Constraints on porosity and mass loss in O-star winds from modeling of X-ray emission line profile shapes

TL;DR

This study models X-ray emission line profiles of Zeta Pup to constrain stellar wind porosity and mass loss, finding moderate porosity effects do not significantly alter mass-loss estimates, supporting X-ray spectroscopy as a reliable calibration tool.

Contribution

It demonstrates that moderate porosity effects do not significantly impact X-ray based mass-loss rate estimates in O-star winds, refining previous models.

Findings

Porosity models with flattened clumps fit poorly to observed line shapes.

Isotropic clump porosity models can fit data if porosity is moderate.

Mass-loss rates are only modestly affected (<40%) by porosity effects.

Abstract

We fit X-ray emission line profiles in high resolution XMM-Newton and Chandra grating spectra of the early O supergiant Zeta Pup with models that include the effects of porosity in the stellar wind. We explore the effects of porosity due to both spherical and flattened clumps. We find that porosity models with flattened clumps oriented parallel to the photosphere provide poor fits to observed line shapes. However, porosity models with isotropic clumps can provide acceptable fits to observed line shapes, but only if the porosity effect is moderate. We quantify the degeneracy between porosity effects from isotropic clumps and the mass-loss rate inferred from the X-ray line shapes, and we show that only modest increases in the mass-loss rate (<~ 40%) are allowed if moderate porosity effects (h_infinity <~ R_*) are assumed to be important. Large porosity lengths, and thus strong porosity effects, are ruled out regardless of assumptions about clump shape. Thus, X-ray mass-loss rate estimates are relatively insensitive to both optically thin and optically thick clumping. This supports the use of X-ray spectroscopy as a mass-loss rate calibration for bright, nearby O stars.