Fundamental parameters of Galactic luminous OB stars V. The effect of microturbulence

We study the effect of microturbulence in the line formation calculations of H and He lines, in the parameter range typical for O and early B stars. We are specially interested in its effect on the determination of stellar parameters: effective temperature, surface gravity and specially on the He abundance. We find that only HeI lines and HeII 4686 are sensibly affected by microturbulence, and that models with lower gravities, the ones suitable for supergiants, are more sensitive to it. We analyze five stars using microturbulence velocities of 0 and 15 km/s. The parameters obtained for 15 km/s differ from the ones at 0 km/s within the limits of the standard error box of our analysis. The quality of the line fits only improves for HeI 4471, but not to the extent of completely solving the so-called dilution effect. Therefore our conclusion is that microturbulence is affecting the derivation of stellar parameters, but its effect is comparable to the adopted uncertainties. Thus it can reduce moderate He overabundances and solve line fit quality differences, but it cannot explain by itself large He overabundances in O stars.