2011/05/04 by J. H. Groh, Jose H. Groh, D. J. Hillier +3
Physics and Astronomy · Social Sciences · #Astronomy and Astrophysical Research #Bistability #Eddington luminosity #Educational Leadership and Practices #Ionization #Luminosity #Luminous blue variable #Omega #Optical bistability #Stellar mass #Stellar, planetary, and galactic studies #astro-ph.GA #astro-ph.SR
paper · pdf · doi:10.1088/0004-637x/736/1/46
13 pages, 8 figures, accepted by ApJ
arxiv created 2011/05/04 · openalex publication_date 2011/07/05 · arxiv updated 2015/05/28 · openalex created_date 2019/06/27 · openalex updated_date 2026/08/05
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 −1 during 1985–1990 which, combined with the high luminosity ( L ⋆ = 1.5 × 10 6 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.