[Paper Review] Seismic diagnostics for transport of angular momentum in stars 1. Rotational splittings from the PMS to the RGB

J. P. Marques, Goupil, M. J.+13 more|arXiv (Cornell University)|Nov 6, 2012

Stellar, planetary, and galactic studies82 references140 citations

TL;DR

This study investigates rotational splittings in solar-like stars from the pre-main sequence to the red-giant branch using seismic diagnostics to constrain angular momentum transport. It finds that standard models overpredict core rotation due to insufficient transport mechanisms, suggesting that horizontal turbulent viscosity may be underestimated or additional physics is needed to explain observed slow core rotation in red giants.

ABSTRACT

Rotational splittings are currently measured for several main sequence stars and a large number of red giants with the space mission Kepler. This will provide stringent constraints on rotation profiles. Our aim is to obtain seismic constraints on the internal transport and surface loss of angular momentum of oscillating solar-like stars. To this end, we study the evolution of rotational splittings from the pre-main sequence to the red-giant branch for stochastically excited oscillation modes. We modified the evolutionary code CESAM2K to take rotationally induced transport in radiative zones into account. Linear rotational splittings were computed for a sequence of $1.3 M_{\odot}$ models. Rotation profiles were derived from our evolutionary models and eigenfunctions from linear adiabatic oscillation calculations. We find that transport by meridional circulation and shear turbulence yields far too high a core rotation rate for red-giant models compared with recent seismic observations. We discuss several uncertainties in the physical description of stars that could have an impact on the rotation profiles. For instance, we find that the Goldreich-Schubert-Fricke instability does not extract enough angular momentum from the core to account for the discrepancy. In contrast, an increase of the horizontal turbulent viscosity by 2 orders of magnitude is able to significantly decrease the central rotation rate on the red-giant branch. Our results indicate that it is possible that the prescription for the horizontal turbulent viscosity largely underestimates its actual value or else a mechanism not included in current stellar models of low mass stars is needed to slow down the rotation in the radiative core of red-giant stars.

Motivation & Objective

• To constrain the internal transport and surface loss of angular momentum in low-mass stars using seismic observations.
• To test the validity of current stellar models in reproducing observed rotational splittings from the pre-main sequence to the red-giant branch.
• To identify discrepancies between predicted and observed core rotation rates in red-giant stars.
• To evaluate the role of meridional circulation, shear turbulence, and alternative transport mechanisms in explaining seismic data.

Proposed method

• Modified the CESAM2K stellar evolution code to include rotationally induced transport in radiative zones.
• Computed linear rotational splittings for a 1.3 M☉ evolutionary sequence from pre-main sequence to red-giant branch.
• Derived rotation profiles from self-consistent evolutionary models and eigenfunctions from linear adiabatic oscillation calculations.
• Applied the turbulent viscosity prescription of Mathis et al. (2004) for horizontal diffusivity and Talon & Zahn (1997) for vertical diffusivity.
• Used the Goldreich-Schubert-Fricke instability criterion to assess its potential role in angular momentum extraction.
• Explored the impact of increasing horizontal turbulent viscosity by two orders of magnitude to test its effect on core rotation.

Experimental results

Research questions

• RQ1 Why do standard stellar models predict excessively high core rotation rates in red-giant stars compared to seismic observations?
• RQ2 To what extent can meridional circulation and shear turbulence explain the observed rotational splittings in low-mass stars?
• RQ3 Does the Goldreich-Schubert-Fricke instability provide sufficient angular momentum extraction to reconcile models with observations?
• RQ4 Can increasing the horizontal turbulent viscosity by two orders of magnitude reduce the predicted core rotation rate to match observations?
• RQ5 What physical mechanisms might be missing in current 1D stellar models to explain the slow core rotation in red giants?

Key findings

• Meridional circulation and shear turbulence alone produce core rotation rates in red-giant models that are too high compared to seismic observations.
• The Goldreich-Schubert-Fricke instability does not extract enough angular momentum to resolve the discrepancy between models and observations.
• Increasing the horizontal turbulent viscosity by two orders of magnitude significantly reduces the central rotation rate on the red-giant branch.
• The results suggest that the standard prescription for horizontal turbulent viscosity may substantially underestimate its true value in stellar interiors.
• An additional, unmodeled mechanism for angular momentum transport may be required to explain the observed slow core rotation in low-mass red-giant stars.
• Seismic diagnostics provide strong constraints on internal rotation profiles and highlight limitations in current stellar evolution models.