A golden age for stellar evolution and evolutionary grids - Commentary on: Schaller G., Schaerer D., Meynet G., and Maeder A., 1992, A\&AS, 96, 269

Model grids represent the basis for an enormous number of astrophysical studies and are a valuable help in many fields. The paper “New grids of stellar models from 0.8 to 120 M at Z = 0.020 and Z = 0.001”, by Schaller, Schaerer, Meynet, & Maeder, has had a strong impact reaching by today (April 14 2009) a total of 1900 citations in the (incomplete) ADS Citation database. What are the reasons for such an impact? I comment first on the reasons why these particular grids of stellar models were computed and made available to the community, and then briefly discuss some problems with the physical inputs of these models and annotate some of their applications in the course of years. Extended grids of stellar models that follow the evolution of the main phases for stars with different masses, possibly until their final stage as either a supernova or white dwarf for different initial chemical compositions, are one of the most useful tools in modern astrophysics, together with their most common derivation, the isochrone tables. They are used in the field of stellar astrophysics as a term of reference either in the study of individual binaries or in the study of the stellar populations of individual clusters. In addition, they are the most useful in studies of galactic and extra-galactic astrophysics to achieve models of population synthesis and to assign ages to stellar systems in the Universe. The nucleosynthesis resulting from stellar evolution and the yields of the matter expelled by stars of different mass at different times, either in winds or in supernova explosions, are the basic ingredients for studying the chemical evolution of galaxies. Computation of a grid is a necessary step in linking stellar physics to the study of the structure and evolution of galaxies. By commenting on the work by Schaller et al., we touch on some of the problems of stellar models concerning their “microphysics” inputs, that is, the opacities, equation of state, nuclear reaction rates, neutrino losses, and whatever else constitutes a physical input in the models that can be (at least formally) derived on the basis of first principles and that is generally made available to the stellar community by researchers dedicated to their specific computation. Models of stellar structure, however, need a proper understanding of other inputs, which we can call “macro-physics” that are not known from first principles (e.g. mass loss, rotation, convection). The best known of these problems – especially for model grids, which require a feasible and flexible model – is how we deal with super-adiabatic convection and what the “extra-mixing” is beyond the formal borders of convective regions, generally called overshooting. In the