High-resolution Near-Infrared Spectroscopy of the B[e] Supergiant LHA 115-S 18: Discovery of Hot Water Vapor Emission
Year: 2026
Type: article
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.
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $2,400
Funders: National Science Foundation, European Commission, Grantová Agentura České Republiky, Consejo Nacional de Investigaciones Científicas y Técnicas, Korea Astronomy and Space Science Institute, Universidad Nacional de La Plata, Ministério da Ciência, Tecnologia, Inovações e Comunicações, Agencia Nacional de Investigación y Desarrollo
Awards: MRI: Development of a High Resolution Infrared Spectrograph with a Large Spectral Grasp, G1102111299, G36660926, G3998373107, G4673193015, G4987610657, Revealing the pulsation-induced mass loss of blue supergiants and its interplay with the interstellar medium, G7214438521, G742691486, Enabling a New Instrument 'IGRINS' for the Gemini South Telescope User Community, G8278124175, G871109655
OpenAlex ID: W7141762244