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 HeI lines, in particular the triplets lambda lambda 4471, 4713 and the singlet lambda 4921, an sensitive to microturbulent velocities in excess of 5 km s(-1). Weaker lines, including most of the HeI singlets, are effectively independent of this parameter, as are the Fowler series He II lines lambda lambda 4199, 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 lambda lambda 4026, 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.