Calibrating chemical mixing induced by internal gravity waves based on hydrodynamical simulations; The chemical evolution of OB-type stars

Abstract

Internal gravity waves (IGWs) have been shown to contribute to the transport of chemical elements in stars with a convective core and radiative envelope. Recent 2D hydrodynamical simulations of convection in intermediate-mass stars have provided estimates of the chemical mixing efficiency of such waves. The chemical diffusion coefficient from IGW mixing is described by a constant A times the squared wave velocity. The value of A, however, remains unconstrained by such simulations. This work aims at investigating what values A can take in order to reproduce the observed nitrogen surface abundances of the most nitrogen-enriched massive stars. Furthermore, we discuss the prevalence of IGW mixing compared to rotational mixing. We provide an implementation of these mixing profiles predicted from hydrodynamical simulations in the one-dimensional stellar evolution code MESA. We compute evolution tracks for stars between 3 and 30Msun with this new implementation for IGW mixing and study the evolution for the surface abundances of isotopes involved in the CNO cycle, particularly the N14 isotope. We show that this 1D framework predicting the chemical diffusion coefficient from IGW mixing yields consistent morphologies of the mixing profile in comparison with hydrodynamical simulations. We find that the value of A must increase with mass in order to reproduce the most nitrogen-enriched stars. Assuming these calibrated values for A, mixing by IGWs is a potential mechanism to reproduce well-mixed stars without needing rapid rotation. We have provided observational limits on the efficiency of IGW mixing for future theoretical studies. Future asteroseismic modelling efforts taking IGW mixing into account will be able to place additional constraints on the convective core mass, as our models predict that the convective core should be significantly more massive if IGW mixing is indeed efficient.

Accepted for publication in Astronomy & Astrophysics. 12 pages. Full abstract in PDF

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