On the wavelength drift of spectral features from structured hot star winds

Spectral lines formed in stellar winds from OB stars are observed to exhibi t profile variations. Discrete Absorption Components (DACs) show a remarkab ly slow wavelength drift with time. In a straightforward interpretation, th is is in sharp contradiction to the steep velocity law predicted by the rad iation-driven wind theory, and by semi-empirical profile fitting. In the pr esent paper we re-discuss the interpretation of the drift rate. We show tha t the Co-rotating Interaction Region (CIR) model for the formation of DACs does not explain their slow drift rate as a consequence of rotation. On the contrary, the apparent acceleration of a spectral CIR feature is even high er than for the corresponding kinematical model without rotation. However, the observations can be understood by distinguishing between the velocity f ield of the matter ow, and the velocity law for the motion of the patterns in which the DAC features are formed. If the latter propagate upstream agai nst the matter ow, the resulting wavelength drift mimics a much slower acce leration although the matter is moving fast. Additional to the DACs, a seco nd type of recurrent structures is present in observed OB star spectra, the so-called modulations. In contrast to the DACs, these structures show a st eep acceleration compatible with the theoretically predicted velocity law. We see only two possible consistent scenarios. Either, the wind is accelera ted fast, and the modulations are formed in advected structures, while the DACs come from structures which are propagating upstream. Or, alternatively , steep and shallow velocity laws may co-exist at the same time in differen t spatial regions or directions of the wind.