We have analyzed the far-ultraviolet spectrum of two Galactic O4 stars , the O4If+ supergiant HD 190429A and the O4V ( ( f ) ) dwarf HD 96715 , using archival FUSE and IUE data .
We have conducted a quantitative analysis using the two NLTE model atmosphere and wind codes , Tlusty and CMFGEN , which incorporate a detailed treatment of NLTE metal line blanketing .
From the far-UV spectrum , we have derived the stellar and wind parameters and the surface composition of the two stars .
The surface of HD 190429A has a composition typical of an evolved O supergiant ( nitrogen-rich , carbon and oxygen-poor ) , while HD 96715 exhibits surface nitrogen enhancement similar to the enrichment found in SMC O dwarfs which has been attributed to rotationally-induced mixing .
Following studies of Magellanic Cloud O stars , we find that homogeneous wind models could not match the observed profile of O v \lambda 1371 and require very low phosphorus abundance to fit the P v \lambda \lambda 1118-1128 resonance lines .
We show , on the other hand , that we are able to match the O v and P v lines using clumped wind models .
In addition to these lines , we find that N iv \lambda 1718 is also sensitive to wind clumping .
For both stars , we have calculated clumped wind models that match well all these lines from different species and that remain consistent with H \alpha data .
In particular , we have achieved an excellent match of the P v resonance doublet , indicating that our physical description of clumping is adequate .
These fits therefore provide a coherent and thus much stronger evidence of wind clumping in O stars than earlier claims .
We show that the success of the clumped wind models in matching these lines results from increased recombination in the clumps , hence from a better description of the wind ionization structure .
We find that the wind of these two stars is highly clumped , as expressed by very small volume filling factors , namely f _ { \infty } = 0.04 for HD 190429A and f _ { \infty } = 0.02 for HD 96715 .
In agreement with our analysis of SMC stars , clumping starts deep in the wind , just above the sonic point .
The most crucial consequence of our analysis is that the mass loss rates of O stars need to be revised downward significantly , by a factor of 3 and more .
These lower mass loss rates will affect substantially the evolution of massive stars .
Accounting for wind clumping is essential when determining the wind properties of O stars .
Our study therefore calls for a fundamental revision in our understanding of mass loss and of O-type star stellar winds .