We have determined accurate values of the product of the mass-loss rate and the ion fraction of P 4+ ,Ṁ q(P 4+ ), for a sample of 40 Galactic O-type stars by fitting stellar-wind profiles to observations of the P v resonance doublet obtained with FUSE, ORFEUS/BEFS, and Copernicus. When P 4+ is the dominant ion in the wind (i.e., 0.5 q(P 4+ ) ≤ 1),Ṁ q(P 4+ ) approximates the mass-loss rate to within a factor of 2. Theory predicts that P 4+ is the dominant ion in the winds of O7-O9.7 stars, though an empirical estimator suggests that the range from O4-O7 may be more appropriate. However, we find that the mass-loss rates obtained from P v wind profiles are systematically smaller than those obtained from fits to Hα emission profiles or radio free-free emission by median factors of ∼130 (if P 4+ is dominant between O7 and O9.7) or ∼20 (if P 4+ is dominant between O4 and O7). These discordant measurements can be reconciled if the winds of O stars in the relevant temperature range are strongly clumped on small spatial scales. We use a simplified two-component model to investigate the volume filling factors of the denser regions. This clumping implies that mass-loss rates determined from "ρ 2 " diagnostics have been systematically over-estimated by factors of 10 or more, at least for a subset of O stars. Reductions in the mass-loss rates of this size have important implications for the evolution of massive stars and quantitative estimates of the feedback that hot-star winds provide to their interstellar environments.

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Context. Cyg OB2 #9 is one of a small set of non-thermal radio emitting massive O-star binaries. The non-thermal radiation is due to synchrotron emission in the colliding-wind region. Cyg OB2 #9 has only recently been discovered to be a binary system, and a multi-wavelength campaign was organized to study its 2011 periastron passage. Aims. We want to better determine the parameters of this system and model the wind-wind collision. This will lead to a better understanding of the Fermi mechanism that accelerates electrons up to relativistic speeds in shocks and its occurrence in collidingwind binaries. We report here on the results of the radio observations obtained in the monitoring campaign and present a simple model to interpret the data. Methods. We used the Expanded Very Large Array (EVLA) radio interferometer to obtain 6 cm and 20 cm continuum fluxes during the Cyg OB2 #9 periastron passage in 2011. We introduce a simple model to solve the radiative transfer in the stellar winds and the colliding-wind region, and thus determine the expected behaviour of the radio light curve. Results. The observed radio light curve shows a steep drop in flux sometime before periastron. The fluxes drop to a level that is comparable to the expected free-free emission from the stellar winds, suggesting that the non-thermal emitting region is completely hidden at that time. After periastron passage, the fluxes slowly increase. We use the asymmetry of the light curve to show that the primary has the stronger wind. This is somewhat unexpected if we use the astrophysical parameters based on theoretical calibrations. But it becomes entirely feasible if we take into account that a given spectral type-luminosity class combination covers a range of astrophysical parameters. The colliding-wind region also contributes to the free-free emission, which can help explain the high values of the spectral index seen after periastron passage. Combining our data with older Very Large Array (VLA) data allows us to derive a period P = 860.0 ± 3.7 days for this system. With this period, we update the orbital parameters that were derived in the first paper of this series. Conclusions. A simple model introduced to explain only the radio data already allows some constraints to be put on the parameters of this binary system. Future, more sophisticated, modelling that will also include optical, X-ray, and interferometric information will provide even better constraints.

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We analyze a 162 ks HETG Chandra observation of the O7.5 III(n)((f)) star ξ Per, together with contemporaneous Hα observations. The X-ray spectrum of this star is similar to other single O stars, and not pathological in any way. Its UV wind lines are known to display cyclical time variability, with a period of 2.086 days, which is thought to be associated with co-rotating interaction regions (CIRs). We examine the Chandra and Hα data for variability on this time scale. We find that the X-rays vary by ∼ 15% over the course of the observations and that this variability is out of phase with variable absorption on the blue wing of the Hα profiles (assumed to be a surrogate for the UV absorption associated with CIRs). While not conclusive, both sets of data are consistent with models where the CIRs are either a source of X-rays or modulate them.

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Aims. We seek to establish evidence in UV P Cygni line profiles that the signs of wind clumping and porosity vary with velocity. We aim to demonstrate empirically that while at most wind velocities optically thick clumps cover only a fraction of the stellar surface, close to the terminal velocity (v ∞ ) where narrow absorption components (NACs) appear in UV lines the covering factor is approximately unity. Methods. SEI line-synthesis models are used to determine the radial optical depths (τ rad (w)) of blue and red components of the Siiv λλ1400 resonance line doublet in a sample of 12 B0 to B4 supergiants. We focus on stars with well developed NACs and relatively low v ∞ so that the Siiv doublet components can be treated as radiatively decoupled and formed independently. Results. For all 12 stars the mean optical depth ratio of the blue to red components is closer to ∼2 (i.e. the ratio of oscillator strengths) in the NACs than at intermediate and lower velocities. The product of mass-loss rate and ion fraction (Ṁ q(Si 3+ )) calculated from the NAC optical depths is a factor of ∼2 to 9 higher compared to mass-loss values sampled at ∼0.4 to 0.6 v ∞ . Conclusions. Since the wind effectively becomes "smooth" at the high NAC velocities and the column density is uniformly distributed over the stellar disk, the optical depths of the NACs are not seriously affected by porosity and this feature thus provides the most reliable measurement of mass-loss rate in the UV lines. Applications of this result to the weak-wind problem of late O-dwarf stars and the "P v mass loss discordance" in early O supergiants are discussed.