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 (TFT). 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 solar-mass, 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.Comment: 20 pages, 15 figures, accepted to Astrophysical Journa

Browning, Matthew K.

Weber, Maria A.

The Astrophysical Journal

English

arXiv

(A) The file wb2016_tables.tar.gz contains a readme file and nine ASCII text files with columns of selected data output from the thin flux tube simulations presented in WB16. More information regarding the data in these files is contained in a readme_notes.txt file. Figures 11, 12, and 13 in WB16 can be generated directly from these data tables. To access more simulation output, send requests to the authors. (B) Files tube_r50per_b30_th05_TLfC3.jpg and tube_r75per_b30_th05_TLfC3.jpg are images of representative magnetic flux tubes, rendered once some portion has reached the simulation upper boundary at 95% of the total stellar radius. These images correspond to Figures 9a and 9d of WB16, respectively. Each tube has an initial magnetic field strength of 30 kG and latitude of 5 degrees, with the former originating at 50% of the stellar radius and the latter at 75%. The flux tube is colored according to the local magnetic field strength, and is given a 3D extent according to the local cross-sectional radius. As the flux tube evolves, convective motions modulate the shape of the initially toroidal ring, promoting buoyantly rising loops. It is speculated that magnetic structures such as these may give rise to visible starspots on stellar surfaces. (C) Movie of a flux tube rising through the interior of a fully convective star (movie_ASHTFT_r75per_b30_th05_TLfC3.mp4), corresponding to the flux tube in tube_r75per_b30_th05_TLfC3.jpg. Time-varying flows modulate the initially toroidal flux tube. Strong downflows pin portions of the tube to deeper layers, while strong upflows may boost portions toward the surface. The flux tube is colored according to its magnetic field strength, with bluer tones representing stronger magnetic field and yellower tones representing weaker magnetic fields. The convective radial velocity field is also shown, with the strongest downflows in blue and strongest upflows in red. Only a portion of the Northern hemisphere is depicted, from the equator to half of the stellar radius in the vertical direction. Each frame represents about 1.3 days of evolution, with the video elapsing about 110 days. The video ends once the fastest rising portion of the flux tube reaches 95% of the total stellar radius, where the simulation terminates. The orientation has been rotated about 90 degrees from that in tube_r75per_b30_th05_TLfC3.jpg such that the fastest rising loop appears in the center of the visualization domain near the equator at the end of the movie. (D) Giant cell convective flows (movie_ASHvr_130days.mp4) from a 3D simulation of fluid motions representative of a fully convective 0.3 solar-mass, main sequence star computed through the Anelastic Spherical Harmonic (ASH) code. The radial velocity field is shown, with the blue tones representing strong downflows and red tones representing strong upflows. Each frame is separated by about 1.3 days of evolution, with only the Northern hemisphere shown. These flows advect flux tubes in the simulations of WB2016, promoting buoyantly rising structures that may be progenitors of starspots. The duration of the video is about 130 days.The article associated with this dataset is available in ORE at: http://hdl.handle.net/10871/23109This repository contains selected output and visualizations from the work presented in Weber & Browning, 2016 (WB16), now published in The Astrophysical Journal. The work investigates how individual flux tubes in a fully convective 0.3 solar-mass star might rise under the combined influence of buoyancy, convection, and differential rotation. Items B, C, and D are derived from the work published in WB16, but were not included in the article. Images and movies were produced by VAPOR (www.vapor.ucar.edu), a product of the Computational Information Systems Laboratory at the National Center for Atmospheric Research (Boulder, Colorado, USA).This work was supported by the European Research Council under ERC grant agreement no. 337705 (CHASM) and by a Consolidated Grant from the UK STFC (ST/J001627/1). Some of the calculations for this paper were performed on the DiRAC Complexity machine, jointly funded by STFC and the Large Facilities Capital Fund of BIS, and the University of Exeter supercomputer, a DiRAC Facility jointly funded by STFC, the Large Facilities Capital Fund of BIS, and the University of Exeter

Open Research Exeter

Modeling the Rise of Fibril Magnetic Fields in Fully Convective Stars (Dataset and Additional Visualizations)

This is the final version of the article. Available from the publisher via the DOI in this record.The dataset associated with this article is available in ORE at: http://hdl.handle.net/10871/25290Many 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 (TFT). 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 solar-mass, 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.This work was supported by the European Research Council
under ERC grant agreement no. 337705 (CHASM) and by a
Consolidated Grant from the UK STFC (ST/J001627/1).
Some of the calculations for this paper were performed on the
DiRAC Complexity machine, jointly funded by STFC and the
Large Facilities Capital Fund of BIS, and the University of
Exeter supercomputer, a DiRAC Facility jointly funded by
STFC, the Large Facilities Capital Fund of BIS, and the
University of Exeter

Modeling the Rise of Fibril Magnetic Fields in Fully Convective Stars

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