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

M. Cantiello$^1$, N. Langer$^{1,2}$, I. Brott$^1$, A. de Koter$^{1,3}$, S. N. Shore$^4$, J. S. Vink$^5$, A. Voegler$^1$, D. J. Lennon$^6$, S.-C. Yoon$^7$

1- Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC, Utrecht, The Netherlands;
2 - Argelander-Institut f"ur Astronomie der Universit"at Bonn, Auf dem H"ugel 71, 53121 Bonn, Germany;
3 - Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands;
4 - Dipartmento di Fisica ``Enrico Fermi'', Universit`a di Pisa, via Buonarroti 2, Pisa 56127 and INFN - Sezione di Pisa, Italy;
5 - Armagh Observatory, College Hill, Armagh, BT61 9DG, Northern Ireland (UK);
6 - Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA;
7 - Department of Astronomy & Astrophysics, University of California, Santa Cruz, High Street, Santa Cruz, CA 95064, USA

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 $10^{3.2}$L$odot$, $10^{3.9}$L$odot$, and $10^{4.2}$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.

Reference: Astronomy & Astrophysics
Status: Manuscript has been accepted