Silver and Palladium - ancient stars tell the story of a second r-process

Camilla Hansen, Heidelberg U.
February 12, 2013, NCSU

A lot of information can be gained from stellar spectra e.g. the stellar ages, velocities, evolutionary stage, abundances, as well as the composition of the environment they were born in. High quality spectra allow us to derive stellar abundances of about 2/3 of the elements in the periodic table. By comparing abundances of elements belonging to different groups in the periodic table, we can gain information about long gone supernovae - maybe even the first ones? These supernovae enriched the gas we observe today in later generations of stars. To date we still do not fully understand how the heavy elements (Z > 37) are formed. Two main production channels are known to create the majority of the heavy elements through neutron captures, namely the rapid neutron capture (r-)process and the slow neutron-capture (s-)process. Recently we have seen that each of these production channels seem to branch into two, a main and a weak process. The nature of weak s-process is fairly well known, while the weak r-process remains a puzzle. Silver and palladium, as well as other elements in the range 40< Z < 50, are thought to form via the weak r-process. Hence, Pd and Ag may carry key information on this process. By studying elements (Sr, Y, Zr, Ba, Eu) with well known formation processes, we can compare their abundances to those of Ag and Pd, and thereby learn about the differences/similarities of various formation processes. Here, I will outline the procedure astronomers go through to derive the stellar abundances, the approximations, and problems we face when doing so.