Abstract The post-main-sequence evolution of massive stars involves phases of intense, often eruptive mass loss, including the Be supergiant phase. These hot stars are surrounded by cool, dense circumstellar disks that host complex chemistry, producing both molecules and dust. Understanding the mass-loss history of Be supergiants is essential for constraining stellar evolution models, particularly regarding their final stages. Near-infrared CO band emission serves as a key tracer of disk dynamics, typically arising from the inner edge of the molecular disk or ring. However, the oxygen-rich environments of these stars also favor the presence of other molecules that trace regions complementary to those probed by CO. In this work, we present high-resolution near-infrared spectra of the Small Magellanic Cloud Be supergiant LHA 115-S 18. Our analysis reveals rotationally broadened CO emission consistent with a Keplerian molecular ring, alongside strong hydrogen wind features in both H and K bands and numerous metallic emission lines. Notably, we report the first detection of hot water vapor emission in a Be supergiant. This finding indicates the existence of extended cool and dense regions in a harsh environment. A radial velocity offset between molecular and Pfund line emission further supports a binary system, with the molecular gas potentially being circumbinary. The discovery of hot H 2 O around the Be supergiant star LHA 115-S 18 challenges classical models on the evolution and chemistry of massive binary stars and provides critical insight into mass-loss processes and molecular enrichment of the interstellar medium.
Arias et al. (2026) studied this question.