Left panel: slice of the S² function (in logarithmic units) along the M_∗ − log q(H) plane with the parameters log q(core) and X_core(C + O) fixed to their optimal values obtained in the best-fit seismic model 1. Right panel: slice of the S² function (in log) along the log q(core) − X_core(C + O) plane with the parameters M_∗ and log q(H) fixed to their optimal values. White contours show regions where the frequency fits have S² values within, respectively, the 1σ, 2σ, and 3σ confidence levels relative to the best-fit solution.