# trying to understand stellar nucleosynthesis

Trying to create a periodic table that shows "where" elements are made. For example big bang, cosmic rays, stars, supernovae etc...but reading a lot about gold,and other heavy neutron rich elements, being formed in colliding neutron stars. Is there a definitive list of elements that may have been formed in neutron stars?

## 1 Answer

The elements you are talking about are created by the r-process - the rapid neutron capture onto heavy elements that can occur in neutron-rich environments. The r-process is probably responsible to some extent for all the most neutron rich (i.e. more neutrons than protons) nuclei with atomic masses above that of iron and is probably responsible for all elements heavier than lead and bismuth. However, there are three broad peaks in the solar-system abundances, centred at Germanium, Xenon and Platinum (the first, second and third r-process peaks), where the r-process is thought to dominate the production over competing processes such as the s- and p-processes (plot taken from here).

If one subtracts the expected contributions from other sources one can estimate the fraction of a given element that comes from the r-process (plot taken from here).

It is a contemporary astrophysical problem (i.e. there is no definitive answer) for the source of the r-process. In other words, it is an unsolved mystery as to what fraction of each r-process element comes from which astrophysical location and it may well vary as a function of cosmic time or location. The two main candidates for the sites of creation for r-process elements are core-collapse supernovae, the neutrino driven winds from supernovae and neutron star mergers.

The main problems to be solved are the detailed physics of supernova explosions and neutron star mergers, the relative populations and numbers of these events as a function of cosmic time and location and even the nuclear physics ingredients that go into calculating the yields of such elements.

A recent paper by van de Voort (2015) performs simulations that suggests most of the r-process elements in the universe could be created by neutron star mergers, and they point out that it is especially difficult for core-collapse supernovae to produce what are known as the "third-peak" r-process nuclei. These claims are all contentious, although the latter point about the heavier r-process elements is echoed in a number of previous works. (e.g. Wanajo 2013). Nevertheless, even this is disputed; for example Nishimura et al. (2015) perform simulations of rapidly rotating supernovae progenitors with strong magnetic fields and find that they can reproduce the abundance pattern of all the r-process elements seen in the Sun.

So if I wanted a vague stab in the dark at directly answering your question, I would choose the group of elements in the third r-process peak, with mass numbers $191 < A< 197$ as being most likely to have been (mostly) produced by neutron star mergers. This includes Osmium (192), Iridium (191, 193), Platinum (194, 195, 196) and possibly gold (197).