Signature of wide-spread clumping in B supergiant winds

Aims. We seek to establish additional observational signatures of the effects of clumping in OB star winds. The action of clumping on strategic wind-formed spectral lines is tested to steer the development of models for clumped winds and thus improve the reliability of mass-loss determinations for massive stars. Methods. The SiIV lambda lambda 1400 resonance line doublets of B0 to B5 supergiants are analysed using empirical line-synthesis models. The focus is on decoding information on wind clumping from measurements of ratios of the radial optical depths (tau(rad)(omega)) of the red and blue components of the SiIV doublet. We exploit in particular the fact that the two doublet components are decoupled and formed independently for targets with relatively low wind terminal velocities. Results. Line-synthesis analyses reveal that the mean ratio of tau(rad)(omega) of the blue to red SiIV components are rarely close to the canonical value of similar to 2 (expected from atomic constants), and spread instead over a range of values between similar to 1 and 2. These results are interpreted in terms of a photosphere that is partially obscured by optically thick structures in the outflowing gas. Conclusions. The spectroscopic signatures established in this study demonstrate the wide-spread existence of wind clumping in B supergiants. The additional information in unsaturated doublet profiles provides a means to quantify the porosity of the winds.