Microturbulence in O supergiants |
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Authors: | Smith & Howarth |
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Institution: | Department of Physics 38 Astronomy, University College London, Gower Street, London WC1E 6BT |
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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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Keywords: | line: formation turbulence stars: abundances stars: atmospheres stars: early-type supergiants |
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