Asteroseismology of solar-type stars: signatures of convective and/or helium cores

TL;DR

This study investigates how small frequency separations in solar-type stars' oscillations can indicate the presence of convective or helium cores, revealing that these signatures are detectable during stellar evolution.

Contribution

The paper provides a systematic analysis of small separation behavior during stellar evolution, linking negative values to helium-rich cores in models of solar-type stars.

Findings

Small separations become negative during certain evolutionary stages.

Negative small separations are signatures of helium-rich cores.

Behavior is consistent across various stellar models.

Abstract

Asteroseismology of solar-type stars is an important tool for constraining stellar parameters and internal structure. Several frequency combinations are largely used for comparisons between models and observations. In particular, the ``small separations'' are very sensitive to stellar cores. We showed in a previous paper that they can change sign, in contradiction with the ``asymptotic theory'', and that this behaviour could correspond to signatures of convective and/or helium cores. Here we analyse this behaviour in detail by systematic modelling during stellar evolution. We computed evolutionary tracks for models with various masses (from 1.05 to 1.25 Msun) and various chemical composition, with and without overshooting. We computed the adiabatic oscillation frequencies of the models and analysed the evolution of their small separations along an evolutionary track. We found that, for all cases, the stars go through a stage, during their evolution time, where the small separations computed between degrees l=0 and l=2 become negative in the observed range of frequencies. This behaviour is clearly related to the signature of a helium-rich core. We discuss the consequences for the interpretation of the acoustic frequencies observed in solar-type stars.