Abstract
Context. The transition between convective and radiative stellar regions is still not fully understood. This currently leads to a poor modelling of the transport of energy and chemical elements in the vicinity of these regions. The sharp variations in sound speed located in these transition regions give rise to a signature in specific seismic indicators, opening the possibility to constrain the physics of convection to radiation transition. Among those seismic indicators, the ratios of the small to large frequency separation for l = 0 and 1 modes (r010) were shown to be particularly efficient to probe these transition regions. Interestingly, in the Kepler Legacy F-type stars, the oscillatory signatures left in the r010 ratios by the sharp sound-speed variation have unexpected large amplitudes that still need to be explained.Aims. We analyse the r010 ratios of stellar models of solar-like oscillating F-type stars in order to investigate the origin of the observed large amplitude signatures of the r010 ratios.Methods. We tested different possibilities that may be at the origin of the large amplitude signatures using internal structures of stellar models. We then derived an analytical expression of the signature, in particular, of the amplitude of variation, that we tested against stellar models.Results. We show that the signature of the bottom of the convective envelope is amplified in the ratios r010 by the frequency dependence of the amplitude compared to the signal seen in the frequencies themselves or the second differences. We also find that with precise enough data, a smoother transition between the adiabatic and radiative temperature gradients could be distinguished from a fully adiabatic region. Furthermore, we find that among the different options of physical input investigated here, large amplitude signatures can only be obtained when convective penetration of the surface convective zone into the underlying radiative region is taken into account. In this case and even for amplitudes as large as those observed in F-type stars, the oscillating signature in the r01 ratios can only be detected when the convective envelope is deep enough (i.e. at the end of the main sequence). Assuming that the origin of the large amplitude glitch signal is due to penetrative convection (PC), we find that the PC must extend downward the convective to radiative transition significantly (about 1 − 2Hp) in order to reproduce the large amplitudes observed for the ratios of F-type stars. This deep extension of the convective envelope causes doubt that the origin of the large amplitudes is due to PC as it is modelled here or implies that current stellar modelling (without PC) leads to an underestimation of the size of convective envelopes. In any case, studying the glitch signatures of a large number of oscillating F-type stars opens the possibility to constrain the physics of the stellar interior in these regions.