Astronomers have detected sulfur-bearing molecules in the harsh environment around one of the hottest and most massive types of stars, challenging long-held assumptions about where complex chemistry can occur in the universe. Using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, an international research team identified sulfur monoxide and sulfur dioxide in the circumstellar environment of the B[e] supergiant HD 87643. This marks the first time that molecules containing sulfur have been observed around a hot evolved massive star.
The discovery, published in The Astrophysical Journal Letters, was led by Cristóbal Bordiu of the Italian National Institute for Astrophysics (INAF) in collaboration with researchers from Spain and Chile. HD 87643 belongs to the rare class of B[e] supergiants — massive, extremely hot stars in an advanced evolutionary stage. Their intense ultraviolet radiation and strong radiative fields create conditions that are generally considered hostile to the formation and survival of molecules. The detection of sulfur oxides in such an environment suggests that the chemistry of these systems is far richer than previously believed.
«HD 87643 is a particularly interesting system because it combines a complex binary structure with extreme physical conditions,» Bordiu explained. The system consists of a binary star where the primary star is surrounded by a disk of material, and the entire system is embedded in a larger cloud of gas and dust. The researchers believe the detected molecules reside within this cloud, though further observations will be needed to pinpoint their exact origin and understand how the companion star influences chemical formation processes.
Chemical analysis indicates that the observed molecular gas has a characteristic age of approximately 10,000 years — a very short timescale by astrophysical standards. This suggests a young, rapidly evolving environment, likely fueled by repeated episodes of mass loss from the central binary system. The team adopted a multidisciplinary approach to interpret the data, combining ALMA observations with radiative transfer modeling, spectral energy distribution reconstruction, and chemical simulations.
«The results show that standard chemical models cannot fully explain the observed abundances,» Bordiu added. «This suggests that non-equilibrium processes are active in the circumstellar region. In particular, the strong ultraviolet radiation from the star may play an important role in modifying the gas chemistry. The presence of sulfur-containing molecules also indicates that chemistry in these environments can be richer than predicted by standard models, although further observations will be needed to clarify the formation and destruction mechanisms of the observed species and the role of binarity.»
Massive stars played a fundamental role in the chemical enrichment of the early universe. Understanding how sulfur-bearing molecules form and transform in their circumstellar regions is therefore a key element in reconstructing the chemical evolution of the cosmos. The detection of sulfur oxides in a B[e] supergiant demonstrates that complex molecular chemistry can develop even under extreme conditions and places new constraints on models describing chemistry in high-energy stellar environments.
The study is part of a broader program dedicated to molecular chemistry in evolved massive stars, including luminous blue variables and Wolf-Rayet stars, with the goal of understanding chemical differences across different evolutionary phases. The research team included scientists from INAF, Ingeniería de Sistemas para la Defensa de España (Isdefe), the Centro de Astrobiología (INTA-CSIC, Spain), and the Joint ALMA Observatory in Chile.
