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

M. L. Arias, A. F. Torres, M. Kraus, L. S. Cidale

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.

High-resolution near-IR 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 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.
Paper Structure (8 sections, 3 figures, 1 table)

This paper contains 8 sections, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. Observations and data reduction
  3. Results
  4. H and K-band atomic spectrum
  5. CO-band emission
  6. Emission from water vapor
  7. Pfund line emission
  8. Discussion and Conclusions

Figures (3)

  • Figure 1: GEMINI/IGRINS H-band (top) and K-band (bottom) IGRINS high-resolution spectrum of the star LHA 115-S 18. Positions of the CO band heads, Pfund emission lines and He I lines are marked with ticks and labeled. We also indicate the identified metallic emission lines.
  • Figure 2: GEMINI/IGRINS observed K-band spectrum of LHA 115-S 18 (black), with the model composed of water vapor (red, top three panels) and water vapor + CO bands + Pfund lines (red, bottom two panels) overplotted, along with the residuals shifted for better visibility (gray).
  • Figure 3: Different fits for selected portions of LHA 115-S 18 K-band spectrum, showing the variation with the model parameteres: temperature and column density. The best fit is for T(H$_2$O) $\approx$ 1800 K and N(H$_2$O) $\approx$$4 - 6 \times 10^{21}$ cm$^{-2}$.