Across the Tachocline Divide: Convection and Dynamo Action In M-Dwarf Stars

Bice, Connor P.

Graduate Thesis Or Dissertation

Across the Tachocline Divide: Convection and Dynamo Action In M-Dwarf Stars Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/ww72bc97s

Abstract

In this thesis, we explore the convective dynamics taking place in the interiors of M-dwarf stars across the transition from partial to full-sphere convection. Additionally, we search for theoretical explanations for the observed Tachocline Divide, whereby fully convective M-dwarfs seem to maintain their magnetic activity for longer and to flare more aggressively than more massive, partially convective M stars. To this end, we employ the open-source Rayleigh code on massively parallel supercomputers to simulate rotating spherical shells of anelastic magnetohydrodynamic convection in 3D with full nonlinearity. These shells include in their computational domains most of the modeled stars’ convection zones, as well as the underlying radiative zone and tachocline where applicable. We consider the dynamics of more than 80 simulations falling into three primary classes of models: partially convective spectral type M2 (0.4M) stars without underlying tachoclines, partially convective M2 stars with tachoclines, and fully convective M4 (0.3M) stars.

We observe the broad characteristics of dynamo action in the convection zones of both spectral types of M-dwarfs to be shared not only between them, but also with more massive G- and K-stars. We find parity in parameter scalings for the efficiency of their dynamos, the timing of their cycles, and the strength of magnetic feedbacks upon the differential rotation. The effect on the differential rotation is of particular note – in many of the models we compute, the shear is strongly quenched by the intense magnetic fields, leading to nearly solid body rotation in the convection zone. We find that the most rapidly rotating models we considered tended to produce prograde traveling nests of enhanced convection. We observe these nests to interact strongly with the magnetic fields through their strong helical turbulence, triggering reversals of the magnetic field and accelerating flux emergence. In one particularly novel case, nest-driven field reversal sets the cycle period of the global dynamo. Among the M-dwarfs, we find a striking tendency toward single hemisphere iii dynamo states, which seem to grow more prominent with the increasing depth of the convection zone. We argue that these states result from weakly nonlinear coupling between even and odd eigenmodes of the dynamo solution. We find the generation of buoyant magnetic loops to be a general property of our simulations, despite their diffusivity. We develop a machine-learning pipeline for the autonomous isolation of these intricate magnetic structures, which we dub LoopNet.

In consideration of the tachocline divide, we find that the presence of a tachocline in M2 stars significantly increases the prominence of large-scale axisymmetric fields in the global magnetic energy budget. We find that this in turn leads to surface fields which are much more dipolar. Through the spin-down interactions of these magnetic fields with the stellar wind, this trend implies shorter magnetic lifetimes than on M2 stars without tachoclines. However, we find that the surface fields of fully convective M4 stars in our simulations are both stronger and more dipolar than those of even the M2 stars with tachoclines. Even after accounting for stellar sizes, the implication of these findings is that the M4 stars should have shorter magnetic lifetimes than M2 stars. This comes as a surprise, due to the distinct inversion of this prediction in observations. We discuss various avenues by which the tension between our findings and observations might be resolved. Using LoopNet, we find that turbulent stirring by convection in the deep interiors of M4 stars allows them to produce significantly more buoyantly rising magnetic flux ropes than similarly active M2 stars. This result may possibly explain the cause for the enhanced flare rate exhibited by fully convective M-dwarfs when compared to more massive active stars.

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