The winds of OBA hypergiants and luminous blue variables

Dynamically consistent atmosphere models reveal multiple wind regimes

¹ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
² Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Universität Heidelberg, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
³ Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, Northern Ireland, UK
⁴ Departamento de Astrofísica, Centro de Astrobiología, (CSIC-INTA), Ctra. Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 23 December 2025
Accepted: 21 February 2026

Abstract

Context. OBA hypergiants are evolved massive stars with notable wind features in their optical spectrum. Located at the cool end of the line-driven wind regime, many of these objects are candidate luminous blue variables (LBVs) and presumably close to the Eddington limit. Despite representing a rather short-lived stage in the life of massive stars, they strongly affect their surroundings and subsequent stellar evolution via their high wind mass loss.

Aims. We investigate the atmospheric conditions and mechanisms that produce the winds and spectral appearance of hypergiants in the OBA temperature regime, thereby also covering typical temperatures of minimum and maximum phases of noneruptive LBVs.

Methods. We used the hydrodynamically consistent version of the atmosphere code PoWR to produce a sequence of atmosphere models with a classical Eddington parameter Γ_e ∼ 0.4 and moderate turbulent pressure, in line with typical parameters obtained in the regime of cool supergiants and hypergiants. We varied the effective temperature at the inner boundary from ∼ 12.5 kK to ∼ 38.0 kK. Our main model series was calculated at solar metallicity, with a few additional tests performed at lower metallicity.

Results. We found a complex pattern of the mass-loss rate as a function of temperature in the hypergiant regime, with regions of higher and lower rates associated with different types of wind solutions, which we call “dense” and “rarefied”. We found known hypergiants and LBVs with spectra resembling the synthetic spectra of our models for all of the wind-solution regions. Around the temperatures where Fe iv recombines to Fe iii and Fe iii recombines to Fe ii, we found a bistability of solutions and sharp increases in the mass-loss rate. In addition, we found decreases when the leading Fe ion begins to change at the wind onset, indicating that the wind solutions switch from dense to rarefied when the driving opacity is insufficient. The resulting velocity fields in the different regions reflect the different atmosphere solutions, where the rarefied solutions agree with the empirical relation of the terminal velocity and effective temperature.

Conclusions. Our results demonstrate the existence of the first and second bistability jump and their association with Fe recombination in the hypergiant regime. However, the jumps are embedded in an overall more complex behavior that agrees but poorly with any existing mass-loss recipe. Toward cooler temperatures, the role of turbulent pressure in the wind acceleration becomes increasingly important, in particular in the rarefied regimes. Combining our results with other recent modeling efforts, we suggest that the switch between rarefied and dense regimes only occurs within a certain proximity to the Eddington limit, while otherwise, only either dense or rarefied solutions will be obtained. Calculations exploring a wider parameter regime and incorporating a more sophisticated treatment of radiatively driven turbulence are necessary to further test this hypothesis.