Modeling the Rise of Fibril Magnetic Fields in Fully Convective Stars

Abstract

Many fully convective stars exhibit a wide variety of surface magnetism, including starspots and chromospheric activity. The manner by which bundles of magnetic field traverse portions of the convection zone to emerge at the stellar surface is not especially well understood. In the solar context, some insight into this process has been gleaned by regarding the magnetism as consisting partly of idealized thin flux tubes (TFTs). Herewe present the results of a large set of TFT simulations in a rotating spherical domain of convective flows representative of a 0.3 M e main-sequence star. This is the first study to investigate how individual flux tubes in such a star might rise under the combined influence of buoyancy, convection, and differential rotation. A time-dependent hydrodynamic convective flow field, taken from separate 3D simulations calculated with the anelastic equations, impacts the flux tube as it rises. Convective motions modulate the shape of the initially buoyant flux ring, promoting localized rising loops. Flux tubes in fully convective stars have a tendency to rise nearly parallel to the rotation axis. However, the presence of strong differential rotation allows some initially low-latitude flux tubes of moderate strength to develop rising loops that emerge in the near-equatorial region. Magnetic pumping suppresses the global rise of the flux tube most efficiently in the deeper interior and at lower latitudes. The results of these simulations aim to provide a link between dynamo-generated magnetic fields, fluid motions, and observations of starspots for fully convective stars.

Magnetism, dynamo action and the solar-stellar connection

“…Finally, as a complement to these global-scale simulations, Weber and Browning (2016) have recently conducted "thin flux tube" simulations of fully convective stars, examining the evolution of fibril fields that are presumed (in their model) to be produced in the interior by dynamo action. Their simulations use methods borrowed from the large body of work in the Solar context that has employed this approximationsee, e.g., Sect.…”

“…(If strong enough internal shear is present, the rising flux tubes can instead emerge closer to the equator.) Thus, the polar spots inferred at the surface of M-dwarfs might conceivably arise either from global-scale dipole fields that locally diminish convective heat transport (as in Yadav et al 2015b), or from a collection of smaller-scale flux tubes that are built in the interior and emerge (as in Weber and Browning 2016), or a combination of both. …”

Connecting the large- and the small-scale magnetic fields of solar-like stars

“…They result from magnetically buoyant flux tubes rising through the convection zone, e.g. Fan (2001); Holzwarth et al (2007); Weber & Browning (2016). The solar flux emergence pattern is characterised by bipolar sunspot pairs that appear between ±35 • latitudinal range (Priest 1982).…”

The Suppression and Promotion of Magnetic Flux Emergence in Fully Convective Stars

“…These simulations 2 Weber et al 2017 assume the dynamo has already built fibril magnetic flux tubes that rise under the combined effects of buoyancy and advection by turbulent flows. Weber & Browning (2016) (hereafter WB16) recently investigated for the first time how flux tubes in a fully convective M dwarf might rise under the joint effects of buoyancy, differential rotation, and convection. The work presented here expands upon the parameter space explored in WB16 by initiating flux tubes at multiple depths between 0.475-0.75R to sample the varying convective flow field structure across this region.…”

“…Some insight into this flux emergence mechanism has been gained by assuming bundles of magnetic field can be represented by idealized thin flux tubes (TFTs). Weber & Browning (2016) have recently investigated how individual flux tubes might evolve in a 0.3M⊙ M dwarf by effectively embedding TFTs in time-dependent flows representative of a fully convective star. We expand upon this work by initiating flux tubes at various depths in the upper ∼50-75% of the star in order to sample the differing convective flow pattern and differential rotation across this region.…”