Surface magnetism of cool stars

Kochukhov, O.; Petit, P.; Strassmeier, K. G.; Carroll, T. A.; Farès, R.; Folsom, C. P.; Jeffers, S. V.; Korhonen, H.; Monnier, J. D.; Morin, J.; Rosén, Lisa; Roettenbacher, Rachael M.; Shulyak, D.

doi:10.1002/asna.201713310

Cited by 25 publications

(10 citation statements)

References 87 publications

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“…Previous studies applied this diagnostic method to atomic and molecular lines in the Stokes I spectra of many low-mass stars and brown dwarfs, detecting fields with typical strengths of 2-4 kG (Johns-Krull & Valenti 1996;Reiners & Basri 2007;Shulyak et al 2014). In a recent study by Shulyak et al (in prep., see also summary in Kochukhov et al 2017) magnetic fields of up to 6.4 kG were found in several active, rapidly rotating (P rot = 0.4-0.8 d) M4-6 dwarfs. The GJ65 components are spinning about a factor of two faster than any of the M dwarf stars with direct magnetic field measurements in the literature.…”

Section: Total Magnetic Fluxmentioning

confidence: 84%

The Global and Small-scale Magnetic Fields of Fully Convective, Rapidly Spinning M Dwarf Pair GJ65 A and B

Kochukhov

Lavail

2017

ApJL

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The nearby M dwarf binary GJ65 AB, also known as BL Cet and UV Cet, is a unique benchmark for investigation of dynamo-driven activity of low-mass stars. Magnetic activity of GJ65 was repeatedly assessed by indirect means, such as studies of flares, photometric variability, X-ray and radio emission. Here we present a direct analysis of large-scale and local surface magnetic fields in both components. Interpreting high-resolution circular polarisation spectra (sensitive to a large-scale field geometry) we uncovered a remarkable difference of the global stellar field topologies. Despite nearly identical masses and rotation rates, the secondary exhibits an axisymmetric, dipolar-like global field with an average strength of 1.3 kG while the primary has a much weaker, more complex and non-axisymmetric 0.3 kG field. On the other hand, an analysis of the differential Zeeman intensification (sensitive to the total magnetic flux) shows the two stars having similar magnetic fluxes of 5.2 and 6.7 kG for GJ65 A and B, respectively, although there is an evidence that the field strength distribution in GJ65 B is shifted towards a higher field strength compared to GJ65 A. Based on these complementary magnetic field diagnostic results we suggest that the dissimilar radio and X-ray variability of GJ65 A and B is linked to their different global magnetic field topologies. However, this difference appears to be restricted to the upper atmospheric layers but does not encompass the bulk of the stars and has no influence on the fundamental stellar properties.

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Section: Total Magnetic Fluxmentioning

confidence: 84%

The Global and Small-scale Magnetic Fields of Fully Convective, Rapidly Spinning M Dwarf Pair GJ65 A and B

Kochukhov

Lavail

2017

ApJL

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“…Like the Sun, virtually every cool star exhibits magnetic activity. This magnetic activity is observable, for example, as X-ray and chromospheric emissions, as light-curve variability owing to starspots or flares, or by direct magnetic field measurements (for reviews see, e.g., Pagano et al 2006;Donati & Landstreet 2009;Reiners 2012;Giampapa 2016;Kochukhov et al 2017). Such activity is mostly due to the large-scale magnetism as found in active regions on the Sun, which harbor large-scale magnetic flux concentrations in the form of sunspots.…”

Section: Introductionmentioning

confidence: 99%

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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“…And, beyond the solar neighborhood, it provides important clues to test whether the Sun can truly serve as a reliable stellar evolution calibrator, or whether it instead is an outlier among stars of its age and mass. In the latter case, it is crucial, also for advancing the field of astrobiology, to clarify in which aspects the Sun may be exceptional, and whether its magnetic activity represents one of its peculiarities (e.g., Adibekyan et al ; Kochukhov et al ).…”

Section: Introductionmentioning

confidence: 99%

The variability of magnetic activity in solar‐type stars

et al. 2017

Self Cite

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This article reviews the current knowledge and status of investigations on the variable magnetic activity of cool stars. We discuss the Sun in the context of solar-type stars, highlighting peculiarities and common features in terms of its magnetic activity and variability over different time scales. We examine how both theory and observations are providing new clues about the main physical processes that generate magnetic fields in the interior of cool stars, as well as about those that lead to evolving stellar surface magnetism and varying chromospheric and coronal phenomena. We then proceed to discuss the relations between stellar age, rotation, and activity throughout the evolution of cool stars. Finally, we touch upon the importance of understanding stellar magnetism also in view of its effect on planetary environments.

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Surface magnetism of cool stars

Cited by 25 publications

References 87 publications

The Global and Small-scale Magnetic Fields of Fully Convective, Rapidly Spinning M Dwarf Pair GJ65 A and B

The Global and Small-scale Magnetic Fields of Fully Convective, Rapidly Spinning M Dwarf Pair GJ65 A and B

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

The variability of magnetic activity in solar‐type stars

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