Gold Creation

One of my favorite astronomy – themed t – shirts says “You are star dust”. It makes the point that we are all made from long dead stars. Of course, everything around us is also made of star stuff. This even includes rare elements such as gold. This paper discusses the process of gold formation.

Most main sequence stars are in the business of making helium from hydrogen. Other elements can be created through fusion . However, this only works for elements as heavy as iron or less. That’s because creation of elements heavier than iron requires additional energy rather than producing energy.

There are two main processes involved in creating the heavier elements. These are called the s – process and the r – process. These are not exotic names — s – process simply means the slow process and r – process means (you guessed it) the rapid process. These processes were largely demonstrated in a seminal paper by Margaret and Geoffrey Burbidge, William Fowler and Fred Hoyle in 1957. This paper is so famous it is often referred to simply as B2FH — the last initials of the authors. Fowler would win the Nobel Prize in 1983 for his work in this area.

You already know that atoms are made up of electrons, neutrons and protons. Any addition or subtraction of an electron that results in a non – neutral atom is called ionization. Any addition or subtraction of a neutron results in a new isotope of an atom. Any addition or subtraction of a proton creates a different element. The process of building up an atom’s nucleus (protons and neutrons) is called nucleosynthesis.

Since protons all have the same charge, it is difficult for an atom to add a proton to an atom’s nucleus because the it has the same kind of charge as the protons already in the nucleus. But a neutron has no charge so adding a neutron is much easier. This is called neutron capture. As we saw above, adding a neutron results in a new isotope of an atom. But the extra neutron is not likely to remain for long. Instead, a process called beta decay results in the neutron turning into a proton (and some other stuff that we will ignore for now).

So the creation of heavy elements requires a supply of neutrons and a lot of energy to shove them into already existing atoms. The s – process can produce some of the heavier elements but s – process elements tend to be much lighter than gold. The s – process can occur during a star’s carbon burning phase. The r – process takes something quite a bit stronger — a supernova. In other words, a star has to be blown to smithereens in order to create gold and other heavy elements. Our solar system has been the benefactor of the r – process. Many rare earths including gold show that to be the case. The evidence is that our solar system was metal enriched by more than one supernova.

But we still don’t know the exact mechanism involved. Here are some possibilities. Each of these has some evidence to support it but also has some reasons for doubt.

MHD Jets

A tightly focused jet of neutrons might create r – process elements. Calculations based on magnetohydrodynamic jets were done for a star having 13 solar masses. In this scenario a supernova explosion does not create a simple spherical outward shock wave but instead a magnetically focused jet passes through oxygen rich layers which kicks off the r – process. The simulations agree with the abundance of r – process elements. But it is not at all clear that there are enough neutrons in this scenario to make it all work.

Quark—Nova

The remnant star from a supernova can be a neutron star. The stellar wind from the new neutron star can increase the likelihood of a successful r – process.The idea that a neutron star is a good place for the r – process makes sense. If we are going to capture a neutron, it has to be easier when a lot of neutrons are available.

Normally we think of neutron stars as the end result of a supernova. But another kind of nova, called a quark – nova, could create the greatest explosions in the universe. It’s possible that quark – novae are the source of gamma – ray bursts.

For r – process elements, the key point about a quark – nova is that there is a transition in the core of a neutron star where matter becomes some kind of strange quark matter. This process is quite violent. It ejects a lot of neutrons at the surface of the neutron star. This matter decompresses creating a neutrino – driven wind. The resulting wave increases the likelihood of r – process nucleosynthesis. But this scenario requires an improbably heavy neutron star and the winds may not be sufficient for the heavier r – process elements such as gold. Also, neutrino – driven winds actually have an inhibitory effect on the r – process.

Neutron Star Mergers

A pair of neutron stars will merge about once per 100,000 years in the Milky Way galaxy. The effects of this rare occurrence are not completely known. The production of r – process elements might be most efficiently done during a neutron star merger. Two nearby stars have to both go supernova without one star gaining enough mass from the other to form a black hole. Then the neutron stars revolve around a common center close enough that they eventually merge. The result might yet create a black hole but a lot of neutrons are available for capture. As in the Proto Neutron Star scenario, the ejecta from the neutron star material and the expected abundances match what we see in the galaxy. However, the rarity of these mergers makes this scenario doubtful.

Collapsar

Another way to synthesize r – process elements is when a black hole is created in a collapsar — a star that collapses. Heavy elements can be synthesized in collapsars. A collapsar is formed when a black hole is formed in a specific way. The original massive star is rotating when it explodes but the explosion is incomplete. Collapsars (like the quark – novae described above) are a leading candidate for gamma – ray bursts. The total amount of heavy elements created in a collapsar can be 1% of a solar mass. Some heavy elements are created in a collapsar much more easily than in a supernova. But the collapsar scenario is a new idea and hasn’t had enough time to be attacked. Previous studies showed that collapsars with slower rotations would not produce the heaviest r – process elements.

Supernova Itself

The energies unleashed in such an explosion are sufficient but there is some doubt that supernovae can account for the abundances of r – process elements.

All of the above involve the aftermath of a supernova. So the simplest answer to “How is gold made?” is still supernovae. It has been reported that the dominant source of the r – process elements are Type II Supernovae with progenitor masses between 20 and 40 solar masses. But it is not certain if one or more of the above processes are necessary or if the supernova itself creates the r – process elements.

There are a couple of specific supernovae scenarios that can create r – process elements. One case involves a star with about 10 solar masses at the time of the supernova. Such a supernova gets two chances to create r – process elements: in the prompt supernova explosion or in a neutrino – driven wind that follows.

(Another version of this article is available with references. See http://ephemeris.sjaa.net/0706/GoldCreation.doc )