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

TL;DR

This study uses Chandra spectra to locate the hottest plasma in zeta Puppis's wind, showing it is far from the star's surface and revealing changes over 18 years, with novel modeling of line blending effects.

Contribution

It provides the first analysis of He-like line ratios in O stars accounting for dielectronic recombination satellite blending and constrains the hot plasma's location in zeta Puppis's wind.

Findings

Hot plasma is located above 1.5 R* in the wind.

Line fluxes and widths have increased over 18 years.

Wind absorption signatures are more pronounced in recent data.

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 ($T \gtrsim{6 \times 10^6}$ K) plasma in the wind of the O supergiant zeta 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 $r \approx 1.5$ R$_{\ast}$, is strongly favored over an origin less than 0.3 R$_{\ast}$ above the photosphere ($r \lesssim 1.3$ R$_{\ast}$), 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 zeta 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.