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 tha t the shapes and strengths of certain saturated HeI lines, in particul ar the triplets lambda lambda 4471, 4713 and the singlet lambda 4921, an sensitive to microturbulent velocities in excess of 5 km s(-1). Wea ker lines, including most of the HeI singlets, are effectively indepen dent of this parameter, as are the Fowler series He II lines lambda la mbda 4199, 4541, 5411. We show that this behaviour is due to interacti on between direct line-broadening effects in the radiative transfer, a nd indirect changes in the atmospheric structure and the populations o f absorbing states. Using an analysis of high-resolution, high signal- to-noise ratio observations of the O9.7 supergiant HD 152003 as an ill ustrative example, we show how the introduction of microturbulence in non-LTE models allows consistent fits to be obtained for all blue-regi on He I lines - including the strong triplets lambda lambda 4026, 4713 , 4471 - at an assumed solar helium abundance, thereby offering a reso lution to the problem of the 'generalized dilution effect' described b y Voels et al. We argue that by extension this result may also have im plications for the so-called 'helium discrepancy' identified in OB-typ e stars by Herrero et al.