Origin and Evolution of Magnetic Field in PMS Stars: Influence of Rotation and Structural Changes

Abstract

During stellar evolution, especially in the PMS, stellar structure and rotation evolve significantly causing major changes in the dynamics and global flows of the star. We wish to assess the consequences of these changes on stellar dynamo, internal magnetic field topology and activity level. To do so, we have performed a series of 3D HD and MHD simulations with the ASH code. We choose five different models characterized by the radius of their radiative zone following an evolutionary track computed by a 1D stellar evolution code. These models characterized stellar evolution from 1 Myr to 50 Myr. By introducing a seed magnetic field in the fully convective model and spreading its evolved state through all four remaining cases, we observe systematic variations in the dynamical properties and magnetic field amplitude and topology of the models. The five MHD simulations develop strong dynamo field that can reach equipartition state between the kinetic and magnetic energy and even superequipartition levels in the faster rotating cases. We find that the magnetic field amplitude increases as it evolves toward the ZAMS. Moreover the magnetic field topology becomes more complex, with a decreasing axisymmetric component and a non-axisymmetric one becoming predominant. The dipolar components decrease as the rotation rate and the size of the radiative core increase. The magnetic fields possess a mixed poloidal-toroidal topology with no obvious dominant component. Moreover the relaxation of the vestige dynamo magnetic field within the radiative core is found to satisfy MHD stability criteria. Hence it does not experience a global reconfiguration but slowly relaxes by retaining its mixed stable poloidal-toroidal topology.

Magnetism, dynamo action and the solar-stellar connection

“…Moss (2003) argues that more massive stars tend to conserve their fossil field more easily than later type stars for which turbulent convection motions have more time to tangle the field to small scales, hence speeding up their Ohmic diffusive decay. More recent work by Arlt (2014) and Emeriau-Viard and Brun (2017) indicate that mixed poloidal/toroidal field may survive this major structural evolution of the star.…”

“…For less massive stars, going through the T-Tauri phase, the situation is different. As the star undergoes an intense fully convective phase, the primordial field captured by the star as it contracts and forms has been reprocessed so efficiently that it is likely forgotten (Moss 2003), see also Emeriau-Viard and Brun (2017). On the main sequence these stars show a contemporary magnetic field that is continuously generated by dynamo action (see Sect.…”

Magnetic and tidal migration of close-in planets

“…The amount of angular momentum to be transferred between the two layers of the star to equilibrate their angular velocities can be expressed as where I r , Ω r are the moments of inertia and the rotation rate of the core, and I c , Ω c are the same quantities assessed in the envelope. Moreover, the expansion of a radiative core during the PMS involves a rapid conversion of convective state to radiative state (Emeriau-Viard & Brun 2017). During this transition phase, a significant mass transfer occurs, which is accompanied by a transport of angular momentum.…”

On the stability of a general magnetic field topology in stellar radiative zones

“…As an example of how fossil fields can form, Figure 1a depicts the process of freezing out the magnetic field as an initially fully convective star evolves along the pre-main-sequence (e.g., Emeriau-Viard & Brun 2017), when the star undergoes gravitational contraction. Once convection has halted in the stably-stratified layers, the field will undergo a slow Ohmic decay if the field has a stable configuration or a fast Alfvénic decay if it is unstable.…”