Since different evolution packages (i.e. MIST, COSMIC, etc) have different definitions of z_solar (0.014 vs 0.02 etc), comparing "metallicity = 0" isn't exactly comparable. Feature request for some mechanism to compare despite the different definitions. For future reference, here's a message from @mwhosek from slack on this:
In SPISEA, we define the user input metallicity [Z] as the metallicity relative to solar. So, that is somewhat agnostic to what a given grid defines “solar” to be. That is why we have a variable called z_solar for each grid which defines what it considers solar to be, and then in each isochrone function we define the metallicity of the grid to be:
z_defined = self.z_solar * (10**metallicity)
where metallicity is the value the user inputs.
So, in my mind, when we tell SPISEA to return an isochrone with metallicity = 0, we are asking to use the solar metallicity grid, whatever the solar metallicity happens to be defined as. Thus, I think it is fine to compare “solar” metallicity for two different evolution models, although in such a comparison we’d need to note that the intrinsic metallicity each grid calls “solar” is slightly different.
In a practical sense, since we don’t yet have metallicity interpolation in the main version of SPISEA, we wouldn’t be able to generate a z = 0.014 model from a grid that only has a z = 0.02 model. Right now, SPISEA just takes the closest model grid in metallicity to the input value, so if you asked for z = 0.014 then it would round to the z = 0.02 grid anyway.
Since different evolution packages (i.e. MIST, COSMIC, etc) have different definitions of z_solar (0.014 vs 0.02 etc), comparing "metallicity = 0" isn't exactly comparable. Feature request for some mechanism to compare despite the different definitions. For future reference, here's a message from @mwhosek from slack on this: