Effects on v sin i determinations of O stars from 3D model atmospheres with high turbulent velocities

When studying massive stars and their life cycles, rotation plays a key role. Hence, understanding the rotation of these stars is crucial when determining their properties, or for constraining evolutionary models. We examined the reliability of the standard methods to derive projected rotation speeds ̌arv i from photospheric spectra of hot massive stars in the presence of high turbulent velocities. We included rotation in the spectral synthesis of O stars by means of three-dimensional (3D) model atmospheres showing significant photospheric turbulent velocities. We then used these as mock observations to back-test the Fourier transform (FT) and goodness-of-fit (GOF) methods commonly used for empirical determination of ̌arv i when the turbulent velocity field is not known. When the expected ̌arv , with the method is no longer reliable. Results from the GOF method show that if one parameter is not significantly larger than the other, empirical best-fit values may be located in a large region of the ̌arv , C = , with C_i i > ̊m macro macro the macroturbulent velocity, FT ̌arv i determinations (most of the time) give reasonable results. However, if ̌arv i < ̊m macro i -- macro parameter space, independent of the true values. The degenerate region closely follows the shape of the empirical formula found by Howarth07 ̊m macro ^2/2+(̌arv i)^2/4 C_e for C_i the inferred value and C_e the expected. Our analysis shows clearly that, generally, only the sum sqrt ̊m macro ^2/2+(̌arv i)^2/4 can be approximated through the standard spectroscopic methods used to infer these parameters individually. Only in the case where one of the two clearly dominates, can good constraints on the dominating parameter be derived. This demonstrates that previously found empirical correlations between ̌arv i and macro as well as the derived statistical distributions of the observed rotation rates for O-star populations will need to be re-analysed and re-interpreted.