Annu. Rev. Astron. Astrophys. 1994. 32: 153-90 Copyright © 1994 by Annual Reviews. All rights reserved

2.3. How to Make Heavy Nuclei

We are now in a position to ask the question "How does nature make heavy nuclei?" We have seen in the previous sections that if we eject nuclei into cold interstellar space, and if these nuclei are always in NSE, then these nuclei must be those with the largest binding energy per nucleon for whatever Y_e is appropriate for the environment in which the nuclei find themselves. From Figure 7, however, we see that for a large range of Y_e, these nuclei are simply iron-group nuclei. If NSE always pertains, stars and supernovae can only eject iron-group nuclei. Our observations of uranium on Earth tell us that this is not what happens.

The escape from this dilemma is the fact that nucleosynthetic systems cannot always be in NSE. There are only two ways this can happen. The first possibility is that the system never has time to come into NSE before the star ejects the nucleons into the interstellar medium. In this case the nucleons assemble themselves part of the way up to iron. This is the "falling short of equilibrium" scenario. The second possibility is that the system begins in NSE at high temperature. As the system expands and cools, the equilibrium changes. As the temperature drops, some nuclear reactions slow down. Eventually these reactions become too slow to allow the system to maintain equilibrium. This is the "freezeout from equilibrium" scenario. These are the only two ways a nucleosynthetic system can be out of equilibrium, and they are the only two options nature has to assemble heavy nuclei.

How in fact does nature make heavy elements in these two scenarios? In the freezeout from equilibrium, a nucleosynthetic system falls out of equilibrium. At this point the composition reflects the last NSE abundance distribution the system attained before it fell out of equilibrium. This composition is a mix of free nucleons, light nuclei, and iron-group nuclei. While some of the reactions necessary to maintain equilibrium become too slow, others such as capture of free nucleons and light nuclei on iron-group nuclei can continue. The iron-group nuclei can serve as "seeds" for the capture of the remaining free nucleons and light nuclei. These captures can produce heavy nuclei. Because the system assembles its own seed nuclei in such a scenario, the system can make heavy elements without any pre-existing seed nuclei. A process that does not require pre-existing seed nuclei to make new nuclei is called primary.

In the other scenario - the falling short of equilibrium scenario - the material never achieves NSE because the timescale to reach equilibrium is always too long compared to the dynamical timescale of the system. The system assembles light nuclei into somewhat heavier nuclei but these are still less massive than the iron-group nuclei characteristic of NSE. An accidental effect of the main nuclear reactions between the light nuclei in this scenario is the liberation of nucleons. If iron-group or heavier nuclei already exist in the system, they can capture these nucleons to produce new heavy nuclei. In this scenario, then, the system must have pre-existing seed nuclei in order to produce heavy elements. We call a process that requires pre-existing seed nuclei to make new elements a secondary process.

These are the only two ways nature can assemble heavy elements. We should not be surprised then that there are two major distributions of heavy nuclei - the r-process and the s-process distributions. We shall see that the r-nuclei in our Solar System likely formed in an environment that experienced a freezeout from equilibrium while the s-nuclei must have formed in an environment that was striving for, but never reached, NSE. The differing character of these scenarios results in the different character of the r- and s-process abundance distributions.

Once an abundance of heavy elements is available, nature may make modifications to it by exposing it to a flux of photons, neutrinos, or nucleons. Such events are probably responsible for the production of the majority of p-nuclei.