Issue |
A&A
Volume 614, June 2018
|
|
---|---|---|
Article Number | A78 | |
Number of page(s) | 19 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361/201731945 | |
Published online | 18 June 2018 |
Simulation of the small-scale magnetism in main-sequence stellar atmospheres
1
Kiepenheuer-Institut für Sonnenphysik,
Schöneckstraße 6,
79104
Freiburg, Germany
e-mail: salhab@leibniz-kis.de;steiner@leibniz-kis.de
2
Istituto Ricerche Solari Locarno (IRSOL),
Via Patocchi 57–Prato Pernice,
6605
Locarno-Monti, Switzerland
3
Department of Physics and Astronomy, Uppsala University,
Box 516,
751 20 Uppsala, Sweden
4
Indian Institute of Astrophysics,
Koramangala II Block,
Bangalore
560 034, India
5
Leibniz-Institut für Astrophysik (AIP),
An der Sternwarte 16,
14482
Potsdam, Germany
Received:
13
September
2017
Accepted:
25
January
2018
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 CO5BOLD 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, Teff, 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 Teff 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 ≈1560 G 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.
Key words: stars: atmospheres / stars: magnetic field / stars: activity / magnetohydrodynamics (MHD) / Sun: atmosphere / Sun: magnetic fields
© ESO 2018
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