A meteorite from a supernova

During an expedition in 1996 in the Libyan desert, which stretches over 700,000 km2 between western Egypt, eastern Libya and northwestern Sudan, an Egyptian geologist, Aly Barakat, discovered a black and shiny stone, about 3 centimeters by 3 and weighing about 30 grams. As he soon realizes with the help of X-rays, the stone contains tiny diamonds. At that point it stops due to a lack of equipment to deepen the investigation. About ten years ago, a geochemist from the University of Johannesburg (South Africa), Jan Kramers, managed to recover a one-gram sample of the rock. Together with his colleagues from different specialties (cosmochemists, petrologists, etc.) and from different backgrounds (France, Switzerland, Germany, Italy, Hungary, Poland), he started a new series of analyses.

The successive works on the stone, now called “Hypatia” in honor of the mathematician, astronomer and ancient philosopher Hypatia of Alexandria (between 350 and 370 – 415), thus show that it was not formed on Earth. – (Kramers & al. 2013) that its composition does not correspond to that of meteorites or comets previously observed, as it is rich in almost pure carbon (Avice & al. 2015) and that it contains minerals, that is older than the formation of our solar system ( appeared about 4.5 billion years ago) (Belyanin & al. 2018). In a new study published in August in the journal Icarus (already available online), Jan Kramers and his colleagues now put forward the idea that Hypatia would indeed be formed from interstellar dust, but whose composition bears traces of a rare and powerful phenomenon: the explosion of a white dwarf in a type Ia supernova.

For this new work, they used proton-induced X-ray emission spectroscopy (PIXE), an extremely sensitive analytical method that involves using a beam of protons to pierce the rock and, using a microscope, measure the nature and amount of the chemical elements contained in the sample . Fifteen elements were identified and their concentrations analyzed. According to the researchers, the chemical signature of this meteorite is unusual. So much so that they had to eliminate several rock formation scenarios before finding the one that could match this signature. Is the parent body from a red giant (a star at the end of its life, very large, luminous and at a low temperature that no longer has enough hydrogen in its core to fuel nuclear fusion)? No, because the meteorite contains too much iron and too little silicon. Could it be a type II supernova (a phenomenon that occurs when a massive star runs out of fuel, collapses and explodes)? Again, Jan Kramers and his colleagues had to rule out this possibility because the proportion of iron, as well as the presence of nickel phosphide in the rock, does not correspond to this type of supernova.

The team then looked at a type of explosion that would explain this strange composition: type IA supernovae. These supernovae occur in a binary system, that is, containing two types of interacting stars, including at least one white dwarf. In this case, Jan Kramers and his team imagine that the other star must be a red giant, because this type of explosion is the source of a large part of the presence of iron in the universe, which corresponds to the unusual chemical composition of the Hypatia Stone .

During the transfer of matter from the red giant to the white dwarf, its mass increases, and when it reaches a certain critical mass, it explodes. After the explosion, a kind of cloud of atoms and subatomic particles is formeddescribes Georgy Belyanin, from the University of Johannesburg and co-author of the study. Then, after a very long cooling process, these atoms stick to the interstellar dust particles and form larger particles. They are still not minerals, but primitive matter.” which would ultimately have given rise to Hypatia’s maternal organ. “Somewhere nearby, the formation of the nebula at the origin of our solar system has begun. Hypatia’s mother body would have been trapped in this nebula and would have continued to cool and grow and incorporate more and more material without affecting the original signature of the supernova explosion in the body. again Georgy Belyanin explains. It would have finally landed on Earth about 28 million years ago. How ? This remains a real mystery. »

If this hypothesis is the most plausible, the correspondence is far from perfect. For 6 of the 15 elements examined, the proportions are actually between 10 and 100 times higher than the models’ predictions. Which prompts Guillaume Avice, cosmochemist and CNRS researcher at the Institut de physique du globe de Paris, who worked on Hypatia a few years ago with Jan Kramers, to say: “The problem with Hypatia is that there can be observational bias. By not looking at an object more and more closely, at a very small scale and doing a lot of measurements on it, much more than on other objects, we can end up finding something in particular, which we cannot refrain from fully explaining.“. The mystery surrounding Hypatia’s origins will continue to fuel scientific research for (at least) a few years…

by Odyssey Pietre

Image: Hypatia, Credit: Georgy Belyanin

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