Implications of Rapid Core Rotation in Red Giants for Internal Angular Momentum Transport in Stars

TL;DR

This paper investigates how internal angular momentum transport affects core rotation in red giants, using asteroseismic data to compare observations with models, revealing inefficient transport in early giants and stronger coupling in later stages.

Contribution

It provides new insights into the timescales and mechanisms of angular momentum transfer in stellar evolution, especially highlighting the role of convective cores and molecular weight gradients.

Findings

Core rotation in early giants is much faster than standard models predict.

Strong core-envelope coupling is observed in evolved secondary clump stars.

Predicted white dwarf rotation periods vary significantly based on decoupling times.

Abstract

Core rotation rates have been measured for red giant stars using asteroseismology. This data, along with helioseismic measurements and open cluster spin down studies, provide powerful clues about the nature and timescale for internal angular momentum transport in stars. We focus on two cases: the metal poor red giant KIC 7341231 ("Otto") and intermediate mass core helium burning stars. For both we examine limiting case studies for angular momentum coupling between cores and envelopes under the assumption of rigid rotation on the main sequence. We discuss the expected pattern of core rotation as a function of mass and radius. In the case of Otto, strong post-main-sequence coupling is ruled out and the measured core rotation rate is in the range of 23 to 33 times the surface value expected from standard spin down models. The minimum coupling time scale (.17 to .45 Gyr) is significantly longer than that inferred for young open cluster stars. This implies ineffective internal angular momentum transport in early first ascent giants. By contrast, the core rotation rates of evolved secondary clump stars are found to be consistent with strong coupling given their rapid main sequence rotation. An extrapolation to the white dwarf regime predicts rotation periods between 330 and .0052 days depending on mass and decoupling time. We identify two key ingredients that explain these features: the presence of a convective core and inefficient angular momentum transport in the presence of larger mean molecular weight gradients. Observational tests that can disentangle these effects are discussed.