Enrichment scheme

Of the 3 sources of metals (SNII, SNIa + PNe), 2 have yields provided in the same way (SNII + PNe). SNIa are much simpler than those since every SNIa outputs the same ejecta, so the only complication is when those are ejected.

Thus, I will describe the SNII + PNe enrichment scheme. In both cases, stellar models are run that result in the stratified abundances of elements inside stars. It is then possible to determine which layers are ejected and which layers are trapped in a core that holds onto the elements forever. The models sum up the masses of elements that are ejected and list those masses for each stellar model of a given mass, metallicity and spin.

For the chemical evolution model, we will create a stellar population comprised of many stars that all have the same (initial) metallicity. We want to know the mass of each element ejected as a function of time. The goal is thus to convert the stellar masses of the stellar models to time based on the lifetimes of stars with those masses, and then to convert the mass ejected from each stellar mass into a fraction of the entire stellar population mass so that it is possible to multiply by the mass of the stellar population to get the yield as a function of time.

One question is what the time interval should be at which the yields are tracked and stored in an enrichment table. Woosley + Weaver (1995) and Limongi & Chieffi (2012) publish models at 11 different masses (WW95: 11, 12, 13, 18,19,20, 22, 25, 30, 35, 40). Karakas (2010) publishes stellar models for 16 masses (1, 1.25, 1.5, 1.75, 1.9, 2, 2.25, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5). Thus, it seems sensible to calculate yields at the 27 times that correspond to those masses and then interpolate the enrichment to some coarser grained scale once the engine is calculating yields coming out of the stars.

The trick is that those masses correspond to vastly varying timescales, so it is necessary to calculate yield rates to be multiplied by the varying the time intervals.