arXiv2007
The ejecta of the first probably very massive stars polluted the Big Bang primordial element mix with the first heavier elements. The resulting ultra metal-poor abundance distribution provided the initial conditions for the second stars of a wide range of initial masses reaching down to intermediate and low masses. The importance of these second stars for understanding the origin of the elements in the early universe are manifold. While the massive first stars have long vanished the second stars are still around and currently observed. They are the carriers of the information about the first stars, but they are also capable of nuclear production themselves. For example, in order to use ultra or extremely metal-poor stars as a probe for the r-process in the early universe a reliable model of the s-process in the second stars is needed. Eventually, the second stars may provide us with important clues on questions ranging from structure formation to how the stars actually make the elements, not only in the early but also in the present universe. In particular the C-rich extremely metal-poor stars, most of which show the s-process signature, are thought to be associated with chemical yields from the evolved giant phase of intermediate mass stars. Models of such AGB stars at extremely low metallicity now exist, and comparison with observation show important discrepancies, for example with regard to the synthesis of nitrogen. This may hint at burning and mixing aspects of extremely metal-poor evolved stars that are not yet included in the standard picture of evolution, as for example the hydrogen-ingestion flash. The second stars of intermediate mass may have also played an important role in the formation of heavy elements that form through slow neutron capture reaction chains (s-process). Comparison of models with observations reveal which aspects of the physics input and assumptions need to be improved. The s-process is a particularly useful diagnostic tool for probing the physical processes that are responsible for the creation of elements in stars, like for example rotation. As new observational techniques and strategies continue to penetrate the field, for example the multi-object spectroscopy, or the future spectroscopic surveys, the extremely metal-poor stars will play an increasingly important role to address some of the most fundamental and challenging, current questions of astronomy.