Evolution of a magnetic field in a differentially rotating radiative zone

Gaurat, Mathieu; Jouve, L.; Lignières, F.; Gastine, T.

doi:10.1051/0004-6361/201526125

Cited by 20 publications

(32 citation statements)

References 32 publications

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“…For examples of the (rather complex) interactions possible, see Mestel et al (1988), Moss et al (1990), or Wei and Goodman (2015). Most recently, Gaurat et al (2015) have investigated the dichotomy between the strong "Ap-type" fields and the sub-gauss magnetism observed in other cases, and argued (similarly to Aurière et al 2007;Lignières et al 2014) that the lower bound of observed field strengths in stably stratified stars might arise essentially from a stability condition: strong enough fields are stable and persist, while weaker ones decay quickly. In all cases, a central role is played by whether the field configurations are stable for intervals comparable to the main-sequence lifetime of the star, so we turn to that topic next.…”

Section: How Strong Should Fossil Fields Be?mentioning

confidence: 99%

“…In some cases these simply shape a pre-existing field, whereas in others it may be that dynamo action is possible. Various authors have investigated these interactions using 2-D and 3-D simulations; among them, we note the papers by , Zahn et al (2007), Arlt and Rüdiger (2011), and the very recent simulations of Jouve et al (2015) and Gaurat et al (2015).…”

Section: Evolution Of Magnetism In Stable Layersmentioning

confidence: 99%

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Magnetism, dynamo action and the solar-stellar connection

Brun

Browning

2017

Living Rev Sol Phys

252

182

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The Sun and other stars are magnetic: magnetism pervades their interiors and affects their evolution in a variety of ways. In the Sun, both the fields themselves and their influence on other phenomena can be uncovered in exquisite detail, but these observations sample only a moment in a single star's life. By turning to observations of other stars, and to theory and simulation, we may infer other aspects of the magnetism-e.g., its dependence on stellar age, mass, or rotation rate-that would be invisible from close study of the Sun alone. Here, we review observations and theory of magnetism in the Sun and other stars, with a partial focus on the "Solar-stellar connection": i.e., ways in which studies of other stars have influenced our understanding of the Sun and vice versa. We briefly review techniques by which magnetic fields can be measured (or their presence otherwise inferred) in stars, and then highlight some key observational findings uncovered by such measurements, focusing (in many cases) on those that offer particularly direct constraints on theories of how the fields are built and maintained. We turn then to a discussion of how the fields arise in different objects: first, we summarize some essential elements of convection and dynamo theory, including a very brief discussion of mean-field theory and related concepts. Next we turn to simulations of convection and magnetism in stellar interiors, highlighting both some peculiarities of field generation in different types of stars and some unifying physical processes that likely influence dynamo action in general. We conclude with a brief

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Section: How Strong Should Fossil Fields Be?mentioning

confidence: 99%

Section: Evolution Of Magnetism In Stable Layersmentioning

confidence: 99%

Magnetism, dynamo action and the solar-stellar connection

Brun

Browning

2017

Living Rev Sol Phys

252

182

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“…In an attempt to interpret this division, Aurière et al (2007) proposed a scenario based on the stability of a large scale magnetic configuration in a differentially rotating star, leading to estimating a critical field strength above which magnetic fields can remain stable on long time scales, while magnetic fields below this limit would likely be destroyed by the internal shear. More detailed models including 2D and 3D numerical simulations Gaurat et al 2015) tend to confirm the existence of a critical field in such configurations, where the pre-main sequence contraction is a possible way to force differential rotation. On the other hand, the magnetic dichotomy might simply be the result of two different magnetic field generation processes.…”

Section: Introductionmentioning

confidence: 96%

Detection of ultra-weak magnetic fields in Am stars:βUrsae Majoris andθLeonis

Blazère

Petit

Lignières

et al. 2016

A&A

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Context. An extremely weak circularly polarized signature was recently discovered in spectral lines of the chemically peculiar Am star Sirius A. A weak surface magnetic field was proposed to account for the observed polarized signal, but the shape of the phaseaveraged signature, dominated by a prominent positive lobe, is not expected in the standard theory of the Zeeman effect. Aims. We aim at verifying the presence of weak circularly polarized signatures in two other bright Am stars, β UMa and θ Leo, and investigating the physical origin of Sirius-like polarized signals further. Methods. We present here a set of deep spectropolarimetric observations of β UMa and θ Leo, observed with the NARVAL spectropolarimeter. We analyzed all spectra with the least squares deconvolution multiline procedure. To improve the signal-to-noise ratio and detect extremely weak signatures in Stokes V profiles, we co-added all available spectra of each star (around 150 observations each time). Finally, we ran several tests to evaluate whether the detected signatures are consistent with the behavior expected from the Zeeman effect. Results. The line profiles of the two stars display circularly polarized signatures similar in shape and amplitude to the observations previously gathered for Sirius A. Our series of tests brings further evidence of a magnetic origin of the recorded signal. Conclusions. These new detections suggest that very weak magnetic fields may well be present in the photospheres of a significant fraction of intermediate-mass stars. The strongly asymmetric Zeeman signatures measured so far in Am stars (featuring a dominant single-sign lobe) are not expected in the standard theory of the Zeeman effect and may be linked to sharp vertical gradients in photospheric velocities and magnetic field strengths.

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“…This has lead them to be referred to as fossil fields. The first theory consists of frozen-in magnetic fields originally present in the interstellar medium during the collapse of the pre-stellar cloud (Moss et al 2001), while the other consists of relaxing the dynamo field that is created during the pre-main sequence (PMS) convective phase in stars that meet specific conditions related to A&A 622, A72 (2019) their rotation (Duez & Mathis 2010;Braithwaite & Spruit 2004;Emeriau & Mathis 2015;Gaurat et al 2015). In this paper, we do not focus on the first scenario, and instead derive new observational constraints to the second scenario by measuring the rotational and magnetic properties of HAeBe and A/B stars' progenitors: the intermediate-mass T Tauri stars (IMTTS).…”

Section: Introductionmentioning

confidence: 99%

Magnetic fields of intermediate-mass T Tauri stars

Villebrun

Alécian

Hussain

et al. 2019

A&A

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Context. The origin of the fossil magnetic fields detected in 5 to 10% of intermediate-mass main sequence stars is still highly debated. Aims. We want to bring observational constraints to a large population of intermediate-mass pre-main sequence (PMS) stars in order to test the theory that convective-dynamo fields generated during the PMS phases of stellar evolution can occasionally relax into fossil fields on the main sequence. Methods. Using distance estimations, photometric measurements, and spectropolarimetric data from HARPSpol and ESPaDOnS of 38 intermediate-mass PMS stars, we determined fundamental stellar parameters (Teff, L and v sin i) and measured surface magnetic field characteristics (including detection limits for non-detections, and longitudinal fields and basic topologies for positive detections). Using PMS evolutionary models, we determined the mass, radius, and internal structure of these stars. We compared different PMS models to check that our determinations were not model-dependant. We then compared the magnetic characteristics of our sample accounting for their stellar parameters and internal structures. Results. We detect magnetic fields in about half of our sample. About 90% of the magnetic stars have outer convective envelopes larger than ∼25% of the stellar radii, and heavier than ∼2% of the stellar mass. Going to higher mass, we find that the magnetic incidence in intermediate-mass stars drops very quickly, within a timescale on the order of few times 0.1 Myr. Finally, we propose that intermediate-mass T Tauri stars with large convective envelopes, close to the fully convective limit, have complex fields and that their dipole component strengths may decrease as the sizes of their convective envelopes decrease, similar to lower-mass T Tauri stars.

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Evolution of a magnetic field in a differentially rotating radiative zone

Cited by 20 publications

References 32 publications

Magnetism, dynamo action and the solar-stellar connection

Magnetism, dynamo action and the solar-stellar connection

Detection of ultra-weak magnetic fields in Am stars:βUrsae Majoris andθLeonis

Magnetic fields of intermediate-mass T Tauri stars

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