CIRCE · Control of Instabilities in Rotating flows Conducting Electricity: dynamo seeds and subcritical transition to MHD turbulence in stellar objects.

Modeling magnetic field generation by dynamo instability in stellar objects is a long-standing challenge with far-reaching implications for stellar evolution theory. Underlying motivations are exemplified by the need to understand stellar spin-down and accretion rates in protostellar discs, which are known to be dynamically impacted by magnetic fields. The interest sparked by recurring discrepancies between predictive evolution models and rapidly-progressing observations drives the current research into the characterization of dynamo mechanisms in stellar objects.This important challenge cannot be solved analytically due to the strong nonlinearities of the magnetohydrodynamics (MHD) equations. Solving it therefore requires the development of innovative numerical approaches. In many astrophysical flows, infinitesimal magnetic seeds cannot be amplified by the flow, whereas finite-amplitude magnetic seeds with a favourable spatial structure can drive, through the Lorentz force nonlinear feedback, the very flow motions on which they subsequently feed by subcritical dynamo instability. This situation is particularly relevant for radiative stellar layers or for the innermost regions of protostellar discs, where the history of perturbations can thus define the magnetic fate of the object. Yet, classical stability methods fail to systematically characterize subcritical dynamo solutions and identify their critical dynamo seeds. The CIRCE project will tackle this theoretical obstacle by developing the recent mathematical tools of nonlinear stability analysis, based on adjoint-based optimal control, for MHD flows. The aim of CIRCE is to identify the least-energy perturbations that can trigger subcritical dynamos and transition to MHD turbulence in models of (a) radiative zones and (b) protostellar discs, and to predict how the resulting transitions determine rotational dynamics and accretion rates.