Measurement errors in can therefore cause a slope in the - diagrams of . However, since the measurement errors in are small compared to the range in , the effect should be small. We choose to minimize the residuals in since we want to predict from . This is in agreement with J97, who found a 0.3-0.4 dex increase for the same interval. The relations are more noisy than for .
Different time scales for star formation.
The progenitor stars of type II SNe are more massive and short-lived than those of type Ia SNe. At this point, only a small fraction of gas is left for new long-lived stars to be formed out of this iron enriched material. A variable IMF slope. Selective loss mechanisms. In the standard picture of SN-driven winds, the outcome is , in contradiction to the observations.