Structure formation in O-type stars and Wolf-Rayet stars

TL;DR

This study investigates the origin of small-scale turbulent structures in the envelopes of massive O and Wolf-Rayet stars, finding that these structures are unlikely caused by sub-surface convection but may result from acoustic or Rayleigh-Taylor instabilities.

Contribution

The paper combines linear stability analysis with 2D simulations to identify the likely mechanisms driving turbulence in massive star envelopes, challenging the common assumption of sub-surface convection as the primary source.

Findings

Structures grow with wavenumber, inconsistent with convective instability.

Simulations suggest acoustic or Rayleigh-Taylor instabilities as possible origins.

Analytical growth rates differ from simulation results, indicating complex dynamics.

Abstract

Turbulent small-scale structures in the envelopes and winds of massive stars have long been suggested as the cause for excessive line broadening that could not be explained by other mechanisms such as thermal broadening. However, the origin of these structures, particularly in the envelope, has not been extensively studied. We study the origin of structures seen in 2D unified stellar atmosphere and wind simulations of O stars and Wolf-Rayet (WR) stars. Particularly, we study whether the structure growth in the simulations is consistent with sub-surface convection, as is commonly assumed to be the origin of this turbulence. Using a linear stability analysis of the optically thick envelopes of massive stars, we identified multiple instabilities that could drive structure growth. We quantified the structure growth in the non-linear simulations of O stars and WR stars by computing density power spectra and tracking their temporal evolution. Then, we compared these results to the analytical results from the stability analysis. The stability analysis leads to two possible instabilities: the convective instability and an acoustic instability. Analytic expressions for the growth rates of these different instabilities are found. In particular, strong radiative diffusion damps the growth rate $\omega$ of the convective instability leading to a distinct $\omega \sim 1/k^2$ dependence on wavenumber $k$. From our power spectra analysis of the simulations, however, we find that structure growth rather increases with $k$ - tentatively as $\omega \sim \sqrt{k}$. Our results suggest that structures in luminous O and WR star envelopes do not primarily develop from the sub-surface convective instability. Rather the growth seems compatible with either the acoustic instability in the radiation-dominated regime or with Rayleigh-Taylor type instabilities, although the exact origin remains inconclusive for now.