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 (Teff ≃ 22,800K) and 2000-2001 (Teff ≃ 17,000K) place the star on different sides of the bistability limit, which occurs in line-driven stellar winds around T eff ∼ 21,000K. Decisive evidence is provided by huge changes in the optical depth of the Lyman continuum in the inner wind as Teff 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-1 during 1985-1990 which, combined with the high luminosity (L⊙ = 1.5 × 106 L⊙), puts AG Car extremely close to the Eddington limit modified by rotation (ΩΓ limit): for an inclination angle of 90°, ΓΩ ≳ 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 ΓΩ 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. © 2011. The American Astronomical Society. All rights reserved.
CITATION STYLE
Groh, J. H., Hillier, D. J., & Damineli, A. (2011). On the nature of the prototype luminous blue variable AG Carinae. II. Witnessing a massive star evolving close to the Eddington and bistability limits. Astrophysical Journal, 736(1). https://doi.org/10.1088/0004-637X/736/1/46
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