High-resolution near-IR spectroscopy of the B[e] supergiant LHA 115-S 18: discovery of hot water vapor emission

TL;DR

This study presents high-resolution near-infrared spectra of the B[e] supergiant LHA 115-S 18, revealing the first detection of hot water vapor emission, which suggests complex circumstellar chemistry and binary interactions affecting stellar evolution.

Contribution

It reports the first observation of hot water vapor emission in a B[e] supergiant, expanding understanding of circumstellar chemistry and mass-loss in massive binary star systems.

Findings

Detection of rotationally broadened CO emission from a Keplerian ring

First identification of hot water vapor emission in a B[e] supergiant

Evidence of a binary system with circumbinary molecular gas

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 which 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 H2 O around the B[e] supergiant star LHA 115-S 18 challenges classical models on evolution and chemistry of massive binary stars and provides critical insight into mass-loss processes and molecular enrichment of the ISM.