Helium-like X-ray line complexes show that the hottest plasma on the O supergiant ζ Puppis is in its wind

Abstract

We present an analysis of Chandra grating spectra of key helium-like line complexes to put constraints on the location with respect to the photosphere of the hottest (T ≳ 6 × 106 K) plasma in the wind of the O supergiant ζ Pup and to explore changes in the 18 years between two sets of observations of this star. We fit two models – one empirical and one wind-shock-based – to the S xv, Si xiii, and Mg xi line complexes and show that an origin in the wind flow, above r ≈ 1.5 R*, is strongly favored over an origin less than 0.3 R* above the photosphere (r ≲ 1.3 R*), especially in the more recent, very long-exposure data set. There is a modest increase in the line and continuum fluxes, line widths, wind absorption signatures, and of the hot plasma’s distance from the photosphere in the 18 years since the first Chandra grating observation of ζ Pup. Both modes of modeling include the effects of dielectronic recombination satellite emission line blending on the helium-like complexes – the first time this has been accounted for in the analysis of He-like line ratios in O stars.

Deriving X-Ray Line Profiles for Massive-star Winds from Momentum-conserving Dynamical Working Surface Solutions

“…Our model implies that to have a Gaussian-like profile from a β = 1 velocity wind, the embedded shocks must be occurring deep within the radiative region near the surface, such as Figure 3 shows for the WS formation radius. Previous studies have inferred such deep radii for the shock radii from He-like fir line ratios (Cassinelli et al 2001;Cohen et al 2022). Wind profile models have also pointed to this, as Cohen et al (2014) found r ≈ 1.5-2 R * to hold across many lines and stars.…”

Survey of X-Rays from Massive Stars Observed at High Spectral Resolution with Chandra

“…These studies tend to fall into two categories: intensive observations of individual or binary stars and coarse observations for larger samples of objects, e.g., characterizing basic X-ray properties for stars in a cluster. A good example of the former would include ζ Pup, with around a ∼1 Ms of data from XMM-Newton (Nazé et al 2012) and a similar amount from Chandra (Cohen et al 2020;Huenemoerder et al 2020;Nichols et al 2021;Cohen et al 2022). The star draws this continued attention because of its relative proximity, X-ray brightness, high mass-loss rate (2.5 × 10 −6 M e yr −1 ; Cohen et al 2020), and high-speed wind v ∞ of 2250 km s −1 (Puls et al 2006;Cohen et al 2010).…”