Supernovae maybe aren't the birth place of heavy elements, like Gold or Silver

We have suspected that elements such as Gold, Silver, and Plutonium are formed during supernovas, when the star dies off, as the nuclear fusion inside stars cannot make such elements, the energy required to synthesize gold is considerably higher which occurs when stars explode in a supernova or when neutron stars collide, only under such extreme conditions, heavy elements for via rapid neutron-capture process, well that's what scientists suspect. But typical supernovas cant explain the quantity of heavy elements in our cosmic neighbors, as a new study suggests.

The study had changes some thoughts of astrophysicists about supernovas. Instead, they think that heavy elements are more likely born in the collision of neutron stars, via neutron capture. This is when an atomic nucleus collides with neutrons and the neutrons merge to form a higher nucleus, and the atomic nuclei swell larger and larger as they rapidly capture neutrons. This series of reactions is called the r-process, but they don't know the site of the r-process. First Scientists thought they had their answer after a neutron star collision was seen in 2017. But heavy elements show up in very old stars, which formed too early for neutron stars to have collided. 

Researchers started with 410-gram samples from pacific ocean crust as if an r-process had recently occurred nearby, some of the elements could have landed on Earth, which we can study to identify. After taking the samples they used a particle accelerator to separate and count atoms, and in one piece of sample scientists searched a variety of plutonium-244, which is produced by r-process and since heavy metals are formed in proportions in the process, plutonium-244 can serve as a proxy for other heavy elements. They found about 180 plutonium-244 atoms deposited into the crust within the last 9 million years.

Credit: Norkiazu Kinoshita

Researchers compared the plutonium count with atoms of iron-60 which is released by supernovas, but it is formed by fusion in stars, not by r-process. In another sample, they detected about 415 iron-60 atoms. Plutonium-244 is radioactive, decaying with a half-life of 80.6 million years, and iron-60 has an even shorter half-life, 2.6 million years, so the elements could not have been there from the time Earth formed, 4.5 billion years ago, which suggests that they came from somewhere else recently. So after some more research, scientists saw two peaks of iron-60 atom count by the depth and were able to see how long they'd been deposited, one peak at about 2.5 million years ago and one about 6.5 million years ago, which suggests two or more supernovas had occurred recently in the past. But they cannot say if the plutonium they detected also came from those supernova, but even if it did, the quantity of plutonium produced in those supernovas would be too small to explain the abundance of heavy elements in our cosmic neighborhood, which just suggests that regular supernovas can't be the source of heavy elements. 

Anna Frebel, Astrophysicist of MIT says, " The supernovae are just not cutting it." That just leaves the other sources for the r-process as the suspects.

Refrence: DOI: 10.1126/science.aax3972