Seismic determination of the helium abundance in low mass stars, insights from modelling the ionisation region
Although seismic inference has proven to be quite valuable when determining stellar parameters of solar-like pulsators, estimating these very parameters can be subject to biases and thus remain uncertain. A particularly important example is the helium-mass degeneracy, where the uncertainties regarding the internal physics cause a poor determination of both the mass and surface helium content. Accordingly, an independent helium estimate is needed to overcome this degeneracy. A promising way to obtain such an estimate is to exploit the so-called ionisation glitch, a deviation from the asymptotic oscillation frequency pattern caused by the rapid structural variation within the ionisation zone. However, although becoming more and more sophisticated, current approaches for modelling glitches face problems such as the need for calibration by realistic stellar models. Accordingly, we focused on an alternative approach to studying glitches using a physically motivated model of the ionisation region. This is done by means of an analytical approximation of the first adiabatic exponent inside an adiabatically stratified region and the derived structure is found to depend on three parameters. They respectively control the surface helium abundance, the electron degeneracy in the convective zone, and the extent of the ionisation region. Characterising the glitch based on such a model would thus allow us to extract physical quantities such as the helium abundance without the need for calibration, as well as other interesting constraints such as the electronic degeneracy in the star’s convection zone.