Gravity mode offset and properties of the evanescent zone in red-giant stars

TL;DR

This study introduces a new method to analyze mixed gravity and pressure modes in red-giant stars, enabling precise measurement of internal properties and the gravity mode offset, which varies among stars.

Contribution

The paper presents an alternative mathematical approach that explicitly accounts for radial order, allowing for the first time the constraint of the gravity mode offset in red-giant stars.

Findings

Determined period spacing and coupling term within a few percent of literature values.

Found that the gravity mode offset varies star by star and should not be fixed.

Discovered a logarithmic relation between the coupling factor and the evanescent region width.

Abstract

The wealth of asteroseismic data for red-giant stars and the precision with which these data have been observed over the last decade calls for investigations to further understand the internal structures of these stars. The aim of this work is to validate a method to measure the underlying period spacing, coupling term and mode offset of pure gravity modes that are present in the deep interiors of red-giant stars. We subsequently investigate the physical conditions of the evanescent zone between the gravity mode cavity and the pressure mode cavity. We implement an alternative mathematical description to analyse observational data and to extract the underlying physical parameters that determine the frequencies of mixed modes. This description takes the radial order of the modes explicitly into account, which reduces its sensitivity to aliases. Additionally, and for the first time, this method allows us to constrain the gravity mode offset for red-giant stars. We determine the period spacing and the coupling term for the dipole modes within a few percent of literature values. Additionally, we find that the gravity mode offset varies on a star by star basis and should not be kept fixed in the analysis. Furthermore, we find that the coupling factor is logarithmically related to the physical width of the evanescent region normalised by the radius at which the evanescent zone is located. Finally, the local density contrast at the edge of the core of red giant branch models shows a tentative correlation with the offset. (abstract abriged)