Evidence for radial dependence in X-ray producing shocks in O star winds

Abstract We fit a new line shape model to Chandra X-ray spectra of the O supergiant ζ Puppis to investigate the spatial distribution of embedded shocks in the wind. 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. We find that this alternative model is equally successful as previous approaches at fitting X-ray spectral lines in the 5 – 17 Å wavelength range. The wind properties described by our model are consistent with the literature, while providing new insights into the spatial heating distribution of the shocked gas. We find that the characteristic radii 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 find that the overall wind mass-loss rates, which we infer from the profile shapes using our heating model, are broadly consistent with prior results, showing that X-ray spectra provide a means of inferring wind mass-loss rates that is robust with respect to changes in the specific heating picture used.