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SUMMARY:In search for red giants' internal magnetic field  - Lisa Bugnet -
  Flatiron Institute
DTSTART:20210222T140000Z
DTEND:20210222T150000Z
UID:TALK153937@talks.cam.ac.uk
CONTACT:Cleo Loi
DESCRIPTION:One of the “legacy" results of the Kepler mission is the int
 erestingly low rotation rate of the core of subgiant (SG) and red giant (R
 G) stars\, which is about 10 times lower than predicted with the current t
 heory for the transport of angular momentum by purely hydrodynamical mecha
 nisms. This discrepancy points out an order of magnitude issue concerning 
 the understanding of the evolution of the stellar angular momentum in evol
 ved Solar-like stars\, a very ubiquitous problem shared by stars of all ty
 pes and ages. The recent discovery of very low-amplitude dipolar oscillati
 ons in a significant fraction of the observed SGs and RGs also points out 
 our misunderstanding of physical processes inside the radiative interiors 
 of evolved Solar-like stars. We thus seek for a missing process taking pla
 ce inside the core of evolved Solar-like stars\, efficient to extract angu
 lar momentum from the core to the surface and to perturbate stellar oscill
 ations. \n\nInternal magnetic fields are one amongst the most serious cand
 idates that are currently studied to solve both problems. Stars more massi
 ve than ~1.1 Ms are known to develop a convective core during the main-seq
 uence: the dynamo process due to this convection could be the origin of a 
 strong magnetic field\, trapped inside the core of the star for the rest o
 f its evolution. Such magnetic fields should impact mixed modes inside the
  core of RG stars\, and their signature should be visible in asteroseismic
  data. To unravel which constraints can be obtained from these observation
 s\, we theoretically investigate the effects of a plausible mixed axisymme
 tric magnetic field with various amplitudes on the mixed-mode frequencies 
 of red giants. Applying a perturbative method\, we estimate the magnetic s
 plitting of the frequencies of simulated mixed dipolar modes that depends 
 on the magnetic field strength and its configuration. A complete asymptoti
 c analysis is derived\, showing the potential of asteroseismology to probe
  the magnetism at each depth as this is done for stellar rotation. The eff
 ects of the mass and the metallicity of the stars are also explored. Final
 ly\, we infer an upper limit for the strength of the field and the associa
 ted lower limit for the timescale of its action to redistribute angular mo
 mentum in stellar interiors.
LOCATION:Online
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