High-resolution Near-Infrared Spectroscopy of the B[e] Supergiant LHA 115-S 18: Discovery of Hot Water Vapor Emission

Abstract The post-main-sequence evolution of massive stars involves phases of intense, often eruptive mass loss, including the B[e] 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 B[e] 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 B[e] 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 B[e] 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 B[e] 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.