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Measurements of explosive nucleosynthesis yields in core-collapse supernovaeprovide tests for explosion models. We investigate constraints on explosiveconditions derivable from measured amounts of nickel and iron after radioactivedecays using nucleosynthesis networks with parameterized thermodynamictrajectories. The Ni/Fe ratio is for most regimes dominated by the productionratio of 58Ni/(54Fe + 56Ni), which tends to grow with higher neutron excess andwith higher entropy. For SN 2012ec, a supernova that produced a Ni/Fe ratio of$3.4\pm1.2$ times solar, we find that burning of a fuel with neutron excess$\eta \approx 6\times 10^{-3}$ is required. Unless the progenitor metallicityis over 5 times solar, the only layer in the progenitor with such a neutronexcess is the silicon shell. Supernovae producing large amounts of stablenickel thus suggest that this deep-lying layer can be, at least partially,ejected in the explosion. We find that common spherically symmetric models of$M_{\rm ZAMS} \lesssim 13$ Msun stars exploding with a delay time of less thanone second ($M_{\rm cut} < 1.5$ Msun) are able to achieve such silicon-shellejection. Supernovae that produce solar or sub-solar Ni/Fe ratios, such as SN1987A, must instead have burnt and ejected only oxygen-shell material, whichallows a lower limit to the mass cut to be set. Finally, we find that theextreme Ni/Fe value of 60-75 times solar derived for the Crab cannot bereproduced by any realistic-entropy burning outside the iron core, andneutrino-neutronization obtained in electron-capture models remains the onlyviable explanation.

Constraints on explosive silicon burning in core-collapse supernovae from measured Ni/Fe ratios