Abstract
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.
Highlights
Oscillations on the surface of solar-type stars have been observed and studied across the Hertzsprung-Russel diagram thanks in large part to the data provided by CoRoT, Kepler, K2, and the TESS missions (e.g. Michel et al 2008; Chaplin et al 2010, 2020; Lund et al 2017; Huber et al 2019; García & Ballot 2019)
We investigate the effects of an axisymmetric mixed poloidal and toroidal fossil field aligned with the rotation axis of the star on mixed-mode frequencies during the SG and red giants (RGs) evolutionary stages
We concentrate on the fossil field scenario, in which a buried magnetism is present inside the radiative interior of the stars
Summary
Oscillations on the surface of solar-type stars have been observed and studied across the Hertzsprung-Russel diagram thanks in large part to the data provided by CoRoT, Kepler, K2, and the TESS missions (e.g. Michel et al 2008; Chaplin et al 2010, 2020; Lund et al 2017; Huber et al 2019; García & Ballot 2019). Michel et al 2008; Chaplin et al 2010, 2020; Lund et al 2017; Huber et al 2019; García & Ballot 2019). While this list is hardly exhaustive, some key results are relevant to this paper and to the importance of the transport of angular momentum in the radiative regions of main sequence (MS) stars. In subgiant (SG) stars and red-giant (RG) stars, some relevant works are the discoveries of the unexpectedly slow rotation rate of their cores There is no clear evolutionary model that yields internal rotation profiles akin to those observed (e.g. Eggenberger et al 2012, 2017, 2019; Ceillier et al 2013; Marques et al 2013, for the loss of angular momentum on the giant branch), nor is there a robust explanation for the observed dipole mode amplitude suppression (Fuller et al 2015; Cantiello et al 2016; Lecoanet et al 2017; Mosser et al 2017; Loi & Papaloizou 2018)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
