Using Shell Models to Investigate Clumping in the WN4 Star HD 50896

TL;DR

This study uses shell models to analyze clumping in the dense wind of the WN4 star HD 50896, revealing that realistic clumps are smaller and more fragmented than traditional models assume, challenging the volume-filling factor approach.

Contribution

It introduces a shell-based modeling method to test the validity of the volume-filling factor approach in dense WR star winds, showing the need for highly decoherent, small-scale clumps.

Findings

Fully intact shells produce spectral dips inconsistent with observations.

Broken shells with lateral decoherence better match observed spectra.

Clumps are smaller than the Sobolev length, indicating highly fragmented wind structures.

Abstract

The spectra of Wolf-Rayet (WR) stars exhibit strong, broad emission lines that originate in the wind. These winds are radiatively driven and are susceptible to hydrodynamic instabilities that result in the formation of clumps. When modelling spectra of WR stars the volume-filling factor (VFF) approach is usually employed to treat clumpy winds. However, it is based on the assumption that the entire wind mass resides in optically thin clumps, which is not necessarily justifiable in dense winds. To test the validity of the VFF approach we use a previously described method of treating clumping, the ``Shell'' approach, to study line and continuum formation in the dense wind of the WN4 star, HD 50896. Our models indicate that fully intact spherical shells are in tension with observed spectra; a persistent ``dip'' in emission lines occurs at line centre. Removing this dip requires our models to use ``broken'' shells -- shells that are highly decoherent laterally. This insinuates that the wind of HD 50896, and by extension the winds of other WR stars, are comprised of small laterally confined and radially compressed clumps -- clumps smaller than the Sobolev length. We discuss some of the conditions necessary for the VFF approach to be valid.