Publication Abstracts

Cameron and Truran 1971

Cameron, A.G.W., and J.W. Truran, 1971: The chemical evolution of the Galaxy. J. Roy. Astron. Soc. Canada, 65, 1-12.

One of the more complicated problems in astrophysics is that of the chemical evolution of the galaxy. There are many different pieces of observational information which bear on this subject, and any general theory must draw on a large number of individual theories which describe the behavior of various parts of galaxies. These include not only theories of galactic structure, but also those of stellar evolution, of nucleosynthesis, of star formation, and of the behavior of the interstellar medium. As these theories are all imperfectly developed at the present time, it might be considered very premature to attempt a grand synthesis of such theories to account for the evolution of the galaxy as a whole. However, we have found it very instructive in our own research to attempt to find a consistent set of assumptions involving these various theories which predicts a large number of observed properties of the galaxy (Truran, Hansen and Cameron 1965; Truran and Cameron 1970). It is very likely that more than one successful set of such assumptions can be found; nevertheless, it is true that the behavior of one aspect of galactic evolution can bring about agreement with the observed properties of the galaxy only provided that various other aspects of different theories are also true. Thus, it is clear that any set of consistent assumptions which is put forward to explain the properties of the galaxy must stand or fall together. Such a set of assumptions provides a set of predictions which can be investigated in further research; if any of these assumptions proves to be false, then it is very likely that other assumptions are also false. Proceeding in this manner, we believe that a useful contribution to galactic research can be made.

The detailed results which we have obtained in our calculations will be presented elsewhere. In this article we present one interesting set of selfconsistent assumptions which has been arrived at in the course of our investigations, and we indicate the consequences of deviations from these assumptions. Most or' our assumptions deal with the consequences of stellar evolution as they affect various processes of nucleosynthesis. Our stellar evolution assumptions are summarized in figure 1, where we have indicated the compositional structure of stars of various mass at the endpoint of their evolution. We also show a successful set of calculations of the abundances of different classes of elements as a function of time in the galaxy in figure 2; these abundances are all given relative to the abundances of the appropriate class of elements in the solar system. We consider these results to represent a successful evolutionary sequence of element abundances in the galaxy because these ratios are all close to unity at a time in galactic evolution which we identify with the formation of the solar system. In the following sections, we elaborate various of the assumptions leading to this rather more successful model.