2011ApJ...736...46G

We show that the significantly different effective temperatures (T_eff) achieved by the luminous blue variable AG Carinae during the consecutive visual minima of 1985-1990 (T_eff≃ 22,800 K) and 2000-2001 (T_eff≃ 17,000 K) place the star on different sides of the bistability limit, which occurs in line-driven stellar winds around T_eff∼ 21,000 K. Decisive evidence is provided by huge changes in the optical depth of the Lyman continuum in the inner wind as T_eff changes during the S Dor cycle. These changes cause different Fe ionization structures in the inner wind. The bistability mechanism is also related to the different wind parameters during visual minima: the wind terminal velocity was 2-3 times higher and the mass-loss rate roughly two times smaller in 1985-1990 than in 2000-2003. We obtain a projected rotational velocity of 220±50 km/s during 1985-1990 which, combined with the high luminosity (L_{sstarf}= 1.5x10⁶ L_☉), puts AG Car extremely close to the Eddington limit modified by rotation (ΩΓ limit): for an inclination angle of 90°, Γ_Ω ≳ 1.0 for M_☉ ≲ 60. Based on evolutionary models and mass budget, we obtain an initial mass of ∼100 M_☉ and a current mass of ∼60-70 M_☉for AG Car. Therefore, AG Car is close to, if not at, the ΩΓ limit during visual minimum. Assuming M = 70 M_☉, we find that Γ_Ω decreases from 0.93 to 0.72 as AG Car expands toward visual maximum, suggesting that the star is not above the Eddington limit during maximum phases.