Angular Momentum Transport by Internal Gravity Waves across Age

Abstract

We present 2D numerical simulations of convection and waves in a 7M⊙ star across stellar ages ranging from zero age to terminal age main sequence. We show that waves efficiently transport angular momentum across the stellar radiative envelope at young ages. However, as the core recedes, leaving behind a “spike” in the Brunt–Väisälä frequency at the convective–radiative interface, the waves are severely attenuated. This, coupled with the changing stratification throughout the radiation zone, leads to significantly reduced angular momentum transport at later stages on the main sequence. Indeed the angular momentum transport at mid–main sequence is typically 3–4 orders of magnitude lower than at zero age, though we expect this to be somewhat mitigated by the chemical mixing also induced by such waves. We provide measures of the angular momentum transport, both in terms of the divergence of the Reynolds stress and a typical “wave luminosity.” However, we caution that the angular momentum transport drives shear flows, resulting in both slowing and speeding up of radiative interiors. While the values of Reynolds stress and angular momentum transport are only within the context of these limited simulations, they are not significantly different to those found previously using simpler prescriptions, providing some confidence in their applicability.