Simulations of Dynamo Action in Fully Convective Stars

Abstract

We present three-dimensional nonlinear magnetohydrodynamic simulations of the interiors of fully convective M dwarfs. Our models consider 0.3 solar-mass stars using the Anelastic Spherical Harmonic code, with the spherical computational domain extending from 0.08 to 0.96 times the overall stellar radius. Like previous authors, we find that fully convective stars can generate kG-strength magnetic fields (in rough equipartition with the convective flows) without the aid of a tachocline of shear. Although our model stars are everywhere unstably stratified, the amplitudes and typical pattern sizes of the convective flows vary strongly with radius, with the outer regions of the stars hosting vigorous convection and field amplification while the deep interiors are more quiescent. Modest differential rotation is established in hydrodynamic calculations, but-unlike in some prior work-strongly quenched in MHD simulations because of the Maxwell stresses exerted by the dynamo-generated magnetic fields. Despite the lack of strong differential rotation, the magnetic fields realized in the simulations possess significant mean (axisymmetric) components, which we attribute partly to the strong influence of rotation on the slowly overturning flows.

Convection and differential rotation properties of G and K stars computed with the ASH code

“…A similarly "banded" differential rotation pattern is evident in a number of previously published ASH simulations (e.g. Bessolaz & Brun 2011;Browning 2008). The differential rotation in the 0.7 and 0.9 M cases are the most solar-like, with the fastest angular rotation rate at the equator and slowest at higher latitudes.…”

“…This code has been extensively tested and used for computing several aspects of the solar interior (e.g., Browning et al 2006;Brun & Toomre 2002;Brun et al 2004;DeRosa et al 2002;Miesch et al 2006), rapidly rotating young stars (Ballot et al 2007;Brown 2009;Brown et al 2008Brown et al , 2011, the convective cores of massive stars (Browning et al 2004a,b;Featherstone et al 2009), fully convective low mass stars (Browning 2008), red giant stars (Brun & Palacios 2009), and pre-main-sequence stars (Bessolaz & Brun, in this volume;Bessolaz & Brun 2011). We briefly describe the basic aspects of the code here, but the reader can find further details of the code in those previous works (see especially, Brun et al 2004;Clune et al 1999).…”

Commission 42: Close Binary Stars

“…They appear to be able to explain the observed effects quite well with reasonable magnetic field strengths and typical spot filling factors (Chabrier et al 2007). Similar progress has been made in our understanding of the origin and strength of magnetic fields in fully convective stars that lack a tachocline (Browning 2008).…”

Towards understanding the global magnetism of the Sun and solar-like stars

“…found for rotation rate above 35 kms −1 , for K type stars at about 10 kms −1 and for M dwarfs around 3-4 kms −1 (Browning 2008;Reiners, Basri & Browning 2009). How stellar magnetic flux scales with rotation rate is thus also important to understand since it is telling us how the magnetic field generated by dynamo action inside the stars emerges and imprints the stellar surface (Rempel 2008) and if it actually saturates.…”