Large-scale magnetic topologies of cool stars

Donati, J.; Morin, J.; Delfosse, X.; Forveille, T.; Farès, R.; Moutou, C.; Jardine, M.

doi:10.1063/1.3099082

Cited by 7 publications

(12 citation statements)

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“…In the latest implementation (Donati 2001;Donati et al 2006c), the magnetic field is decomposed in its poloidal and toroidal components, both expressed as spherical harmonics expansions; this newer method is found to be not only much more robust (especially for low-order large-scale fields like dipoles) and more physical (for the field description), but also more convenient (e.g., allowing to fine tune the respective weight of spatial scales) and informative (poloidal and toroidal field components are key ingredients in most theoretical studies on magnetic stars, Mestel 1999). While most efficient for rapidly rotating stars, this method is also applicable to slow rotators, though limited to low-order spherical harmonic modes (e.g., Donati et al 2006cDonati et al , 2008b. Note that magnetic mapping is practical both for stars with no intrinsic field variations and for stars with variable fields, provided that the typical timescale on which the field evolves is long compared to the rotation period.…”

Section: Parametric Modelling and Tomographic Imaging Of Magnetic Fieldsmentioning

confidence: 99%

“…It is also worthwhile noting that X-ray luminosities of M dwarfs (relative to their bolometric luminosities) are roughly equal (at similar Ro) on both sides of the full-convection threshold, while the strengths of their large-scale fields feature a clear discontinuity (at a mass of about 0.4 M ⊙ , Donati et al 2008c). All this suggests that dynamo processes become much more efficient at producing large-scale mainly-axisymmetric poloidal fields essentially as a response to the rapid growth in convective depths with decreasing stellar masses; this is qualitatively compatible with the idea that the geometry of the CZ may control the kind of dynamo wave that a cosmic body can excite (Goudard & Dormy 2008).…”

Section: Benchmarking Dynamo Models With Observations Of Cool Starsmentioning

confidence: 99%

“…It allowed in particular the large-scale field properties of M dwarfs to be investigated for the first time on both sides of the full convection threshold (presumably occurring at spectral type M4, i.e., at a mass of 0.35 M ⊙ , Baraffe et al 1998). Comparing to partly-convective early M dwarfs reveals that the transition in the large-scale field properties is fairly sharp and located at a mass of about 0.4 to 0.5 M ⊙ (Donati et al 2008c),…”

Section: Magnetic Properties Of Cool Stars: Field Strengths Large-scmentioning

confidence: 99%

“…To make it more synthetic, the plot focusses only on a few basic properties of the reconstructed magnetic topologies, namely the reconstructed magnetic energy density e (actually the integral of Results for stars with M ⋆ < 0.2 M ⊙ are preliminary (from Donati et al 2008b). …”

Section: Magnetic Properties Of Cool Stars: Field Strengths Large-scmentioning

confidence: 99%

“…Most cool stars also host magnetic fields, whose large-and medium-scale topologies can be revealed through tomographic techniques (e.g., Landstreet 1992;Donati et al 2008b). Magnetic fields of low-mass stars power a wide variety of energetic phenomena (referred to as activity) such as flares, prominences, coronae and winds; the fields themselves most likely result from the interaction of convection and rotation (the so-called dynamo processes) and are highly variable in nature on timescales ranging from minutes (e.g., flares) to years (e.g., activity cycles).…”

Section: Introductionmentioning

confidence: 99%

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Magnetic Fields of Nondegenerate Stars

Donati

Landstreet

2009

Annu. Rev. Astron. Astrophys.

657

656

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Magnetic fields are present in a wide variety of stars throughout the HR diagram and play a role at basically all evolutionary stages, from very-low-mass dwarfs to very massive stars, and from young star-forming molecular clouds and protostellar accretion discs to evolved giants/supergiants and magnetic white dwarfs/neutron stars. These fields range from a few µ G (e.g., in molecular clouds) to TG and more (e.g., in magnetic neutron stars); in non-degenerate stars in particular, they feature large-scale topologies varying from simple nearly-axisymmetric dipoles to complex non-axsymmetric structures, and from mainly poloidal to mainly toroidal topology.After recalling the main techniques of detecting and modelling stellar magnetic fields, we review the existing properties of magnetic fields reported in cool, hot and young non-degenerate stars and protostars, and discuss our understanding of the origin of these fields and their impact on the birth and life of stars.

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Section: Parametric Modelling and Tomographic Imaging Of Magnetic Fieldsmentioning

confidence: 99%

Section: Benchmarking Dynamo Models With Observations Of Cool Starsmentioning

confidence: 99%

Section: Magnetic Properties Of Cool Stars: Field Strengths Large-scmentioning

confidence: 99%

Section: Magnetic Properties Of Cool Stars: Field Strengths Large-scmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Magnetic Fields of Nondegenerate Stars

Donati

Landstreet

2009

Annu. Rev. Astron. Astrophys.

657

656

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Magnetic torques on T Tauri stars: Accreting versus non-accreting systems

Pantolmos

Zanni

Bouvier

2020

A&A

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Context. Classical T Tauri stars (CTTs) magnetically interact with their surrounding disks, a process that is thought to regulate their rotational evolution. Aims. We compute torques acting on the stellar surface of CTTs that arise from different accreting (accretion funnels) and ejecting (stellar winds and magnetospheric ejections) flow components. Furthermore, we compare the magnetic braking due to stellar winds in two different systems: isolated (i.e., weak-line T Tauri and main-sequence) and accreting (i.e., classical T Tauri) stars. Methods. We use 2.5D magnetohydrodynamic, time-dependent, axisymmetric simulations that were computed with the PLUTO code. For both systems, the stellar wind is thermally driven. In the star-disk-interaction (SDI) simulations, the accretion disk is Keplerian, viscous, and resistive, and is modeled with an alpha prescription. Two series of simulations are presented, one for each system (i.e., isolated and accreting stars). Results. In classical T Tauri systems, the presence of magnetospheric ejections confines the stellar-wind expansion, resulting in an hourglass-shaped geometry of the outflow, and the formation of the accretion columns modifies the amount of open magnetic flux exploited by the stellar wind. These effects have a strong impact on the stellar-wind properties, and we show that the stellar-wind braking is more efficient in the SDI systems than in the isolated ones. We further derive torque scalings over a wide range of magnetic field strengths for each flow component in an SDI system (i.e., magnetospheric accretion and ejections, and stellar winds), which directly applies a torque on the stellar surface. Conclusions. In all the performed SDI simulations, the stellar wind extracts less than 2% of the mass accretion rate and the disk is truncated by up to 66% of the corotation radius. All simulations show a net spin-up torque. We conclude that in order to achieve a stellar-spin equilibrium, we need either more massive stellar winds or disks that are truncated closer to the corotation radius, which increases the torque efficiency of the magnetospheric ejections.

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Magnetic Structure of Rapidly Rotating Fk Comae-Type Coronae

Cohen

Drake

Kashyap

et al. 2010

ApJ

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We present a three-dimensional simulation of the corona of an FK Com-type rapidly rotating G giant using a magnetohydrodynamic model that was originally developed for the solar corona in order to capture the more realistic, non-potential coronal structure. We drive the simulation with surface maps for the radial magnetic field obtained from a stellar dynamo model of the FK Com system. This enables us to obtain the coronal structure for different field topologies representing different periods of time. We find that the corona of such an FK Com-like star, including the large scale coronal loops, is dominated by a strong toroidal component of the magnetic field. This is a result of part of the field being dragged by the radial outflow, while the other part remains attached to the rapidly rotating stellar surface. This tangling of the magnetic field, in addition to a reduction in the radial flow component, leads to a flattening of the gas density profile with distance in the inner part of the corona. The three-dimensional simulation provides a global view of the coronal structure. Some aspects of the results, such as the toroidal wrapping of the magnetic field, should also be applicable to coronae on fast rotators in general, which our study shows can be considerably different from the well-studied and well-observed solar corona. Studying the global structure of such coronae should also lead to a better understanding of their related stellar processes, such as flares and coronal mass ejections, and in particular, should lead to an improved understanding of mass and angular momentum loss from such systems.

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Large-scale magnetic topologies of cool stars

Cited by 7 publications

References 0 publications

Magnetic Fields of Nondegenerate Stars

Magnetic Fields of Nondegenerate Stars

Magnetic torques on T Tauri stars: Accreting versus non-accreting systems

Magnetic Structure of Rapidly Rotating Fk Comae-Type Coronae

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