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We analyze the production of the element Cr in galactic chemical evolution(GCE) models using the NuGrid nucleosynthesis yields set. We show that theunusually large [Cr/Fe] abundance at [Fe/H] $\approx 0$ reported by previousstudies using those yields and predicted by our Milky Way model originates fromthe merging of convective Si-burning and C-burning shells in a 20 $M_\odot$model at metallicity $Z=0.01$, about an hour before the star explodes. Thismerger mixes the incomplete burning material in the Si shell, including$^{51}$V and $^{52}$Cr, out to the edge of the carbon/oxygen (CO) core. Theadopted supernova model ejects the outer 2 $M_\odot$ of the CO core, whichincludes a significant fraction of the Cr-rich material. When including this 20$M_\odot$ model at $Z=0.01$ in the yields interpolation scheme of our GCE modelfor stars in between 15 and 25 $M_\odot$, we overestimate [Cr/Fe] by an orderof magnitude at [Fe/H] $\approx$ 0 relative to observations in the Galacticdisk. This raises a number of questions regarding the occurrence of Si-C shellmergers in nature, the accuracy of different simulation approaches, and theimpact of such mergers on the pre-supernova structure and explosion dynamics.According to the conditions in this 1D stellar model, the substantialpenetration of C-shell material into the Si-shell could launch aconvective-reactive global oscillation, if a merger does take place. In anycase, GCE provides stringent constraints on the outcome of this stellarevolution phase.

Chromium Nucleosynthesis and Silicon-Carbon Shell Mergers in Massive Stars