detection of hot water vapor (H₂O) emission. This is the first time molecules of water have been registered around a B[e] supergiant in the entire observational history of this class of stars.

Until now, the environment around such stars was considered too hostile for water to survive: intense radiation and harsh ultraviolet light were expected to destroy molecular compounds quickly. The presence of H₂O suggests otherwise — even in such extreme conditions, there exist extended regions cool and dense enough for water molecules to persist and emit.

This forces a rethink of the circumstellar structure of B[e] supergiants: alongside the hot wind and the Keplerian CO ring, there must be additional, cooler pockets of gas that current models have not accounted for.

A velocity offset hints at a binary system

Another detail in the spectra adds further intrigue. The radial velocity of the molecular gas turned out to be offset relative to the velocity of the hydrogen Pfund line emission. Such an offset is a typical signature of a binary system, where two stars orbit a common center of mass.

If confirmed, the molecular ring around LHA 115-S 18 is most likely circumbinary — encircling both stars of the system rather than just one.

The discovery of hot H2O around the B[e] supergiant star LHA 115-S 18 challenges classical models on evolution and chemistry of massive binary starsM. L. Arias et al., arXiv:2603.22334

Confirming the binarity and the circumbinary nature of the disk would carry implications well beyond this single object. Classical models of massive binary star evolution did not anticipate such complex outflow chemistry. The presence of water alongside the CO ring means that the molecular enrichment of the interstellar medium by such stars is far more diverse than previously assumed.