Constraining the Rotation Profile in a Low-Luminosity Subgiant with a Surface Rotation Measurement

TL;DR

This study models the internal rotation profile of a low-luminosity subgiant star using observed rotational splittings, demonstrating that auxiliary surface rotation measurements significantly improve the precision of the inferred rotation gradient.

Contribution

The paper introduces a forward modeling approach assuming a step-like rotation profile and shows how surface rotation priors enhance the constraints on the rotation discontinuity.

Findings

Surface rotation priors improve the rotation profile constraints.

The model recovers core and surface rotation rates consistent with previous studies.

Discontinuity in the radiative zone is most supported by the data.

Abstract

Rotationally-induced mode splitting frequencies of low-luminosity subgiants suggest that angular momentum transport mechanisms are 1-2 orders of magnitude more efficient in these stars than predicted by theory. Constraints on the rotation profile of low-luminosity subgiants could be used to identify the dominant mechanism for angular momentum transport. We develop a forward model for the rotation profile given observed rotational splittings, assuming a step-like rotation profile. We identify a consistent degeneracy between the position of the profile discontinuity and the surface rotation rate. We perform mock experiments that show the discontinuity position can be better constrained with a prior on the surface rotation rate, which is informed by star spot modulations. We finally apply this approach to KIC 12508433, a well-studied low-luminosity subgiant, as an example case. With the observed surface rotation prior, we obtain a factor of two increase in precision of the position of strong rotation gradient. We recover the literature values of the core and surface rotation rates and find the highest support for a discontinuity in the radiative zone. Auxiliary measurements of surface rotation could substantially improve inferences on the rotation profile of low-luminosity subgiants with already available data.