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Simulation of the small-scale magnetism in main-sequence stellar atmospheres
Kiepenheuer Inst Sonnenphys, Schoneckstr 6, D-79104 Freiburg, Germany.
Kiepenheuer Inst Sonnenphys, Schoneckstr 6, D-79104 Freiburg, Germany;Ist Ric Solari Locarno IRSOL, Via Patocchi 57 Prato Pernice, CH-6605 Locarno, Switzerland.
Kiepenheuer Inst Sonnenphys, Schoneckstr 6, D-79104 Freiburg, Germany.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 614, article id A78Article in journal (Refereed) Published
Abstract [en]

Context: Observations of the Sun tell us that its granular and subgranular small-scale magnetism has significant consequences for global quantities such as the total solar irradiance or convective blueshift of spectral lines.

Aims: In this paper, properties of the small-scale magnetism of four cool stellar atmospheres, including the Sun, are investigated, and in particular its effects on the radiative intensity and flux.

Methods: We carried out three-dimensional radiation magnetohydrodynamic simulations with the (COBOLD)-B-5 code in two different settings: with and without a magnetic field. These are thought to represent states of high and low small-scale magnetic activity of a stellar magnetic cycle.

Results: We find that the presence of small-scale magnetism increases the bolometric intensity and flux in all investigated models. The surplus in radiative flux of the magnetic over the magnetic field-free atmosphere increases with increasing effective temperature, T-eff, from 0.47% for spectral type K8V to 1.05% for the solar model, but decreases for higher effective temperatures than solar. The degree of evacuation of the magnetic flux concentrations monotonically increases with T-eff as does their depression of the visible optical surface, that is the Wilson depression. Nevertheless, the strength of the field concentrations on this surface stays remarkably unchanged at approximate to 1560G throughout the considered range of spectral types. With respect to the surrounding gas pressure, the field strength is close to (thermal) equipartition for the Sun and spectral type F5V but is clearly sub-equipartition for K2V and more so for K8V. The magnetic flux concentrations appear most conspicuous for model K2V owing to their high brightness contrast.

Conclusions: For mean magnetic flux densities of approximately 50 G, we expect the small-scale magnetism of stars in the spectral range from F5V to K8V to produce a positive contribution to their bolometric luminosity. The modulation seems to be most effective for early G-type stars.

Place, publisher, year, edition, pages
EDP SCIENCES S A , 2018. Vol. 614, article id A78
Keywords [en]
stars: atmospheres, stars: magnetic field, stars: activity, magnetohydrodynamics (MHD), Sun: atmosphere, Sun: magnetic fields
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-358517DOI: 10.1051/0004-6361/201731945ISI: 000435438700003OAI: oai:DiVA.org:uu-358517DiVA, id: diva2:1244624
Available from: 2018-09-03 Created: 2018-09-03 Last updated: 2018-09-03Bibliographically approved

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Freytag, Bernd

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