Metal-poor Stars Shed Light on the Origin of Gold

Neutron star mergers produce r-process elements
Neutron star mergers produce uncommon heavy components like gold. It is just not but clear whether or not collapsing stars additionally produce such components.
ESO / L. Calçada / M. Kornmesser

“We finally know where gold comes from!” introduced the headlines in 2017 following the detection of gravitational waves from the neutron star collision referred to as GW170817.

But do we actually?

The recipe to make components heavier than iron sounds easy sufficient: Bombard a lighter nucleus with neutrons and watch it develop. But there’s a catch — to provide heavy components like gold, platinum, and uranium, a nucleus has to develop actually quick, in any other case it decays into lighter components earlier than it reaches a secure kind. This fast course of produces about half of all components heavier than iron.

The cosmic origin of these rapid-process, or r-process components has lengthy been topic to debate. The fortuitous case of GW170817 precipitated an important leap ahead. Short-lived seen and infrared mild accompanying the neutron star merger carried clear signatures of r-process components. While just one ingredient, particularly strontium, has been recognized in the knowledge, scientists nonetheless estimated that this occasion alone doubtless produced between 3 and 13 Earth plenty’ value of gold.

But whereas there’s little doubt that neutron star mergers produce r-process components, the jury continues to be out on how necessary these occasions are in the grand scheme of issues. After all, different cosmic occasions would possibly produce these components, too. For instance, the violent deaths of large stars may additionally play a task. In a latest research to look in The Astrophysical Journal (preprint available here), a group of scientists exhibits that we should not low cost supernovae simply but.

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 research.

One long-standing situation considerations metal-poor stars present in the galactic halo. These sparse stars encompass the galaxy’s spiral disk and fashioned a very long time in the past from almost pristine gas that was barely touched by earlier generations of stars. Yet these metal-poor stars have a comparatively excessive quantity of r-process components of their atmospheres. How did these components get into the gas from which the stars have been born?

It normally takes billions of years for 2 stars in a binary system to turn into neutron stars, spiral towards one another, and merge. By the time the merger seeds the surrounding gas with r-process components, the metal-poor star had already been born.

The collapse of an enormous star nearing the finish of its transient life may additionally create situations conducive to the formation of r-process components, however on shorter time scales than that of a binary merger. The thought works in theory however hasn’t been confirmed immediately.

Brauer and her colleagues determined to check whether or not the collapsing star situation may account for the abundances of r-process components, specifically the europium noticed 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 group constructed a easy but self-consistent mannequin of a galaxy, represented by an enormous ball of gas wherein a quantity of stars collapse. Each stellar explosion enriches the gas with metals like iron, and a few of these supernovae additionally produce r-process components. The mannequin efficiently reproduces the relative abundances of europium and iron in metal-poor stars.

One key query is, what number of supernovae need to explode to account for the noticed abundances of r-process components? “[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 related to the most excessive explosions of big stars. The outcome implies that not each supernova can be producing r-process components, solely the most excessive ones.

Gamma-ray burst
This illustration of a gamma-ray burst coming from the collapse of an enormous star, which could be the sort of collapsing star probably to provide r-process components.
NASA / GSFC

Despite the promising outcomes, it’s too early to attract robust conclusions. “The team looks only at one element, europium, but it could also be possible to use barium, for example,” says Watson. Barium is comparatively straightforward to detect in the metal-poor stars and will assist constrain the mannequin. Furthermore, Brauer is already finding out how the complicated mixing of components in the gas from which the stars are born impacts the outcomes.

Watson additionally attracts consideration to a different often-overlooked line of proof: nanodiamonds. Some of these tiny, sub-micron diamonds present in meteorites comprise traces of r-process components.  “The question is, where is that coming from?” asks Watson. “Probably from a core-collapse supernova, but who knows?”

Ultimately, scientists should sort out the complicated query of the origin of r-process components from completely different angles. The way issues stand now, it appears that evidently multiple sort of cosmic supply contributes to the general abundance of gold and associated components in the universe.


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