Simulations of massive star atmospheres and winds during giant eruptive and quiescent luminous blue variable phases

Mass loss from massive stars located in the part of the Hertzsprung-Russell diagram (HRD) where we find luminous blue variables (LBVs) is profoundly important for stellar evolution yet very poorly understood. We used time-dependent radiation-hydrodynamic (RHD) simulations to examine the atmosphere and wind properties of massive stars in this region of the HRD. We computed 2D and 1D RHD models of the coupled envelopes, atmospheres, and wind outflows of massive stars, tuned to the region in the HRD where LBVs are located. Our unified simulations, which start deep in the stellar envelope (below the iron opacity bump) and range well into the outflowing wind, account for line-driving as well as radiative enthalpy and photon tiring. Mass-loss rates and wind speeds are thereby emergent properties in the simulations. A grid of models was created by slightly increasing the stellar energy at the lower boundary, which then controls the emergent radiative luminosity at the photosphere. Increasing total stellar energy results in a natural transition from very turbulent atmospheres with line-driven winds to effectively stationary super-Eddington massive outflows. In the latter case, radiative enthalpy keeps the star below the Eddington limit in the deep layers where the wind is launched. However, as energy transport by enthalpy becomes inefficient as we approach the surface, the star was observed to become highly super-Eddington at its recombined photosphere. Our sub-Eddington models are essentially blue hypergiant stars (T_ ̊m eff ∼ 10 000-20 000 K) with very variable surfaces, effective mass-loss rates of dot M ∼ 2-5 ̊m M_⊙/yr, and wind speeds of ̌arv_∞ ∼ 200-300 km/s, resembling quiescent LBVs, like P Cygni. The super-Eddington models have optically thick wind envelopes and extremely inflated red or yellow surfaces 10^ -5 ̊m eff ∼ 5000 K), with dot M ∼ 0.1-1 ̊m M_⊙/yr and ̌arv_∞ ∼ 400-500 km/s, thus resembling a massive star during a great eruption, such as that of η Carinae. Although not tuned to any specific star, our RHD atmosphere and wind models naturally reproduce the overall characteristic stellar and wind parameters inferred for massive stars in their quiescent LBV and yellow giant eruptive phases. It remains an open question whether the energy increase needed to trigger a giant eruption can be obtained solely by the internal evolution of the star itself or if it requires an external energy source such as a stellar merger or a close binary interaction.