The study of the impact of angular momentum transport in low mass red giant stars using asteroseismology

Computational and analytical models predict several mechanisms of transport of angular momentum (AM) and of mixing of chemical elements capable of describing some of the physical phenomena in star’s interiors through their evolution. Nonetheless, recent measurements of core rotation rates of red giants enabled by the Kepler mission, indicating a possible missing AM transfer mechanism in the current theory of stellar interiors.

In the first part of this work, we use the Modules for Experiments in Stellar Astrophysics (MESA) stellarevolution code to compute models of low mass rotating stars from the zero-age main sequence (ZAMS) to the red giant branch (RGB) calibrating them to a specific Kepler asteroseismic target, KIC8579095.

We include transport of AM and rotation-induced chemical mixing due to magnetic fields in radiative zones, using the Tayler-Spruit dynamo formalism and the recent revision by Fuller et al. (2019) (named Fuller-formalism), which has showed very promising results for red giant models. We also make use of the adiabatic pulsation code, GYRE, to further study this mechanisms using asteroseismic observables. We find that only the models including the Fuller prescription for AM transport were able to predict the observed core rotation rates on the RGB. The efficiency of transport of AM increases with inclusion of mixing, predicting lower rotation rates in the RGB, despite the mixing induced by the Fuller-formalism being extremely small as expected.

In the second part, we tested the efficiency of AM transport of Fuller-formalism models with different input physics: stellar mass, metallicity, convective overshooting and the free parameter of this theory. We found models particularly sensible to variations in initial mass, but less to metallicity and overshooting. Lastly, using a sample of 1093 stars with masses in the 1-2 solar mass range that include all the stellar evolution phases from main-sequence (MS) up to the red clump (RC), we study the transfer of AM through these phases. To that end, we modeled a typical star with a mass of 1.5 M and compared the results obtained with asteroseismic observations for this group of stars. We found that a wider range for the α parameter [0.5;5] in the Fuller-formalism is needed to reproduce the particular measurements of RGB and RC stars.