Glitches in solar-like oscillating F-type stars: Possible contribution of non-linear terms

Abstract

The glitch signatures in $r_{010}$ for F-type stars (higher amplitude and period of the oscillatory component) are very different from those of G-type stars. The aim of this work is to analyse the signatures of these glitches and understand the origin of the differences in these signatures between G-type and F-type stars. We fit the glitch signatures in the frequencies, second differences, and $r_{010}$ ratios while assuming either a sinusoidal variation or a more complex expression. The fit provides the acoustic depth, and hence the position, of the bottom of the convective envelope for nine \textit{Kepler} stars and the Sun. We find that for F-type stars, the most commonly used fitting expressions for the glitch of the bottom of the convective envelope provide different measurements of the position of the bottom of the convective envelope for the three seismic indicators, while it is not the case for G-type stars. When adding an additional term in the fitting expression with twice the acoustic depth of the standard term (a contribution that accounts for the highly non-sinusoidal shape of the signature in the $r_{010}$ ratios), we find better agreement between the three seismic indicators and with the prediction of stellar evolution models. While the origin of this additional term is not yet understood, this may be an indication that the transition between the convective envelope and the underlying radiative zone is different for G- and F-type stars. This outcome brings new insight into the physics in these regions.