Recent results strongly challenge the canonical picture of massive star winds : various evidence indicates that currently accepted mass-loss rates , { \dot { M } } , may need to be revised downwards , by factors extending to one magnitude or even more .
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 .
We present a first attempt concerning this objective , by means of constraining 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 our own and archival data for H _ { \alpha } , IR , mm and radio fluxes , and using approximate methods , calibrated to more sophisticated models .
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 can not 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 { \dot { M } } derived from H _ { \alpha } 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 .
H _ { \alpha } turned out to be a useful tool to derive the clumping properties inside r < 3 { \ldots } 5 R _ { \star } .
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 \pm 1.4 ) , whereas thinner winds have similar clumping properties in the inner and outer regions . Our findings are compared with theoretical predictions , and the implications are discussed in detail , by assuming different scenarios regarding the still unknown clumping properties of the outer wind .