Bright OB stars in the Galaxy - III. Constraints on the radial stratification of the clumping factor in hot star winds from a combined Halpha, IR and radio analysis

TL;DR

This study investigates the radial distribution of clumping in the winds of 19 Galactic O-type stars by analyzing multi-wavelength data, revealing that denser winds are more strongly clumped in their inner regions.

Contribution

It provides the first constraints on how clumping varies with radius in hot star winds using combined Halpha, IR, mm, and radio observations.

Findings

Denser winds show stronger inner-region clumping (factor ~4.1).

Thinner winds have similar clumping levels throughout.

Radio fluxes serve as upper limits for mass-loss rates.

Abstract

Recent results strongly challenge the canonical picture of massive star winds: various evidence indicates that currently accepted mass-loss rates, Mdot, may need to be revised downwards significantly. This is because the most commonly used mass-loss diagnostics are affected by ``clumping'' (small-scale density inhomogeneities), influencing our interpretation of observed spectra and fluxes. Such downward revisions would have dramatic consequences for the evolution of, and feedback from, massive stars, and thus robust determinations of the clumping properties and mass-loss rates are urgently needed. Here, we present a first attempt to constrain the radial stratification of the so-called clumping factor. To this end, we have analyzed a sample of 19 Galactic O-type supergiants/giants, by combining data for Halpha, IR, mm and radio fluxes, and using appropriate analysis methods. Clumping has been included into our analysis in the ``conventional'' way, by assuming the inter-clump matter to be void. Because (almost) all our diagnostics depends on the square of density, we cannot derive absolute clumping factors, but only factors normalized to a certain minimum. This minimum was usually found to be located in the outermost, radio-emitting region, i.e., the radio mass-loss rates are the lowest ones, compared to Mdot derived from Halpha and the IR. The radio rates agree well with those predicted by theory, but are only upper limits, due to unknown clumping in the outer wind. Our most important result concerns a (physical) difference between denser and thinner winds: for denser winds, the innermost region is more strongly clumped than the outermost one (with a normalized clumping factor of 4.1+/-1.4), whereas thinner winds have similar clumping properties in the inner and outer regions.