Radial mode widths in red giant stars spectra observed by<i>Kepler</i>

Vrard, M.; Mosser, B.; Barban, C.

doi:10.1051/epjconf/201716004012

Cited by 4 publications

(2 citation statements)

References 19 publications

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“…For comparison, we represent the position in the diagram of different typical observational frequencies: the frequency resolutions of the Kepler, TESS, and PLATO data, and the typical value of = 0 mode line widths as estimated by Vrard et al (2017) and Mosser et al (2018). The line width of = 1 mixed modes of normal amplitude was shown by Benomar et al (2014) to be linked to that of = 0 modes by…”

Section: Detectability Of the Magnetic Signaturementioning

confidence: 99%

Magnetic signatures on mixed-mode frequencies

Bugnet

Prat

Mathis

et al. 2021

A&A

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Context. The discovery of moderate differential rotation between the core and the envelope of evolved solar-like stars could be the signature of a strong magnetic field trapped inside the radiative interior. The population of intermediate-mass red giants presenting surprisingly low-amplitude mixed modes (i.e. oscillation modes that behave as acoustic modes in their external envelope and as gravity modes in their core) could also arise from the effect of an internal magnetic field. Indeed, stars more massive than about 1.1 solar masses are known to develop a convective core during their main sequence. The field generated by the dynamo triggered by this convection could be the progenitor of a strong fossil magnetic field trapped inside the core of the star for the remainder of its evolution. Aims. Observations of mixed modes can constitute an excellent probe of the deepest layers of evolved solar-like stars, and magnetic fields in those regions can impact their propagation. The magnetic perturbation on mixed modes may therefore be visible in asteroseismic data. To unravel which constraints can be obtained from observations, we theoretically investigate the effects of a plausible mixed axisymmetric magnetic field with various amplitudes on the mixed-mode frequencies of evolved solar-like stars. Methods. First-order frequency perturbations due to an axisymmetric magnetic field were computed for dipolar and quadrupolar mixed modes. These computations were carried out for a range of stellar ages, masses, and metallicities. Conclusions. We show that typical fossil-field strengths of 0.1 − 1 MG, consistent with the presence of a dynamo in the convective core during the main sequence, provoke significant asymmetries on mixed-mode frequency multiplets during the red giant branch. We provide constraints and methods for the detectability of such magnetic signatures. We show that these signatures may be detectable in asteroseismic data for field amplitudes small enough for the amplitude of the modes not to be affected by the conversion of gravity into Alfvén waves inside the magnetised interior. Finally, we infer an upper limit for the strength of the field and the associated lower limit for the timescale of its action in order to redistribute angular momentum in stellar interiors.

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Section: Detectability Of the Magnetic Signaturementioning

confidence: 99%

Magnetic signatures on mixed-mode frequencies

Bugnet

Prat

Mathis

et al. 2021

A&A

View full text Add to dashboard Cite

show abstract

“…For comparison, we represent the position in the diagram of different typical observational frequencies: the frequency resolu-tions of the Kepler, TESS and PLATO data, and the typical value of = 0 mode linewidths as estimated by Vrard et al (2017) and Mosser et al (2018). The linewidth of = 1 mixed modes of normal amplitude was shown by Benomar et al (2014) to be linked to that of = 0 modes by…”

Section: Detectability Of the Magnetic Signaturementioning

confidence: 99%

Magnetic signatures on mixed-mode frequencies. I. An axisymmetric fossil field inside the core of red giants

Bugnet,

Prat,

Mathis

et al. 2021

Preprint

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Context. The discovery of the moderate differential rotation between the core and the envelope of evolved solar-like stars could be the signature of a strong magnetic field trapped inside the radiative interior. The population of intermediate-mass red giants presenting a surprisingly low-amplitude of their mixed modes (i.e. oscillation modes that behave as acoustic modes in their external envelope and as gravity modes in their core) could also arise from the effect of an internal magnetic field. Indeed, stars more massive than about 1.1 solar masses are known to develop a convective core during their main sequence. The field generated by the dynamo triggered by this convection could be the progenitor of a strong fossil magnetic field trapped inside the core of the star for the rest of its evolution. Aims. The observations of mixed modes can constitute an excellent probe of the deepest layers of evolved solar-like stars. Thus, magnetic fields in those regions can impact their propagation. The magnetic perturbation on mixed modes may thus be visible in asteroseismic data. To unravel which constraints can be obtained from observations, we theoretically investigate the effects of a plausible mixed axisymmetric magnetic field with various amplitudes on the mixed-mode frequencies of evolved solar-like stars. Methods. The first-order frequency perturbations due to an axisymmetric magnetic field are computed for dipolar and quadrupolar mixed modes. These computations are carried out for a range of stellar ages, masses, and metallicities. Results. We show that typical fossil-field strengths of 0.1 − 1 MG, consistent with the presence of a dynamo in the convective core during the main sequence, provoke significant asymmetries on mixed-mode frequency multiplets during the red-giant branch. We provide constraints and methods for the detectability of such magnetic signatures. We show that these signatures may be detectable in asteroseismic data for field amplitudes small enough for the amplitude of the modes not to be affected by the conversion of gravity into Alfvén waves inside the magnetised interior. Finally, we infer an upper limit for the strength of the field, and the associated lower limit for the timescale of its action, to redistribute angular momentum in stellar interiors.

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