Microturbulence in O supergiants

We investigate the role of classical microturbulence in the non‐LTE He  i /He  ii line formation problem for luminous O‐type stars. We find that the shapes and strengths of certain saturated He  i lines, in particular the triplets λλ4471, 4713 and the singlet λ4921, are sensitive to microturbulent velocities in excess of 5 km s −1 . Weaker lines, including most of the He  i singlets, are effectively independent of this parameter, as are the Fowler series He  ii lines λλ4199, 4541, 5411. We show that this behaviour is due to interaction between direct line‐broadening effects in the radiative transfer, and indirect changes in the atmospheric structure and the populations of absorbing states. Using an analysis of high‐resolution, high signal‐to‐noise ratio observations of the O9.7 supergiant HD 152003 as an illustrative example, we show how the introduction of microturbulence in non‐LTE models allows consistent fits to be obtained for all   blue‐region He  i lines — including the strong triplets λλ4026, 4713, 4471 — at an assumed solar helium abundance, thereby offering a resolution to the problem of the ‘generalized dilution effect’ described by Voels et al. We argue that by extension this result may also have implications for the so‐called ‘helium discrepancy’ identified in OB‐type stars by Herrero et al.