2024A&A...681A..17D

Context. The fate of stars largely depends on the amount of mass lost during the end stages of evolution. For single stars with an initial mass between ∼8-30 M_⊙, most mass is lost during the red supergiant (RSG) phase, when stellar winds deplete the H-rich envelope. However, the RSG mass-loss rate (Ṁ) is poorly understood theoretically, and so stellar evolution models rely on empirically derived mass-loss rate prescriptions. However, it has been shown that these empirical relations differ largely, with differences up to 2 orders of magnitude.
Aims. We aim to derive a new mass-loss rate prescription for RSGs that is not afflicted with some uncertainties inherent in preceding studies.
Methods. We have observed CO rotational line emission towards a sample of RSGs in the open cluster RSGC1 that all are of a similar initial mass. The ALMA CO(2-1) line detections allowed us to retrieve the gas mass-loss rates (Ṁ_CO). In contrast to mass-loss rates derived from the analysis of dust spectral features (Ṁ_SED), the data allowed us a direct determination of the wind velocity and no uncertain dust-to-gas correction factor was needed.
Results. Five RSGs in RSGC1 have been detected in CO(2-1). The retrieved Ṁ_CO values are systematically lower than Ṁ_SED. Although only five RSGs in RSGC1 have been detected, the data allow us to propose a new mass-loss rate relation for M-type red supergiants with effective temperatures between ∼3200 and 3800 K that is dependent on the luminosity and initial mass, and that is valid during the phase where nuclear burning determines the evolution along the RSG branch. The new mass-loss rate relation is based on the new Ṁ_CO values for the RSGs in RSGC1 and on prior Ṁ_SED values for RSGs in four clusters, including RSGC1. The new Ṁ-prescription yields a good prediction for the mass-loss rate of some well-known Galactic RSGs that are observed in multiple CO rotational lines, including α Ori, μ Cep and VX Sgr. Moreover, there are indications that a stronger, potentially eruptive, mass-loss process is occurring during some fraction of the RSG lifetime, suggesting that RSGs might experience a phase change in mass loss leading to the wind mass-loss rate dominating the RSG evolution at that stage.
Conclusions. Implementing a lower mass-loss rate in evolution codes for massive stars has important consequences as to the nature of their end-state. A reduction of the RSG mass-loss rate implies that quiescent RSG mass loss is not enough to strip a single star's hydrogen-rich envelope. Upon core collapse such single stars would explode as RSGs. Mass-loss rates of order ∼6 times higher would be needed to strip the H-rich envelope and produce a Wolf-Rayet star while evolving back to the blue side of the Hertzsprung-Russell diagram. Future observations of a larger sample of RSGs in open clusters should allow a more stringent determination of the Ṁ_CO-luminosity relation and a sharper diagnostic as to when the phase change in mass loss is occurring.