Bright OB stars in the Galaxy

Abstract

Context. Recent results strongly challenge the canonical picture of massive star winds: various evidence indicates that currently accepted mass-loss rates,Ṁ, 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. Aims. 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. Methods. To this end, we have analyzed a sample of 19 Galactic O-type supergiants/giants, by combining our own and archival data for H α , 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 cannot derive absolute clumping factors, but only factors normalized to a certain minimum. Results. 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 toṀ derived from H α 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 α turned out to be a useful tool to derive the clumping properties inside r < 3. . .5 R . 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. Conclusions. 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.

Bright OB stars in the Galaxy

“…Since Vink et al predict an increase inṀ of a factor of five, this would imply that the clumping factors for hotter objects are larger by factors of 4 (most optimistic case) to 156 (worst case) compared to those of cooler ones 20 . Given our present knowledge (see Fullerton et al 2006;Puls et al 2006, and references therein), this is not impossible, but raises the question about the physical origin of such a difference. This hypothesis would also imply that all B-SG mass-loss rates are overpredicted, though to a lesser extent for cooler subtypes.…”

“…Since we assume an unclumped wind, the actual mass-loss rates of our sample stars might, of course, be lower. In case of small-scale clumping, this reduction would be inversely proportional to the square root of the effective clumping factor being present in the H α forming region (e.g., Puls et al 2006 and references therein). In Figs.…”

“…Besides the open question of small-scale clumping (which can change the H α morphology quite substantially 12 , cf. Puls et al 2006), the fact that in two of the four problematic cases convincing evidence has been found for the presence of time-dependent large-scale structure (HD 190603, Rivinius et al 1997) A comparison of present results with such from previous studies (Crowther et al 2006;Barlow & Cohen 1977) for three stars in common indicates that the parameters derived by Crowther et al (2006) for HD 190603 and HD 198478 are similar to ours (accounting for the fact that higher T eff and M V result in larger log g andṀ, respectively, and vice versa). The massloss rates from Barlow & Cohen (1977) (derived from the IRexcess!)…”

“…However, recent observational studies of clumping in O-SGs have revealed inconsistencies both between results originating from different wind diagnostics, such as UV resonance lines, H α and the IR-/radioexcess (Fullerton et al 2006;Puls et al 2006), and between theoretical predictions and observed constraints on the radial stratification of the clumping factor (Bouret et al 2005;Puls et al 2006). In addition, there are observational results which imply that clumping might depend on wind density.…”

Long term variability of Cygnus X-1

“…Early-type supergiants show strong line driven winds (CAK mechanism; Castor et al 1975;Morton 1967;Lucy & Solomon 1970) and typical mass loss rates of a few 10 −6 M /yr (e.g., Puls et al 2006). These high velocity winds that can reach terminal velocities ∞ > 2000 km s −1 (e.g., Muijres et al 2012) are perturbed and clumpy (Owocki et al 1988;Feldmeier et al 1997;Puls et al 2006;Oskinova et al 2012; with over 90% of the wind mass concentrated in less than ∼10% of the wind volume (Sako et al 1999;Rahoui et al 2011, for Vela X-1 and Cyg X-1, respectively).…”