Discovery of a transitional type of evolved massive star with a hard ionizing flux

Wolf-Rayet (WR) stars are the evolved descendants of the most massive stars and show emission-line-dominated spectra formed in their powerful winds. Marking the final stage before core collapse, the standard picture of WR stars has been that they evolve through three well-defined spectral subtypes known as WN, WC and WO. Here we present a detailed analysis of five objects that defy this scheme, demonstrating that WR stars can also evolve directly from the WN stage to the WO stage (WN/WO). Our study reveals that this direct transition is connected to low metallicity and weaker winds. The WN/WO stars and their immediate WN precursors are hot and emit a high flux of photons capable of fully ionizing helium. The existence of these stages unveils that high-mass stars that manage to shed off their outer hydrogen layers in a low-metallicity environment can spend a considerable fraction of their lifetime in a stage that is difficult to detect in integrated stellar populations, but at the same time yields a hard ionizing flux. The identification of the WN-to-WO evolution path for massive stars has significant implications for understanding the chemical enrichment and ionizing feedback in star-forming galaxies, in particular at earlier cosmic times.

Recommended citation: Sander et al. (2025), Discovery of a transitional type of evolved massive star with a hard ionizing flux, Nature Astronomy
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