Smith, KC; Howarth, ID; (1998) Microturbulence in O supergiants. Monthly Notices of the Royal Astronomical Society , 299 (4) 1146 - 1158. 10.1046/j.1365-8711.1998.01873.x.

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Abstract

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.

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