Astronomers are on the hunt for the remnants of the neutron-star collision that gave Earth its precious metals.
When neutron stars merge, they spew a wealth of short-lived elements into their surroundings, and these materials become part of later-forming solar systems. Now scientists are trying to close in on the merger that seeded our solar system by tracing the elements produced by the original decaying material. From that work, they believe the responsible merger occurred 100 million years before and 1,000 light-years away from the birth of our solar system.
"It was close," the project's lead scientist, Szabolcs Marka, who is a physicist at Columbia University, told Space.com. "If you look up at the sky and you see a neutron-star merger 1,000 light-years away, it would outshine the entire night sky."
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Marka and his colleague Imre Bartos, an astrophysicist at the University of Florida, used meteorites from the dawn of the solar system to track down the collision. They analyzed the isotopes — flavors of elements with different numbers of neutrons in their atoms — in these rocks.
First, they calculated the quantity of radioactive isotopes in the early solar system; then the researchers compared their measurements with the amount of isotopes produced by neutron-star mergers. Marka presented the results of their research in January at the winter meeting of the American Astronomical Society in Honolulu.
"Our" neutron-star merger
The universe's heavy elements, such as gold, platinum and plutonium, form when neutrons bombard existing atoms. During such collisions, a neutral neutron can emit a negatively charged electron, becoming a positively charged proton and changing the atom's identity.
This process, known as rapid neutron capture, occurs only during the most powerful explosions, such as supernovas and neutron-star mergers. But scientists continue to debate which of these extreme events is responsible for the bulk of heavy elements in the universe.
So Marka and Bartos turned to ancient meteorites in an effort to understand which type of event may have seeded the early solar system. Locked inside of those rocks from the young solar system is material that spewed from an explosion, and although those initial elements were radioactive and rapidly decayed, they left behind signatures of their past presence.
And as the Laser Interferometer Gravitational-Wave Observatory (LIGO) begins to identify potential neutron-star mergers, scientists are applying its observations to help identify the most likely contributors of material formed in a nearby merger, what Marka called "the witch's brew of the galaxy," the slowly decaying material that made its way to the solar system.
Previous studies estimated that a supernova occurs in the Milky Way once every 50 years or so. LIGO's new observations suggest that neutron-star mergers occur much less frequently, approximately once every 100,000 years. The amount of heavy elements in the solar system suggested that they came from a nearby neutron-star merger, as supernova origins would have yielded more material.
From there, the pair relied on the individual isotopes to determine where and when the solar system's local neutron-star merger had occurred.
"Each isotope is a stopwatch starting at the explosion," Marka said. By studying how much of each isotope was left when the material was captured, he was able to pin down the age of the collision that showered the solar system. "There is only one point in time when they all agree," he said. That point occurred roughly 100 million years before the solar system formed, an eye blink in astronomical time scales. The team also calculated how far away the stars collided, a distance of 1,000 light-years, based on how much material ended up in the solar system.
What the team could not figure out was the direction at which these heavy elements entered the neighborhood that would become our solar system, a discovery that could theoretically allow scientists to pinpoint the remnants of the collision. The problem is that the sun hasn't been sitting still for the 4.5 billion years since it formed; instead, it's been traveling around the galaxy.
Along the way, it has left behind the stars that formed near it in the same cluster, stars that astronomers have long hunted in vain. Marka hopes that one day, astronomers will find those sister stars and the remnants of the neutron-star merger that formed the solar system.
According to Marka, the new discovery hit close to home. "People were actually crying," he said, referring to members of his team.
He said he thinks that strong emotional reaction arose because this neutron-star merger wasn't just an event that happened out in space. It was one that contributed to each of us, personally.
"This is not esoteric, it's ours," Marka said. "Not ours in the galaxy but ours in the solar system."
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The calculated age for the neutron stars merger suggest it took place about 4.7 billion years ago, the precise location of the merger is not known because the location of the solar nebula or proto-sun at that time is not known in the Milky Way as the report indicates. However, the model does use an assumed mass for the solar nebula to explain the r-process elements, using meteorite studies too. Today we have a variety of stars documented with dust disks around them, many in the mass range of 1-3 earth masses, some quite large, more than 40,000 earth masses, quite a variety it appears. Is there spectra evidence for neutron star merger elements in those disks and gas like gold and uranium? For example, DH Tau system with DH Tau b, FU Orionis disk < =22 earth masses. There are *baby stars* forming documented in Orion nebula with <=26 earth masses.
The report provides an interesting, r-process calculation for our solar system to explain gold on Earth as an example.
It's hard to find solar sister stars birthed from the same molecular cloud region, with 4.5 billion years and 200 million years orbit for Sun it has done 20+ orbits. Meanwhile the RMS velocity spread of the orbit speed is something like 50/220 on average or 20 %. So the spread is more than an orbit, and the sisters could be anywhere.
The single neutron star event simplifies things. From last year's article:
"Neutron star mergers are thought to be pretty rare in our galaxy, occurring only a few times every million years, the researchers wrote. Supernovas, on the other hand, are much more common; according to a 2006 study from the European Space Agency, a massive star explodes in our galaxy once every 50 years or so.
That supernova rate is much too high to account for the levels of heavy elements observed in early solar system meteors, Bartos and Marka concluded, ruling them out as the likely source of those elements. A single nearby neutron star merger, however, fits the story perfectly."
But the proof is in the pudding.
First I note that the meeting proceedings are likely not yet published in peer review, but the older paper gets an 80 +/- 40 Myrs period and 300 +/- 100 pc distance between merger and our system birth so essentially the same. The one event dominance they get from the rarity of mergers and dispersion in between.
Second, the cloud process that birthed our system is complicated. In the canonical model, a vast molecular cloud self attracts. Eventually it births a first generation of very massive, shortlived stars that go supernova and both seed and compress the cloud parts. That sets of a second generation of massive, active stars whose solar winds blow up spherical shells of compressed gas. Such shells break up into solar nebula and eventually form disks that birth on average some five hundred star "sisters" of more common mass, among them our Sun.
The difference here is that the model timing has the neutron binary merger seed the cloud, before or at the same time as the nebula formation (perhaps before, since massive stars mature and die within 10s of milions of years). The paper show it seeding the individual nebula in Fig. 1, but I think that is just the illustration of the model run, not an entire model of a merger interacting with a molecular cloud. I wouldn't take the extent of the model too seriously - it works either way.
They rule out more frequent processes being dominant contributors since our system has a low abundance of radioactive isotopes, which is a funny constraint since the metal level is average (IIRC GAIA results - it used to be labeled "high"). But that is the beauty of the merger, since it produces a lot of such isotopes!