The emergence of a [WC] star in Sakurai’s object

Sakurai’s object provides a rare opportunity to observe stellar evolution on human time-scales. Since its born-again event and detection in 1996, its evolution has been extensively monitored, and recent optical spectroscopy has suggested the emergence of [WR]-type emission features. In this letter, we present a secure spectroscopic identification of its central star based on new VLT/FORS2 observations and NLTE expanding atmosphere models. Several observed emission lines arise in a [WR]-type stellar wind, establishing the central star as a [WR] object. The strongest stellar features are due to C ii–iii and He i. We derive the stellar and wind parameters of Sakurai’s object and discuss its evolutionary status. The [WR] star is much cooler (Tₑff ~ 30.5 kK) than the born-again star V605 Aql (Tₑff ~ 95 kK), which erupted about 80 yr earlier. The inferred temperature is lower than predicted by models suppressing convective mixing efficiency during the VLTP, but is consistent with higher time-resolution calculations involving slightly lower-mass (0.6 M_) remnants. Continued spectroscopic monitoring and atmospheric modelling will be crucial for tracing its reheating curve and constraining the physics of this rapid evolutionary phase.