Simulations of Core Convection in Rotating A‐Type Stars: Magnetic Dynamo Action

Core convection and dynamo activity deep within rotating A-type stars of 2solar masses are studied with 3--D nonlinear simulations. Our modelingconsiders the inner 30% by radius of such stars, thus capturing within aspherical domain the convective core and a modest portion of the surroundingradiative envelope. The MHD equations are solved using the ASH code to examineturbulent flows and magnetic fields, both of which exhibit intricate timedependence. By introducing small seed magnetic fields into our progenitorhydrodynamic models rotating at one and four times the solar rate, we assesshere how the vigorous convection can amplify those fields and sustain themagainst ohmic decay. Dynamo action is indeed realized, ultimately yieldingmagnetic fields that are in energy equipartion with the flow. Such magnetismreduces the differential rotation obtained in the progenitors, partly byMaxwell stresses that transport angular momentum poleward and oppose theReynolds stresses in the latitudinal balance. In contrast, in the radialdirection we find that the Maxwell and Reynolds stresses may act together totransport angular momentum. The central columns of slow rotation established inthe progenitors are weakened, with the differential rotation waxing and waningin strength as the simulations evolve. We assess the morphology of the flowsand magnetic fields, their complex temporal variations, and the manner in whichdynamo action is sustained. Differential rotation and helical convection areboth found to play roles in giving rise to the magnetic fields. The magnetismis dominated by strong fluctuating fields throughout the core, with theaxisymmetric (mean) fields there relatively weak.Comment: 58 pages (ApJ refereeing format), 20 figures (low res), published in ApJ August 2005 (abstract slightly modified to fit in 24 lines limit