The complex dependencies of Wolf-Rayet winds -- Insights from detailed radiative transfer models

Abstract

With their emission-line dominated spectra, the appearance of Wolf-Rayet stars is shaped by their strong stellar winds. Yet, the physical mechanisms behind their high mass loss have long remained enigmatic. While we know nowadays that radiative driving is sufficient to explain WR-type outflows, a coherent description of them is still lacking, not least to the complex physical conditions invalidating some of the approximations sufficient for other hot-star winds. One promising instrument towards a better understanding of WR winds are comoving-frame, non-LTE stellar atmosphere models including a consistent solution of the hydrodynamics. While so far limited to 1D, their detailed treatment of the radiative transfer and the population numbers is key to overcome the traditional problem of connecting stellar structure models with observed spectra. By creating larger model sequences, we can identify previously unknown scalings and describe trends of WR wind quantities with fundamental stellar parameters and abundances. This article will present a summary of recent insights on WR-type winds, revealing a complex picture with various remaining challenges. Beside covering classical, hydrogen-free WR stars, we present new results to uncover dependencies of later-type WR stars and the presence of hydrogen-containing envelopes. We further discuss oncoming challenges and insights from 2D and 3D RHD simulations which need to be mapped into 1D dynamical atmosphere models.

Figures (3)

• Figure 1: A hydrodynamically-consistent solution for the velocity field in a 1D atmosphere model without requiring force multipliers: The grey solid line denotes the sound speed, while the red and blue solid lines show the inward and outward part the resulting velocity field. The corresponding velocity gradients, which are obtained from the equation of motion, are shown as dashed curves of the same color. The black circles mark the course grid of the atmosphere model to which the obtained solution is eventually mapped.
• Figure 2: Mass-loss rates plotted over different radii for a sequence of hydrodynamically-consistent atmosphere models using a fixed $L$, $M$, and chemical composition: The blue dash-dotted curve denotes the radius at the inner model boundary ($R_\ast$), while the green solid curve shows the radius at the critical point as defined by Eq. (\ref{['eq:hydro']}). The red dashed curve shows the radius of $\tau_\mathrm{Ross} = 2/3$, reflecting where the light in the continuum leaves the star. The black dashed curve shows results from stellar structure calculations with a sonic boundary conditions from Grassitelli+2018.
• Figure 3: Mass-loss rates versus effective temperature at the critical point obtained from sequences of hydrodynamically-consistent atmosphere models assuming different hydrogen-containing envelopes on top of a $20\,M_\odot$ He star. For all models with envelopes, a surface hydrogen mass fraction of $0.2$ (by mass) is assumed. Individual models are shown by dots except for the sequence where an additional shell-burning luminosity of $0.1\,$dex is assumed (red triangles). The upper x-axis denotes $R_\mathrm{crit}$ in $R_\odot$ for the models with $\log L/L_\odot = 5.7$.