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Mixed modes observed in red giants allow for investigation of the stellarinterior structures. One important feature in these structures is the buoyancyspike caused by the discontinuity of the chemical gradient left behind duringthe first dredge-up. The buoyancy spike emerges at the base of the convectivezone in low-luminosity red giants and later becomes a glitch when the g-modecavity expands to encompass the spike. Here, we study the impact of thebuoyancy spike on the dipolar mixed modes using stellar models with differentproperties. We find that the applicability of the asymptotic formalisms for thecoupling factor, q, varies depending on the location of the evanescent zone,relative to the position of the spike. Significant deviations between the valueof q inferred from fitting the oscillation frequencies and either of theformalisms proposed in the literature are found in models with a largefrequency separation in the interval 5 to 15 microHz, with evanescent zoneslocated in a transition region which may be thin or thick. However, it is stillpossible to reconcile q with the predictions from the asymptotic formalisms, bychoosing which formalism to use according to the value of q. For starsapproaching the luminosity bump, the buoyancy spike becomes a glitch andstrongly affects the mode frequencies. Fitting the frequencies withoutaccounting for the glitch leads to unphysical variations in the inferred q, butwe show that this is corrected when properly accounting for the glitch in thefitting.

Evolution of Dipolar Mixed-mode Coupling Factor in Red Giant Stars: Impact of Buoyancy Spike