Massive stars near the Eddington Limit

• Applicant: Dr. Götz Gräfener
• Subject Area: Astrophysics and Astronomy
• Term: from 2016 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 325018366

I am presenting a project to investigate the properties of very massive stars (VMS) in the upper Hertzsprung-Russel Diagram (HRD). The question how massive stars evolve and end their lives is of fundamental importance for many branches of astronomy, including the evolution of galaxies and star clusters, supernovae (SNe) and gamma-ray bursts (GRBs), and the enrichment of the interstellar medium (ISM) with nucleosynthetic products since the Big Bang. The most massive stars, with their extreme luminosities and strong stellar winds, dominate the evolution of star-forming galaxies by ionising and shaping their ISM. Their strong mass loss, in the form of stationary stellar winds or episodic outbursts, feeds freshly processed material back into the ISM, and decides whether massive stars end their lives as neutron stars or black holes after their final collapse and SN explosion.The physical properties of VMS (above ~40 solar masses) are largely determined by their proximity to the Eddington limit. The dominance of radiation pressure is believed to be responsible for effects like variability and mass loss in this regime. A number of recent theoretical studies focused on the envelope inflation effect, an inflation of the outer stellar envelope near the Eddington limit that changes stellar radii and effective temperatures by large factors. This effect is particularly interesting as it implies radius variations and stability limits reminiscent of the S-Dor type variability in the Luminous Blue Variable (LBV) phase. Currently there is increasing interest in this enigmatic evolutionary phase as it may dominate the mass-loss budget of the most massive stars, and because of new evidence for LBV-type behaviour in direct SN progenitor phases.The evolutionary status of LBVs, and the nature of their enigmatic giant eruptions, is currently unclear. A major goal of the proposed research programme is to provide combined stellar envelope/wind models that can be used to identify LBV-type behaviour in evolutionary computations. Current stellar structure and evolution models either suppress the inflation effect, or include only part of the relevant physics. Moreover they neglect the interplay between envelope and wind near the Eddington limit. Through the proposed connection of improved stellar evolution models with boundary conditions for optically-thick radiation-driven stellar winds, it will be possible to predict the properties of stars in these phases, and to identify phases with LBV-type behaviour. Furthermore, the models will give insight in the conditions at the wind base of strong, radiatively-driven stellar winds that could help to resolve outstanding problems with the numerical modelling of this important type of mass loss. Ultimately, the models will be used to test scenarios for the origin of giant LBV eruptions (or SN impostors), one of the major unsolved problems in stellar astrophysics.

• DFG Programme: Research Grants