Nucleosynthesis of Gold in Neutron Star Collisions

Gold is rare on Earth and it is also rare in the Universe. Unlike elements like carbon, silicon or iron, gold cannot be created within the core of a star. Instead, the creation of gold requires a more energetically cataclysmic event. Short-duration gamma-ray bursts (SGRBs) are intense flashes of gamma-rays lasting less than ~2 seconds. They are believed to be produced following the merger of compact object binaries involving two neutron stars (NS-NS) or a neutron star and a black hole (NS-BH).

A compact object binary forms when both massive stars in a binary system separately explode as supernovae and leave behind their collapsed cores as a tightly bound NS-NS, NS-BH or BH-BH pair. As both compact objects circle each other, they radiate away gravitational waves and draw closer to each other until they eventually collide. Nevertheless, only collisions involving NS-NS and NS-BH pairs can produce SGRBs. “It’s a very fast, catastrophic, extremely energetic type of explosion,” says Edo Berger, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA).

NS-NS and NS-BH mergers are expected to create significant quantities of neutron-rich radioactive nuclei via the r-process, also known as the rapid neutron capture process, from the ejection of neutron-rich material. These radioactive nuclei will decay and produce a faint transient, known as a “kilonova”, in the days following the SGRB. It is believed that NS-NS and NS-BH mergers may be the dominant source for stable r-process elements in the Universe. All r-process elements are heavier than iron, a list that includes gold, mercury, platinum, uranium, thorium and more.

Berger et al. (2013) present the first detection of a kilonova following a SGRB. The SGRB is identified as GRB 130603B and it was initially detected by NASA’s Swift satellite on 3 June 2013 at 15:49:14 UTC. Although the burst event itself lasted for less than two-tenths of a second, GRB 130603B displayed a slowly fading afterglow dominated by infrared light. Over the next few days, telescopes in Chile and the Hubble Space Telescope (HST) performed optical and near-infrared observations of the afterglow.

A kilonova model with estimated ejecta mass ~10,000 to 30,000 Earth masses travelling at ~10 to 30 percent the speed of light is consistant with the observed properties of the afterglow from GRB 130603B. Assuming 10 parts per million of the ejecta mass is in the form of gold, that works out to ~10 times the mass of the Moon in gold alone. In comparison, the total amount of gold that have been mined in human history is roughly equivalent in terms of volume to a cube 21 metres on a side.

GRB 130603B is the first SGRB with evidence for r-process rich ejecta and it provides a clear signature for a compact object merger event involving either a NS-NS collision or a NS-BH collision. Based on the ejecta mass estimated for GRB 130603B and on the known frequency of SGRBs, compact object mergers are likely to be the dominant site for the nucleosynthesis of stable r-process elements in the Universe. “It’s possible that supernovae still produce a small contribution, but they do not appear to be the dominant process,” says Berger. After being created and ejected outward, these heavy elements eventually become incorporated into the formation of subsequent generations of stars and planets elsewhere in the galaxy. “To paraphrase Carl Sagan, we are all star stuff, and our jewellery is colliding-star stuff,” says Berger.

Reference:

Berger et al. (2013), “An r-Process Kilonova Associated with the Short-Hard GRB 130603B”, arXiv:1306.3960 [astro-ph.HE]