Properties of small-scale magnetism of stellar atmospheres

Steiner, O.; Salhab, R.; Freytag, B.; Rajaguru, S. P.; Schaffenberger, W.; Steffen, M.

doi:10.1093/pasj/psu083

Cited by 13 publications

(22 citation statements)

References 14 publications

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“…Comparing row 12 with row 1 we readily find that the strength of the magnetic flux concentrations, by far, do not reach thermal equipartition for the K-type atmospheres, but reach close to equipartition for the model F5V. In fact, maximal field strengths can reach superequipartition for models G2V and F5V as already noted by Steiner et al (2014). This is now confirmed in the present work, which shows max(|B z (z 0 )|) in row 3.…”

Section: Characteristics Of the Magnetic Flux Concentrationssupporting

confidence: 88%

“…Beeck et al (2011) reported on MHD simulations, starting from the same six models mentioned above but introducing initial homogeneous vertical magnetic fields with strengths of 20 G, 100 G, and 500 G. These authors found distinctive differences between solar-like small-scale magnetic structures and those forming in M dwarfs, where the latter are more pore-like. Steiner et al (2014) analyzed MHD models of spectral types K8V to F5V 1 , concluding that the field strength of the strongest flux concentrations (i) was fairly independent of spectral type when measured at the optical depth level τ R = 1; (ii) assumed thermal superequipartition for the Sun and warmer atmosphere, while staying much weaker for cool atmospheres; and (iii) that their presence correlates with enhanced bolometric radiative intensity and flux. Beeck et al (2015a,b) thoroughly analyzed their mainsequence models of spectral types M2V to F3V and of various initial magnetic field strengths and, inter alia, confirmed the findings (i) to (iii), extending their validity to a wider range of spectral types and magnetic fluxes.…”

Section: Introductionmentioning

confidence: 99%

“…In this paper, we present simulations and their analysis of the same spectral sequence that was considered in the preliminary study of Steiner et al (2014) but now augmented by equivalent comparison runs without magnetic field and for a much longer simulation time for improving reliability. A central question to be answered is whether and to what extent the presence of small-scale magnetism enhances radiative loss from stellar surfaces other than the Sun.…”

Section: Introductionmentioning

confidence: 99%

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Simulation of the small-scale magnetism in main-sequence stellar atmospheres

Salhab

Steiner

Berdyugina

et al. 2018

A&A

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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.

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Section: Characteristics Of the Magnetic Flux Concentrationssupporting

confidence: 88%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Simulation of the small-scale magnetism in main-sequence stellar atmospheres

Salhab

Steiner

Berdyugina

et al. 2018

A&A

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“…This is not a surprising result since the thermal pressure in the photosphere of the simulated white dwarfs is only slightly larger than that in the Sun, and a similar magnetic pressure is necessary to inhibit convective flows. Studies of the impact of magnetic fields on surface convection in the Sun and Sun-like stars by numerous RMHD simulations (Rempel et al 2009;Cheung et al 2010;Freytag et al 2012;Beeck et al 2013;Steiner et al 2014) can also be used to learn about the same process in white dwarfs, even though the origins and large-scale structures of magnetic fields are very different. Figure 2 presents the temperature profiles of our simulations, drawn from the average of T 4 á ñ over surfaces of constant τ R for 12 snapshots.…”

Section: Magnetohydrodynamic Simulationsmentioning

confidence: 99%

On the Evolution of Magnetic White Dwarfs

Tremblay

Fontaine

Freytag

et al. 2015

ApJ

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We present the first radiation magnetohydrodynamic simulations of the atmosphere of white dwarf stars. We demonstrate that convective energy transfer is seriously impeded by magnetic fields when the plasma-β parameter, the thermal-to-magnetic-pressure ratio, becomes smaller than unity. The critical field strength that inhibits convection in the photosphere of white dwarfs is in the range B = 1-50 kG, which is much smaller than the typical 1-1000 MG field strengths observed in magnetic white dwarfs, implying that these objects have radiative atmospheres. We have employed evolutionary models to study the cooling process of high-field magnetic white dwarfs, where convection is entirely suppressed during the full evolution (B  10 MG). We find that the inhibition of convection has no effect on cooling rates until the effective temperature (T eff ) reaches a value of around 5500 K. In this regime, the standard convective sequences start to deviate from the ones without convection due to the convective coupling between the outer layers and the degenerate reservoir of thermal energy. Since no magnetic white dwarfs are currently known at the low temperatures where this coupling significantly changes the evolution, the effects of magnetism on cooling rates are not expected to be observed. This result contrasts with a recent suggestion that magnetic white dwarfs with T eff  10,000 K cool significantly slower than non-magnetic degenerates.

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“…Until a few years ago, MHD simulations of this kind were only available for the Sun. Recently, the first comprehensive 3D radiative MHD simulations for stars other than the Sun were presented (Beeck et al 2011;Wedemeyer et al 2013;Steiner et al 2013Steiner et al , 2014Beeck et al 2015b). …”

Section: Introductionmentioning

confidence: 99%

Three-dimensional simulations of near-surface convection in main-sequence stars

Beeck

Schüßler

Cameron

et al. 2015

A&A

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Context. The convective envelopes of cool main-sequence stars harbour magnetic fields with a complex global and local structure. These fields affect the near-surface convection and the outer stellar atmospheres in many ways and are responsible for the observable magnetic activity of stars. Aims. Our aim is to understand the local structure in unipolar regions with moderate average magnetic flux density. These correspond to plage regions covering a substantial fraction of the surface of the Sun (and likely also the surface of other Sun-like stars) during periods of high magnetic activity. Methods. We analyse the results of 18 local-box magnetohydrodynamics simulations covering the upper layers of the convection zones and the photospheres of cool main-sequence stars of spectral types F to early M. The average vertical field in these simulations ranges from 20 to 500 G. Results. We find a substantial variation of the properties of the surface magnetoconvection between main-sequence stars of different spectral types. As a consequence of a reduced efficiency of the convective collapse of flux tubes, M dwarfs lack bright magnetic structures in unipolar regions of moderate field strength. The spatial correlation between velocity and the magnetic field as well as the lifetime of magnetic structures and their sizes relative to the granules vary significantly along the model sequence of stellar types.

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Properties of small-scale magnetism of stellar atmospheres

Cited by 13 publications

References 14 publications

Simulation of the small-scale magnetism in main-sequence stellar atmospheres

Simulation of the small-scale magnetism in main-sequence stellar atmospheres

On the Evolution of Magnetic White Dwarfs

Three-dimensional simulations of near-surface convection in main-sequence stars

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