The NPR News item Astronomers Strike Gravitational Gold In Colliding Neutron Stars mentions and quotes "Daniel Kasen, a theoretical astrophysicist at the University of California, Berkeley:"

He spent late nights watching the data come in and says the colliding stars spewed out a big cloud of debris.

"That debris is strange stuff. It's gold and platinum, but it's mixed in with what you'd call just regular radioactive waste, and there's this big radioactive waste cloud that just starts mushrooming out from the merger site," Kasen says. "It starts out small, about the size of a small city, but it's moving so fast — a few tenths of the speed of light — that after a day it's a cloud the size of the solar system."

According to his estimates, this neutron star collision produced around 200 Earth masses of pure gold, and maybe 500 Earth masses of platinum. "It's a ridiculously huge amount on human scales," Kasen says. He personally has a platinum wedding ring and notes that "it's crazy to think that these things that seem very far out and kind of exotic actually impact the world and us in kind of intimate ways."

Has the merger of neutron star binaries been necessary to explain abundances of the heavy elements such as gold and platinum, or is this just an anecdotal item? How important are binary neutron stars for the abundances of heavy elements such as gold? Is there a particular or notable paper I can read on this?

I have already read this answer but I'm looking for a better explanation of the need for this kind of merger to explain abundances. I'm pretty sure there is nothing in any observed gamma ray events that shows spectral lines of gold or any identifiable heavy element (due to the incredible doppler broadening), so the connection must actually come from simulations.


1 Answer 1


The creation of some very heavy neutron-rich elements, like gold and platinum, requires the rapid capture of neutrons. This will only occur in dense, explosive conditions where the density of free neutrons is large. For a long time, the competing theories and sites for the r-process have been inside core-collapse supernovae and during the merger of neutron stars.

My understanding is that it has become increasingly difficult for supernovae to produce (in theoretical models) sufficient r-process elements to match both the quantity and detailed abundance ratios of r-process elements in the solar system (see for example Wanajo et al. 2011; Arcones & Thielmann 2012). The conditions required, particularly a very neutron-rich environment in the neutrino-driven winds, are just not present without the fine tuning of parameters (see below).

Instead, the models invoking neutron star mergers are much more robust to theoretical uncertainties and successfully produce r-process elements. The question mark appears to be only over their frequency at various times in the evolution of a galaxy and exactly how much enriched material is ejected.

The announcement of GW170817 makes this all more plausible. A neutron star merger has been seen. The behaviour of the optical and infrared emission after the event matches the expectations of merging neutron star models (e.g. Pian et al. 2017; Tanvir et al. 2017). Of particular note is the developing opacity and fading in the blue and visible, with the spectrum becoming dominated by the infrared with broad spectral features. This is the expectation for an expanding cloud of material that is heavily polluted by the presence of lanthanides and other r-process elements (Chornock et al. 2017). The reasonable agreement between the observations and models suggests that indeed a large quantity of r-process elements were produced in this explosion.

To go from there to the claim that the origin of gold is solved (as claimed in the press conference) is a step too far. The amount of r-process material produced has large uncertainties and is model-dependent. The rate of mergers is only constrained to about an order of magnitude in the local universe and is not measured/known in the early universe. What could be said is that this channel for r-process production has been directly observed and so must be taken into account.

On the other hand, r-process production by the supernova channel is not yet ruled out. Some simulations at least, that involve rotation and magnetic fields appear to be still "in the game" (e.g. Nishimura et al. 2016). It could be that the presence of significant r-process material in very old metal-poor stars requires a supernova channel, since the merger of neutron stars takes some considerable time to occur (e.g. Cescutti et al. 2015; Cote et al. 2017).

The overall picture is still uncertain. A review by Siegel (2019) concludes that the best fit to the available evidence is that some rare type(s) of core collapse supernovae (known as "collapsars") are still the best bet to explain the Milky Way r-process elements. The primary evidence for this is the presence of Europium (an r-process element) enhancements in some very old halo stars and the general trend of decreasing Eu/Fe with increasing Fe, suggesting a more alpha-element-like production site for the r-process - i.e. supernovae.


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