“We finally know where gold comes from!” announced the headlines in 2017 following the detection of gravitational waves from the neutron star collision known as GW170817.
But do we really?
The recipe to make elements heavier than iron sounds simple enough: Bombard a lighter nucleus with neutrons and watch it grow. But there’s a catch — to produce heavy elements like gold, platinum, and uranium, a nucleus has to grow really fast, otherwise it decays into lighter elements before it reaches a stable form. This rapid process produces about half of all elements heavier than iron.
The cosmic origin of these rapid-process, or r-process elements has long been subject to debate. The fortuitous case of GW170817 precipitated a great leap forward. Short-lived visible and infrared light accompanying the neutron star merger carried clear signatures of r-process elements. While only one element, namely strontium, has been identified in the data, scientists nonetheless estimated that this event alone likely produced between 3 and 13 Earth masses’ worth of gold.
But while there’s no doubt that neutron star mergers produce r-process elements, the jury is still out on how important these events are in the grand scheme of things. After all, other cosmic events might produce these elements, too. For example, the violent deaths of massive stars could also play a role. In a recent study to appear in The Astrophysical Journal (preprint available here), a team of scientists shows that we shouldn’t discount supernovae just yet.
History, as Told by Metal-poor Stars
“There are a lot of problems with neutron star mergers as a source of heavy elements in the early universe,” explains Kaley Brauer (Massachusetts Institute of Technology), who led the new study.
One long-standing issue concerns metal-poor stars found in the galactic halo. These sparse stars surround the galaxy’s spiral disk and formed a long time ago from nearly pristine gas that was barely touched by earlier generations of stars. Yet these metal-poor stars have a relatively high amount of r-process elements in their atmospheres. How did these elements get into the gas from which the stars were born?
It usually takes billions of years for two stars in a binary system to become neutron stars, spiral toward each other, and merge. By the time the merger seeds the surrounding gas with r-process elements, the metal-poor star had already been born.
The collapse of a massive star nearing the end of its brief life could also create conditions conducive to the formation of r-process elements, but on shorter time scales than that of a binary merger. The idea works in theory but hasn’t been proven directly.
Brauer and her colleagues decided to test whether the collapsing star scenario could account for the abundances of r-process elements, in particular the europium observed in metal-poor stars. “We started with a simple assumption,” says Brauer. “What if you said all heavy elements were formed in this way in the early universe?”
Europium, Barium & Nanodiamonds
The team constructed a simple yet self-consistent model of a galaxy, represented by a giant ball of gas in which a number of stars collapse. Each stellar explosion enriches the gas with metals like iron, and some of these supernovae also produce r-process elements. The model successfully reproduces the relative abundances of europium and iron in metal-poor stars.
One key question is, how many supernovae have to explode to account for the observed abundances of r-process elements? “[The researchers] come to some interesting conclusions,” says Darach Watson (University of Copenhagen). “They find frequencies which are similar to those of long gamma-ray bursts.” Such gamma-ray bursts are associated with the most extreme explosions of giant stars. The result implies that not every supernova would be producing r-process elements, only the most extreme ones.
Despite the promising results, it’s too early to draw strong conclusions. “The team looks only at one element, europium, but it could also be possible to use barium, for example,” says Watson. Barium is relatively easy to detect in the metal-poor stars and could help constrain the model. Furthermore, Brauer is already studying how the complex mixing of elements in the gas from which the stars are born affects the results.
Watson also draws attention to another often-overlooked line of evidence: nanodiamonds. Some of these tiny, sub-micron diamonds found in meteorites contain traces of r-process elements. “The question is, where is that coming from?” asks Watson. “Probably from a core-collapse supernova, but who knows?”
Ultimately, scientists will have to tackle the complex question of the origin of r-process elements from different angles. The way things stand now, it seems that more than one type of cosmic source contributes to the overall abundance of gold and related elements in the universe.