Hillier et al. This is a consequence of the fact that in the Pauldrach et al. model, helium is mostly only singly ionized in the outer regions of the wind, so that the photoelectric opacity is very high at soft X-ray energies. The data provide a number of important constraints on the nature and location of the X-ray emitting material in the Pup wind. Of particular interest is the fact that we see a suppression of the forbidden line and enhancement of the intercombination line in the helium-like triplets of nitrogen, oxygen, and neon. We show that this is a natural consequence of the intense ultraviolet radiation field in the wind, and that it allows us to place constraints on the location of the X-ray emitting shocks relative to the star.
Additional constraints come from the emission line velocity profiles.
We show that the data are consistent with the predictions of the Pauldrach et al. model. 2, we describe the details of the RGS observation, the nature of our data reduction and analysis, and the key observational features of the spectrum. 3, we consider the implications of our results for our understanding of Pup, and of the X-ray emission from O stars in general.