The robustness of bi-stability jump predictions

The bi-stability (BS) jump comes from a long-standing theoretical prediction of radiatively driven wind theory, associated with Fe iv/Fe iii recombination around T_eff ≈ 21–25 kK. While the majority of theoretical approaches predict a strong increase in mass-loss rates across the BS jump, most empirical mass-loss studies of OB supergiants have not revealed the expected signature. We computed new, hydrodynamically consistent PoWR models at low and intermediate Eddington parameters (Γ_e ≈ 0.2−0.3) to test whether the BS jump persists in the canonical B-supergiant regime. The PoWR models presented in this work predict a robust BS jump, with an increase in mass-loss rates by more than an order of magnitude and a simultaneous drop in the terminal wind velocity, in line with Monte Carlo models and other co-moving frame calculations. The jump coincides with a transition in the dominant line driver from Fe iv to Fe iii. The presence of the BS jump is not restricted to high-Γ_e objects and it remains relevant for models well below the luminous blue variable (LBV) and hypergiant regimes. The persistence of the BS jump in hydrodynamically consistent models at lower Γ_e values supports the interpretation of this jump as a temperature-driven ionisation effect that is activated once a stationary line-driven wind solution is present. The continuing discrepancy between predictions and empirical population studies motivates further code-comparison work and controlled observational tests using individual objects such as LBVs.

Recommended citation: Vink et al. (2026), The robustness of bi-stability jump predictions, Astronomy and Astrophysics
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