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# Sub-surface convection zones in hot massive stars and their observable consequences

Abstract

We study the convection zones in the outer envelope of hot massive stars which are caused by opacity peaks associated with iron and helium ionization. We determine the occurrence and properties of these convection zones as function of the stellar parameters. We then confront our results with observations of OB stars. A stellar evolution code is used to compute a grid of massive star models at different metallicities. In these models, the mixing length theory is used to characterize the envelope convection zones. We find the iron convection zone (FeCZ) to be more prominent for lower surface gravity, higher luminosity and higher initial metallicity. It is absent for luminosities below about $$10^{3.2}{\,{\rm L}_\odot}$$, $$10^{3.9}{\,{\rm L}_\odot}$$, and $$10^{4.2}{\,{\rm L}_\odot}$$ for the Galaxy, LMC and SMC, respectively. We map the strength of the FeCZ on the Hertzsprung-Russell diagram for three metallicities, and compare this with the occurrence of observational phenomena in O stars: microturbulence, non-radial pulsations, wind clumping, and line profile variability. The confirmation of all three trends for the FeCZ as function of stellar parameters by empirical microturbulent velocities argues for a physical connection between sub-photospheric convective motions and small scale stochastic velocities in the photosphere of O- and B-type stars. We further suggest that clumping in the inner parts of the winds of OB stars could be caused by the same mechanism, and that magnetic fields produced in the FeCZ could appear at the surface of OB stars as diagnosed by discrete absorption components in ultraviolet absorption lines.

# Introduction

Massive stars, in a general sense, have convective cores and radiative envelopes (Kippenhahn 1990). The introduction of the so called “

While the envelope convection zones may, at first glance, be negligible for the internal evolution of hot massive stars, they may cause observable phenomena at the stellar surface. The reason is that the zones are located very close to the photosphere for some mass interval (see below). Here, we will discuss which observed features in hot stars might be produced by these near surface convection zones. In particular, we examine whether a link exists between these convective regions and observable small scale velocity fields at the stellar surface and in the stellar wind,“microturbulence”. A similar idea has been used to explain microturbulence in low mass stars (Edmunds 1978), in which deeper envelope convection zones reach the photosphere. While (Edmunds 1978) concludes that the same mechanism cannot explain microturbulent velocities in O and B stars, the iron-peak induced sub-photospheric convection zones in these stars had not yet been discovered. We demonstrate in this paper that these convection zones may not only cause motions which are observable, but possibly even directly affect the evolution: First, we discuss how photospheric velocity fields may affect the structure of massive star winds by inducing clumping at the base of the wind and thereby affecting the stellar mass-loss. And second, we argue that the near surface convection zones may generate magnetic fields which – if they migrate to the surface – further affect the stellar wind mass-loss and, more significantly, the associated stellar angular momentum loss.

We construct grids of massive main sequence star models, for various metallicities, that allow us to predict the occurrence and properties of sub-surface convection zones as function of the stellar parameters (Sect. \ref{results}). We then compare the model predictions with observed stellar properties, e.g., empirically derived microturbulent velocities and observations of wind clumping in hot massive stars (Sect. \ref{comparison}).