Upper Mass-Loss Limits and Clumping in the Intermediate and Outer Wind Regions of OB stars

Abstract

We probe the radial clumping stratification of OB stars in the intermediate and outer wind regions (r>~2 R*) to derive upper limits for mass-loss rates, and compare to current mass-loss implementation. Together with archival multi-wavelength data, our new far-infrared continuum observations for a sample of 25 OB stars (including 13 B Supergiants) uniquely constrain the clumping properties of the intermediate wind region. We derive the minimum radial stratification of the clumping factor through the stellar wind, fclmin(r), and the corresponding maximum mass-loss rate, Mdotmax, normalising clumping factors to the outermost wind region (clfar=1). The clumping degree for r>~2 R* decreases or stays constant with increasing radius for almost the whole sample. There is a dependence on luminosity class and spectral type at the intermediate region relative to the outer ones: O Supergiants (OSGs) present a factor 2 larger clumping factors than B Supergiants (BSGs). The maximum clumping of roughly 1/3 of the OB Supergiants occurs close to the wind base (r<~2 R*) and then decreases monotonically. This contrasts with the more frequent case where the lowermost clumping increases towards a maximum, and needs to be addressed by theoretical models. Additionally, the estimated Mdotmax for BSGs is at least one order of magnitude lower than theoretical values, whereas for OSGs our results and predictions agree within errors. Assuming values of clfar=4-9 from hydrodynamical models would imply a reduction of mass-loss rates included in stellar evolution models by a factor 2-3 for OSGs and by factors 6-200 for BSGs below the first bi-stability jump. This implies large reductions of mass-loss rates applied in evolution-models for BSGs, independently of the actual clumping properties of these winds, and a thorough re-investigation of BSG mass-loss rates and their effects on stellar evolution.