Assessing the Deep Interior Dynamics and Magnetism of A-type Stars

A-type stars have both a shallow near-surface zone of fast convection that can excite acoustic modes and a deep zone of core convection whose properties may be studied through asteroseismology. Many A stars also exhibit large magnetic spots as they rotate. We have explored the properties of core convection in rotating A-type stars and their ability to build strong magnetic fields. These 3-D simulations using the ASH code may serve to inform asteroseismic deductions of interior rotation and magnetism that are now becoming feasible. Our models encompass the inner 30% by radius of a 2 solar mass A-type star, capturing both the convective core and some of the overlying radiative envelope. Convection can drive a column of strong retrograde differential rotation and yield a core prolate in shape. When dynamo action is admitted, the convection is able to generate strong magnetic fields largely in equipartition with the dynamics. Introducing a modest external field (which may be of primordial origin) into the radiative envelope can substantially alter the turbulent dynamics of the convective core, yielding magnetic fields of remarkable super-equipartition strength. The turbulent convection involves a complex assembly of helical rolls that link distant portions of the core and stretch and advect magnetic field into broad swathes of strong toroidal field. These simulations reveal that supercomputing is providing a perspective of the deep dynamics that may become testable with asteroseismology for these stars.