Angular momentum transport by heat-driven g-modes in slowly pulsating B stars

Abstract

Motivated by recent interest in the phenomenon of waves transport in massive\nstars, we examine whether the heat-driven gravity (g) modes excited in\nslowly-pulsating B (SPB) stars can significantly modify the stars' internal\nrotation. We develop a formalism for the differential torque exerted by g\nmodes, and implement this formalism using the GYRE oscillation code and the\nMESASTAR stellar evolution code. Focusing first on a $4.21$ $M_\\odot$ model, we\nsimulate 1,000 years of stellar evolution under the combined effects of the\ntorque due to a single unstable prograde g mode (with an amplitude chosen on\nthe basis of observational constraints), and diffusive angular momentum\ntransport due to convection, overshooting, and rotational instabilities.\n We find that the g mode rapidly extracts angular momentum from the surface\nlayers, depositing it deeper in the stellar interior. The angular momentum\ntransport is so efficient that by the end of the simulation the initially\nnon-rotating surface layers are spun in the retrograde direction to\n$\\approx30\\%$ of the critical rate. However, the additional inclusion of\nmagnetic stresses in our simulations, almost completely inhibits this spin-up.\n Expanding our simulations to cover the whole instability strip, we show that\nthe same general behavior is seen in all SPB stars. After providing some\ncaveats to contextualize our results, we hypothesize that the observed slower\nsurface rotation of SPB stars (as compared to other B-type stars) may be the\ndirect consequence of the angular momentum transport that our simulations\ndemonstrate.\n