CONICET | Buscador de Institutos y Recursos Humanos

The advanced stages of high-mass stars are characterized by episodic mass loss shed during phases of instability. Key for assigning these stars a proper evolutionary state is to assess the composition and geometry of their ejecta alongsidethe stellar properties. We selected five hot LBV candidates in M33 to refine their classification, investigate their circumstellar environments and explore their evolutionary properties. Being accessible targets in the near-infrared, we conducted medium-resolution K-band spectroscopy with GNIRS mounted on Gemini North to investigate their molecular circumstellar environments. Two stars were found to display CO emission, which was modeled to emerge from a circumstellar orcircumbinary Keplerian disk/ring. The identification of the carbon isotope 13C and, for one of the two stars, a significantly low 12CO/13CO ratio, implies an evolved stellar state. As both CO emission stars are highly luminous and hence do not undergo a red supergiant phase, we suggest that stripping processes and equatorial high-density ejecta due to fast rotation are responsible for the enrichment of the stellar surface with processed material from the core. A candidate B[e]SG displays an absorption CO profile, which may be attributed tojet or stellar pulsations. The featureless GNIRS spectra of two stars suggestlow-density gas shell or dissipation of the molecule due to the ionizing temperature of the star. We highlight the importance of the infrared data to resolve the evolutionary status of massive stars in transition and thus, to constrain thephysics of the diverse pre-supernova stellar states.