Star-in-a-box simulations of fully convective stars

Abstract

Context. Main-sequence late-type stars with masses of less than 0.35 M⊙ are fully convective. Aims. The goal is to study convection, differential rotation, and dynamos as functions of rotation in fully convective stars. Methods. Three-dimensional hydrodynamic and magnetohydrodynamic numerical simulations with a star-in-a-box model, in which a spherical star is immersed inside of a Cartesian cube, are used. The model corresponds to a 0.2 M⊙ main-sequence M5 dwarf. A range of rotation periods (Prot) between 4.3 and 430 d is explored. Results. The slowly rotating model with Prot = 430 days produces anti-solar differential rotation with a slow equator and fast poles, along with predominantly axisymmetric quasi-steady large-scale magnetic fields. For intermediate rotation (Prot = 144 and 43 days) the differential rotation is solar-like (fast equator, slow poles), and the large-scale magnetic fields are mostly axisymmetric and either quasi-stationary or cyclic. The latter occurs in a similar parameter regime as in other numerical studies in spherical shells, and the cycle period is similar to observed cycles in fully convective stars with rotation periods of roughly 100 days. In the rapid rotation regime the differential rotation is weak and the large-scale magnetic fields are increasingly non-axisymmetric with a dominating m = 1 mode. This large-scale non-axisymmetric field also exhibits azimuthal dynamo waves. Conclusions. The results of the star-in-a-box models agree with simulations of partially convective late-type stars in spherical shells in that the transitions in differential rotation and dynamo regimes occur at similar rotational regimes in terms of the Coriolis (inverse Rossby) number. This similarity between partially and fully convective stars suggests that the processes generating differential rotation and large-scale magnetism are insensitive to the geometry of the star.

A Babcock–Leighton-type Solar Dynamo Operating in the Bulk of the Convection Zone

“…Fan & Fang (2014), Guerrero et al (2016), and Käpylä et al (2017) demonstrate that solar-like large-scale magnetic fields can be produced entirely within a convection zone without extending the simulation down to the tachocline. Käpylä (2021) shows the similarity of large-scale field generation between partially and fully convective stars. Progress in MHD simulations provides an impetus to rethink the roles of the tachocline in the BL-type dynamo.…”

Solar-like Dynamos and Rotational Scaling of Cycles from Star-in-a-box Simulations

“…The star-in-a-box model described in Käpylä (2021) is used; see also Dobler et al (2006). A star of radius R is embedded into a Cartesian cube with side length H = 2.2R.…”

“…The setup of the simulations is otherwise identical to those in Käpylä (2021) except that the amplitude of the radiative conductivity K 0 is enhanced such that in the thermodynamically saturated state, the star has a radiative core (CZ) that encompasses roughly two-thirds (one-third) of the stellar radius. Furthermore, the diffusion coefficients η, ν, and χ SGS have radial profiles such that their values in the radiative core are 10 2 smaller than in the CZ to avoid diffusive spreading of magnetic fields and flows into the core.…”

Solar-like dynamos and rotational scaling of cycles from star-in-a-box simulations

“…The star-in-a-box model described in Käpylä (2021) is used; see also Dobler et al (2006). A star of radius R is embedded into a Cartesian cube with a side length H = 2.2R.…”

“…The set-up of the simulations is otherwise identical to those in Käpylä (2021) except that the amplitude of the radiative conductivity K 0 is enhanced such that in the thermodynamically saturated state the star has a radiative core (CZ) that encompasses roughly two thirds (one third) of stellar radius. Furthermore, the diffusion coefficients η, ν, and χ SGS have radial profiles such Table 1.…”