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We use the line ratios of resolved X-ray resonance and satellite line pairs from Si  xiii and Mg  xi ions to test the widely held assumption that the shocked plasma in the wind of hot stars is in collisional ionization equilibrium. We specifically apply this test to the prototypical O supergiant ζ Puppis, as its line ratios are well resolved in its total 813 ks Chandra observation. This satellite-to-resonance line ratio is known to be a reliable diagnostic of plasma temperature but has yet to be applied to hot stars due to needing a long exposure time to see both lines above the continuum. In testing the assumed plasma state, we also provide evidence against significant ionization and excitation from proton collisions. Since dielectronic recombination (DR) allows somewhat lower energy free electrons to excite resonance lines than by the usual collisional pathways, we also use the DR flux ratios to extend diagnostics of the ionization fraction of the parent ion to lower temperatures than any other method can achieve. Also, the DR ratio supports a concept of an average free electron temperature, for each target ion used, which can constrain heating models. [ABSTRACT FROM AUTHOR]

We fit a new line shape model to Chandra X-ray spectra of the O supergiant ζ Puppis to test the robustness of mass-loss rates derived from X-ray wind line profiles against different assumed heating models. Our goal is to track the hot gas by replacing the common assumption that it is proportional to the cool gas emission measure. Instead of assuming a turn-on radius for the hot gas (as appropriate for the line-deshadowing instability internal to the wind), we parametrize the hot gas in terms of a mean-free path for accelerated low-density gas to encounter slower high-density material. This alternative model is equally successful as previous approaches at fitting X-ray spectral lines in the 5–17 Å wavelength range. We find that the characteristic radius where the hottest gas appears is inversely proportional to line-formation temperature, suggesting that stronger shocks appear generally closer to the surface. This picture is more consistent with pockets of low density, rapid acceleration at the lower boundary than with an internally generated wind instability. We also infer an overall wind mass-loss rate from the profile shapes with a technique used previously in the literature. In doing so, we find evidence that the mass-loss rate derived from X-ray wind line profiles is not robust with respect to changes in the specific heating picture used. [ABSTRACT FROM AUTHOR]

We attempt to determine the driver for clumping in hot-star winds by extending the measure of the spectral variability level of Galactic Wolf–Rayet stars to by far the hottest known among them, the WN2 star WR 2 and the WO2 stars WR 102 and WR 142. These three stars have T⋆ = 140 and 200 kK, the last two being well above the bulk of WR stars with T⋆ ∼ 40–120 kK. This full temperature range for WR stars is much broader than that of their O-star progenitors (∼30–50 kK), so is better suited to look for any temperature dependence of wind clumping. We have obtained multiple observations with high signal-to-noise, moderate-resolution spectroscopy in search of small-scale variability in the strong emission lines from the dense winds of these three extreme stars, and find a very low-level of variability in both stars. Temperature and terminal velocity are correlated, so faster winds show a lower variability, though this trend goes against any predictions made involving Line Deshadowing Instability (LDI) only, implying that instabilities intrinsic to LDI are not the main source of wind clumping. Instead, it could be taken as support for the suggestion that clumps are caused by a subsurface convection zone (SSCZ) at T ∼ 170 kK, since such an SSCZ would have little opportunity to operate under the hydrostatic surface of these hottest WR stars. It is still possible, however, that an SSCZ-related driver could interact with nonlinear line instability effects to enhance or possibly even produce clumps. [ABSTRACT FROM AUTHOR]