# Constraining transport of angular momentum in stars: Combining asteroseismic observations of core helium burning stars and white dwarfs

## TL;DR

This study investigates the missing angular momentum transport process in stars by comparing models with asteroseismic observations of core helium burning stars and white dwarfs, revealing the need for a decreasing efficiency of the process during evolution.

## Contribution

It constrains the efficiency of the unknown angular momentum transport process in stars by combining asteroseismic data with stellar evolution models, and suggests the process's effectiveness decreases during core helium burning.

## Key findings

- Tayler-Spruit dynamo cannot account for observed rotation rates.
- An artificial viscosity of ~10^7 cm^2/s matches core helium burning star data.
- The missing transport process likely decreases in efficiency during evolution.

## Abstract

Transport of angular momentum has been a challenging topic within the stellar evolution community, even more since the recent asteroseismic surveys. All published studies on rotation using asteroseismic observations show a discrepancy between the observed and calculated rotation rates, indicating there is an undetermined process of angular momentum transport active in these stars. We aim to constrain the efficiency of this process by investigating rotation rates of 2.5 M$_{\odot}$ stars. First, we investigated whether the Tayler-Spruit dynamo could be responsible for the extra transport of angular momentum for stars with an initial mass of 2.5 M$_{\odot}$. Then, by computing rotating models including a constant additional artificial viscosity, we determined the efficiency of the missing process of angular momentum transport by comparing the models to the asteroseismic observations of core helium burning stars. Parameter studies were performed to investigate the effect of the stellar evolution code used, initial mass, and evolutionary stage. We evolved our models into the white dwarf phase, and provide a comparison to white dwarf rotation rates. The Tayler-Spruit dynamo is unable to provide enough transport of angular momentum to reach the observed values of the core helium burning stars investigated in this paper. We find that a value for the additional artificial viscosity $\nu_{\rm{add}}$ around 10$^7$ cm$^2$ s$^{-1}$ provides enough transport of angular momentum. However, the rotational period of these models is too high in the white dwarf phase to match the white dwarf observations. From this comparison we infer that the efficiency of the missing process must decrease during the core helium burning phase. When excluding the $\nu_{\rm{add}}$ during core helium burning phase, we can match the rotational periods of both the core helium burning stars and white dwarfs.