Left panel: illustration of a situation such that the assumed core mass of a model (not necessarily converged) is too small. In the right panel the assumed core mass is too large. The solid lines represent y in the model while the dashed ones are extrapolations. Case 1 stands for a model without discontinuity at the core boundary when the Schwarzschild criterion is used. Case 2 represents situations such that y has a small discontinuity. Case 3 stands for a larger discontinuity.