Gravitational Waves, Nuclear Fire, Rocks, and Love

The old trope that ‘we are starstuff’ has ingrained itself in our minds to such an degree that it tends to lose some of its poetry. Yes, components heavier than hydrogen and helium in our earthly environment were created as part of the differed life-cycles of long-gone generations of stars. Many of these cosmic heating systems expunged their guts into the void, contaminating our galaxy with traces of the atomic nuclei we call oxygen, carbon, iron and more. And over the eons gravity has caused the re-condensation of this interstellar matter. As a result the aspects have actually been segregated, permitting the starstuff to become extraordinarily focused – making brand-new stars, planets, and the clusters of heavy nuclei that make up human beings and their ludicrous complexity.

It is truly fantastic, but repeat this tale enough times and it begins to noise a little ordinary. Part of the factor is that the narrative can end up being unclear – from talking in broad terms about earlier generations of now hidden stars, to our loose descriptions of the nature of interstellar matter. It’s a bit like when an aged relative tells you about your extended family tree. There can be little to recognize with, even however you really want to make the connection.

The story gets a lot more fascinating when you look closer though. For one thing, not all aspects are produced in the same method. Possibly the most intriguing example is that of the so-called ‘r-process’ aspects. These have nuclei much heavier than iron and are developed by a mechanism called rapid neutron-capture. As the name suggests, you requirement something to capture the neutrons, in the form of ‘seed’ nuclei, and you need a fearsome flux of neutrons – enough coming in quick sufficient to construct up nuclei beyond any extremely unsteady intermediate setups.

But where do such environments exist?

In 2017 the gravitational wave observatories LIGO and Virgo made headlines by finding the telltale signature of a binary neutron-star merger. Two stellar-mass balls of nuclear material spiralling together with an escalating scream of spacetime oscillations.

Unlike binary black hole mergers this event churned out a prodigious quantity of electro-magnetic radiation in what’s called a kilonova (literally a thousand times the output of an common outstanding nova). Telescopic study of the kilonova produced engaging assistance for a picture where merging neutron stars are an r-process paradise. This suggests that these catastrophic events play a major role in providing some of the heaviest components to our galactic landscape. From gold, platinum, and iridium, to thorium, uranium, and brief components like plutonium.

Now, a brand-new piece of research by Bartos and Marka, published this week in Nature, supplies an ingenious and somewhat shocking insight to the origins of r-process components in our own solar system. To achieve this they combine two key analyses. One is data from meteorites that maintain evidence of the elemental mix in our forming solar system some 4.6 billion years back. The other is a creative analytical model of the Galaxy’s history of neutron star mergers.

What the research points to is a very close by neutron star crash that took place at the dawn of our regional cosmic history. Traces of this one event seem to be present in the information of radioisotopes coming from the r-process that got sprayed into our forming system after the neutron stars collided.

Reaching this conclusion requires some nimble thinking and difficult foot-work. Neutron star-on-neutron star mergers are cosmically rare in the Milky Way, with in between one and a hundred occurring per million years across its entire expanse. Particular r-process aspects, like the actinides (including Curium-247, Plutonium-244, and Iodine-129), not just have relatively short half-lives, measured in the 10s of millions of years, however have left distinct signatures in ancient solar system meteoritic material that enable us to measure their original abundances. So, the amount of these aspects that existed during the window of time that our solar system was forming uses a lever arm on not just how recently those elements had actually been forged, however also how close-by the forge need to have been. 

By building a simulation of neutron star mergers throughout our galaxy, and throughout its history leading up to our solar system’s development (about 9 billion years into the Milky Method’s existence), Bartos and Marka are able to analyze what scenarios might have produced the actinide mix presumed from meteoritic analyses.

The result is that it appears that there was a single kilonova from a neutron star merger that happened within 80 plus-or-minus 40 million years of the development of the solar system and was about 1,000 light years away. The scientists price quote that such a close-by kilonova occasion would outshine whatever in the night sky for over a day. 4 and a half billion years ago, as the merger’s newly made aspects exploded outwards and diffused across interstellar area, a total of about 1020 kilograms wound up being transferred into our young system.

From there you can work out how much of the Earth’s repository of r-process components came from that one occasion. For example, about one eyelash’s worth of the iodine in your body will have come from those neutron stars. A T esla Model 3 contains a total of about 5 grams of the nuclei generated by this specific neutron star merger. A modern fission reactor, making use of enriched uranium, will have about 200 kilograms of material that was produced in this singular cosmic surge.

Critically, this study also appears to rule out events such as core-collapse supernova – where huge stars implode – as the primary producers of r-process aspects throughout the Galaxy. Those occasions, which occur hundreds or even thousands of times more often than neutron star mergers, simply don’t seem to fit the evidence.

Taken entirely it looks like we can upgrade the tale of our origins in ‘starstuff’. Not only are we indebted to even more esoteric and extreme physics than we maybe pictured, we now have 2 really specific members of our ancestral tribe to put on the family tree, a set of neutron- star-crossed enthusiasts whose welcome literally ended in fire.

[Full disclosure: I am acknowledged in the paper by Bartos and Marka, and they are both associates. However my contribution to their work was completely in the kind of making motivating noises.] 


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