Revisiting and assessing uncertainties in stellar populations synthesis models

In this review we address the uncertainties implicit in evolutionary synthesis model computations. After describing the general structure of synthesis codes, we discuss several source of uncertainties that may affect their results. In particular, we discuss the uncertainties arising in the computation of isochrones from evolutionary tracks; those related to atmosphere models; those that are a consequence of the incompleteness of the input ingredients; and those associated with the computational aspect used in synthesis codes. We also discuss the issue of the inclusion of distributed properties in synthesis models, an issue that will become relevant in the next future; as a paradigm of this case, we illustrate the difficulties implied by the inclusion of tracks with rotation in synthesis models. Finally, we describe several examples of the statistical approach to population synthesis. We report on the failure of the fuel consumption theorem (FCT) and the isochrone synthesis code to produce mutually consistent results. However, we argue that FCT and isochrone synthesis results are reliable for application to real systems in the wavelength range where they coincide. On the constructive side, we derive several useful survival strategies to bypass uncertainties. We show that single stellar populations at the turn-off ages of the tabulated tracks can be safely compared, as they are scarcely affected by the interpolation scheme used to compute isochrones. Finally, we suggest to use derivative quantities, such as the SN-rate, as bug detectors. On the recommendation side, we advocate for greater transparency and more documentation in synthesis modeling. We also ask stellar model makers to think of us and include more mass values in the tracks.