Magnetic signatures on mixed-mode frequencies

Bugnet, L.; Prat, V.; Mathis, S.; Astoul, Aurélie; Augustson, Kyle; García, R. A.; Mathur, S.; Amard, Louis; Neiner, C.

doi:10.1051/0004-6361/202039159

Cited by 31 publications

(74 citation statements)

References 160 publications

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“…There is also the potential to look at the timing of the extra mixing, as well as mixing rate as a function of time. Further, it may be possible to investigate whether mixing can be connected to the sorts of asteroseismic diagnostics that probe the interior structure of a star, including its density profile (Kjeldsen & Bedding 1995), chemical discontinuities (Verma et al 2017), internal rotation (Gehan et al 2018), and magnetic fields (Bugnet et al 2021).…”

Section: One-dimensional Stellar Evolution Models Suggestmentioning

confidence: 99%

Characterizing Observed Extra Mixing Trends in Red Giants using the Reduced Density Ratio from Thermohaline Models

Fraser¹,

Joyce²,

Anders³

et al. 2022

Preprint

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Observations show an almost ubiquitous presence of extra mixing in low-mass upper giant branch stars. The most commonly invoked explanation for this is the thermohaline instability. One dimensional stellar evolution models include prescriptions for thermohaline mixing, but our ability to make direct comparisons between models and observations has thus far been limited. Here, we propose a new framework to facilitate direct comparison: Using carbon to nitrogen measurements from the SDSS-IV APOGEE survey as a probe of mixing and a fluid parameter known as the reduced density ratio from one dimensional stellar evolution programs, we compare the observed amount of extra mixing on the upper giant branch to predicted trends from three-dimensional fluid dynamics simulations. By applying this method, we are able to place empirical constraints on the efficiency of mixing across a range of masses and metallicities. We find that the observed amount of extra mixing is strongly correlated with the reduced density ratio and that trends between reduced density ratio and fundamental stellar parameters are robust across choices for modeling prescription. We show that stars with available mixing data tend to have relatively low density ratios, which should inform the regimes selected for future simulation efforts. Finally, we show that there is increased mixing at low values of the reduced density ratio, which is consistent with current hydrodynamical models of the thermohaline instability. The introduction of this framework sets a new standard for theoretical modeling efforts, as validation for not only the amount of extra mixing, but trends between the degree of extra mixing and fundamental stellar parameters is now possible.

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Section: One-dimensional Stellar Evolution Models Suggestmentioning

confidence: 99%

Characterizing Observed Extra Mixing Trends in Red Giants using the Reduced Density Ratio from Thermohaline Models

Fraser¹,

Joyce²,

Anders³

et al. 2022

Preprint

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“…Van Beeck et al (2020) made a systematic exploration of this magnetic signature along the evolution of intermediatemass pulsating γ Doradus (γ Dor hereafter) and slowly pulsating B (SPB) stars and showed that the magnetic signatures differ appreciably from those due to rotation alone. They found that these signatures are measurable for dipolar gravity mode oscillations in terminal-age main-sequence (TAMS) stars for a magnetic field with a near-core strength larger than 10 5 G. Next, Bugnet et al (2021) and Loi (2021) studied the perturbation of mixed gravito-acoustic modes propagating in evolved low-and intermediate-mass stars by axisymmetric and non-axisymmetric inclined mixed dipolar fossil fields, respectively. They identified asymmetries in the frequency splittings induced by the field.…”

Section: Introductionmentioning

confidence: 99%

Detecting deep axisymmetric toroidal magnetic fields in stars

Dhouib

Mathis

Bugnet

et al. 2022

A&A

Self Cite

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Context. Asteroseismology has revealed small core-to-surface rotation contrasts in stars in the whole Hertzsprung–Russell diagram. This is the signature of strong transport of angular momentum (AM) in stellar interiors. One of the plausible candidates to efficiently carry AM is magnetic fields with various topologies that could be present in stellar radiative zones. Among them, strong axisymmetric azimuthal (toroidal) magnetic fields have received a lot of interest. Indeed, if they are subject to the so-called Tayler instability, the accompanying triggered Maxwell stresses can transport AM efficiently. In addition, the electromotive force induced by the fluctuations of magnetic and velocity fields could potentially sustain a dynamo action that leads to the regeneration of the initial strong axisymmetric azimuthal magnetic field. Aims. The key question we aim to answer is whether we can detect signatures of these deep strong azimuthal magnetic fields. The only way to answer this question is asteroseismology, and the best laboratories of study are intermediate-mass and massive stars with external radiative envelopes. Most of these are rapid rotators during their main sequence. Therefore, we have to study stellar pulsations propagating in stably stratified, rotating, and potentially strongly magnetised radiative zones, namely magneto-gravito-inertial (MGI) waves. Methods. We generalise the traditional approximation of rotation (TAR) by simultaneously taking general axisymmetric differential rotation and azimuthal magnetic fields into account. Both the Coriolis acceleration and the Lorentz force are therefore treated in a non-perturbative way. Using this new formalism, we derive the asymptotic properties of MGI waves and their period spacings. Results. We find that toroidal magnetic fields induce a shift in the period spacings of gravity (g) and Rossby (r) modes. An equatorial azimuthal magnetic field with an amplitude of the order of 105 G leads to signatures that are detectable in period spacings for high-radial-order g and r modes in γ Doradus (γ Dor) and slowly pulsating B (SPB) stars. More complex hemispheric configurations are more difficult to observe, particularly when they are localised out of the propagation region of MGI modes, which can be localised in an equatorial belt. Conclusions. The magnetic TAR, which takes into account toroidal magnetic fields in a non-perturbative way, is derived. This new formalism allows us to assess the effects of the magnetic field in γ Dor and SPB stars on g and r modes. We find that these effects should be detectable for equatorial fields thanks to modern space photometry using observations from Kepler, TESS CVZ, and PLATO.

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“…In parallel, the possibility to detect magnetic fields through their effect on the oscillation frequencies is under investigation (see e.g. Bugnet et al 2021). Awaiting observational evidence, theoretical work strongly supports the presence of magnetic fields in stars that possess a convective core (MS stars with M 1.2 M ).…”

Section: Introductionmentioning

confidence: 99%

Angular momentum transport in a contracting stellar radiative zone embedded in a large-scale magnetic field

Gouhier

Jouve

Lignières

2022

A&A

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Context. Some contracting or expanding stars are thought to host a large-scale magnetic field in their radiative interior. By interacting with the contraction-induced flows, such fields may significantly alter the rotational history of the star. They thus constitute a promising way to address the problem of angular momentum transport during the rapid phases of stellar evolution. Aims. In this work, we aim to study the interplay between flows and magnetic fields in a contracting radiative zone. Methods. We performed axisymmetric Boussinesq and anelastic numerical simulations in which a portion of the radiative zone was modelled by a rotating spherical layer, stably stratified and embedded in a large-scale (either dipolar or quadrupolar) magnetic field. This layer is subject to a mass-conserving radial velocity field mimicking contraction. The quasi-steady flows were studied in strongly or weakly stably stratified regimes relevant for pre-main sequence stars and for the cores of subgiant and red giant stars. The parametric study consists in varying the amplitude of the contraction velocity and of the initial magnetic field. The other parameters were fixed with the guidance of a previous study. Results. After an unsteady phase during which the toroidal field grew linearly and then back-reacted on the flow, a quasi-steady configuration was reached, characterised by the presence of two magnetically decoupled regions. In one of them, magnetic tension imposes solid-body rotation. In the other, called the dead zone, the main force balance in the angular momentum equation does not involve the Lorentz force and a differential rotation exists. In the strongly stably stratified regime, when the initial magnetic field is quadrupolar, a magnetorotational instability is found to develop in the dead zones. The large-scale structure is eventually destroyed and the differential rotation is able to build up in the whole radiative zone. In the weakly stably stratified regime, the instability is not observed in our simulations, but we argue that it may be present in stars. Conclusions. We propose a scenario that may account for the post-main sequence evolution of solar-like stars, in which quasi-solid rotation can be maintained by a large-scale magnetic field during a contraction timescale. Then, an axisymmetric instability would destroy this large-scale structure and this enables the differential rotation to set in. Such a contraction-driven instability could also be at the origin of the observed dichotomy between strongly and weakly magnetic intermediate-mass stars.

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Magnetic signatures on mixed-mode frequencies

Cited by 31 publications

References 160 publications

Characterizing Observed Extra Mixing Trends in Red Giants using the Reduced Density Ratio from Thermohaline Models

Characterizing Observed Extra Mixing Trends in Red Giants using the Reduced Density Ratio from Thermohaline Models

Detecting deep axisymmetric toroidal magnetic fields in stars

Angular momentum transport in a contracting stellar radiative zone embedded in a large-scale magnetic field

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