Expanded view of the log q(core) − X_core(C + O) region where the two model solutions are found. The map represents , a “projection” of the 4-dimensional S² function (in log scale): at each log q(core), X_core(C + O) position, the value given is the minimum of log S² that can be found among the values obtained for all M_∗ and log q(H). White contours show regions where the frequency matches have S² values within, respectively, the 1σ , 2σ , and 3σ confidence levels relative to the best-fit solution. The yellow marks indicate the positions of the two solutions found by the optimization code.