Asteroseismology of evolved stars to constrain the internal transport of angular momentum V. Efficiency of the transport on the red giant branch and in the red clump

Context. Asteroseismology provides constraints on the core rotation rate for hundreds of low- and intermediate-mass stars in evolved phases. Current physical processes tested in stellar evolution models cannot reproduce the evolution of these core rotation rates. Aims. We investigate the efficiency of the internal angular momentum redistribution in red giants during the hydrogen-shell and corehelium burning phases based on the asteroseismic determinations of their core rotation rates. Methods. We computed stellar evolution models with rotation and model the transport of angular momentum by the action of a sole dominant di ffusive process parameterised by an additional viscosity in the equation of angular momentum transport. We constrained the values of this viscosity to match the mean core rotation rates of red giants and their behaviour with mass and evolution using asteroseismic indicators along the red giant branch and in the red clump. Results. For red giants in the hydrogen-shell burning phase, the transport of angular momentum must be more e fficient in more massive stars. The additional viscosity is found to vary by approximately two orders of magnitude in the mass range M similar to 1-2.5 M-circle dot. As stars evolve along the red giant branch, the efficiency of the internal transport of angular momentum must increase for low-mass stars (M less than or similar to 2 M-circle dot) and remain approximately constant for slightly higher masses (2.0 M-circle dot less than or similar to M less than or similar to 2.5 M-circle dot). In red clump stars, the additional viscosities must be an order of magnitude higher than in younger red giants of similar mass during the hydrogen-shell burning phase. Conclusions. In combination with previous e fforts, we obtain a clear picture of how the physical processes acting in stellar interiors should redistribute angular momentum from the end of the main sequence until the core-helium burning phase for low- and intermediate-mass stars to satisfy the asteroseismic constraints.

Automated summary

In this paper, the efficiency of internal angular momentum redistribution in red giants during the hydrogen-shell and core-helium burning phases was investigated based on asteroseismic determinations of their core rotation rates. It was found that the additional viscosity required for angular momentum transport varies with stellar mass and evolutionary stage, with higher efficiencies needed in more massive stars and older red clump stars.