Fast core rotation in red-giant stars revealed by gravity-dominated mixed modes

TL;DR

This paper reports the detection of rapid core rotation in red-giant stars, revealing a steep gradient in rotation rates from surface to core, consistent with theoretical predictions.

Contribution

It provides the first observational evidence of non-rigid rotation in red giants using mixed mode frequency splittings, confirming theoretical models.

Findings

Core rotates at least ten times faster than surface.

Rotation rate increases from surface to core.

Supports models predicting steep internal rotation gradients.

Abstract

When the core hydrogen is exhausted during stellar evolution, the central region of a star contracts and the outer envelope expands and cools, giving rise to a red giant, in which convection occupies a large fraction of the star. Conservation of angular momentum requires that the cores of these stars rotate faster than their envelopes, and indirect evidence supports this. Information about the angular momentum distribution is inaccessible to direct observations, but it can be extracted from the effect of rotation on oscillation modes that probe the stellar interior. Here, we report the detection of non-rigid rotation in the interiors of red-giant stars by exploiting the rotational frequency splitting of recently detected mixed modes. We demonstrate an increasing rotation rate from the surface of the star to the stellar core. Comparing with theoretical stellar models, we conclude that the core must rotate at least ten times faster than the surface. This observational result confirms the theoretical prediction of a steep gradient in the rotation profile towards the deep stellar interior.