Austrian Early Career Conference 2024

Contribution:
Talk

Authors:
L. Barrault [1]; L. Bugnet [1]; S. Mathis [2]; A. Serenelli [3]

Affiliations:
1: Institute of Science and Technology Austria (ISTA); 2: AIM, CEA, CNRS, Université Paris-Saclay; 3: Institute of Space Sciences (ICE, CSIC) Barcelona

Title:
Using red clump stars as probes of stellar interior dynamics: focus on overshooting and magnetism

Abstract:
Low-mass stars undergoing Helium burning in their core represent a well defined and populated feature of the HR diagram, the "red clump" (RC). RC stars are unique probes of their past-evolution stages, as their structure and hence seismic and photometric observables vary according to the modelling parameters chosen along the Main Sequence and the Red Giant Branch. Their study gives constraints on the dynamical processes mixing the internal stellar plasma. Namely, the overshooting phenomenon and magnetism are two cornerstones of the physics of stellar interiors. Overshooting brings more mixing, hence extending the lifespan of stars. Magnetism distributes plasma along the field lines, often inhibiting convection and mixing. Both processes are key to understand stellar dynamics and better constrain stellar evolution and ages in the Universe.
First, we show using the stellar evolution code GARSTEC and the APOKASC-3 survey that the variation of the overshooting parameter plays a significant role in the relative size of the Helium core, hence in the morphology of the RC and the transition mass towards the Secondary Red Clump, formed of higher mass core Helium burning stars in which the onset of He burning would occur into non-degenerate conditions. We conclude on giving constraints for the overshooting parameter that are independent of previously derived estimations, and anticipate a mass dependence of such a parameter, that will be derived in the near-future.
Second, we use such corrected modelling prescriptions to evolve low-mass stars up to the RC with the stellar evolution code MESA, and investigate the persistence of a stable magnetic field previously detected in the radiative zone of early Red Giants. We use GYRE (state-of-the-art stellar oscillation code) coupled to the newly developed MAGSPLITPY code computing magnetic frequency splittings. We show that even for the expected weak amplitude of a stable magnetic field, the magnetic signature is non-negligible in the low-frequency range of the detectable mixed p-g modes and follow the overall tendency previously observed in RGB stars. However, unlike for the case of their younger counterparts, the magnetic splitting obtained is highly dependant on the discontinuities of the Brunt-Väisälä frequency near the convective core, arising primarily from the adopted prescription on extra-mixing. Such results in turn strengthen our interest in using RC stars as unique probes for stellar physics and prove that constraints in the modelling of dense core convection and overshooting will have an appreciable impact on the measurement of magnetic fields inside RC stars in the future, and therefore on our knowledge of stellar evolution.