Silicon carbide "star dust" in meteorites helps to understand the erupting stars

A collaboration between two scientists from Arizona State University – cosmochemist Maitrayee Bose and astrophysicist Sumner Starrfield, both from the School of Earth and Space Exploration – uncovered the link and put in place evidence the type of stellar explosion at the origin of the star grains.

Their study has just been published in the Astrophysical Journal.

The microscopic grains of silicon carbide, a thousand times smaller than the average width of a human hair, were among the building materials that built the solar and planetary system. Born in nova explosions, which are cataclysmic eruptions repeated by some types of white dwarf stars, the silicon carbide grains are now found in primitive meteorites.

"Silicon carbide is one of the most resistant elements found in meteorites," Bose said. "Unlike other elements, these star dust grains have survived unchanged since the birth of the solar system."

Violent birth

A star becomes a nova – a "new star" – when it suddenly illuminates multiple magnitudes. The Novae occur in pairs of stars, one of the stars being a hot and compact residue called a white dwarf. The other is a giant and cool star so large that its extended external atmosphere is feeding the white dwarf with gas. When enough gas accumulates on the white dwarf, a thermonuclear eruption then ensues and the star becomes a nova.

Although powerful, the eruption does not destroy the white dwarf or its mate, so novae can explode again and again, repeatedly throwing space in the grains of gas and dust from the air. explosion. From there, the dust grains mingle with clouds of interstellar gas to become the ingredients of new star systems.

The sun and the solar system were born about 4.6 billion years ago from such an interstellar cloud, sown with dust grains of earlier stellar eruptions by many types of "solar". stars. Almost all the original grains have been consumed to make the sun and the planets, but a tiny fraction has remained. Today, these star dust particles, or presolar grains, can be identified in primitive materials of the solar system such as chondritic meteorites.

"The key that solved this problem for us was the isotopic composition of star dust grains," Bose said. Isotopes are varieties of chemical elements that have extra neutrons in their nuclei. "The isotope analysis allows us to trace the raw materials together to form the solar system."

She added, "Each grain of silicon carbide bears a signature of the isotopic composition of its parent star, which provides a probe of that star's nucleosynthesis – how it fabricated the elements."

Bose collected published data on thousands of grains and found that almost all grains were naturally grouped into three main categories, each of which can be assigned to one type of star or another.

But there were about 30 grains that could not be attributed to a particular star origin. In the initial analyzes, these beans were reported as possibly coming from explosions of nova.

But did they do it?

Make Star Dust

As a theoretical astrophysicist, Starrfield uses computational calculations and simulations to study various types of stellar explosions. These include novae, recurrent novae, X-ray bursts and supernovae.

By working with other astrophysicists, he was developing a computer model to explain the ejected materials in the spectrum of a nova discovery in 2015. He then attended a symposium speech given by Bose before that. she does not join the faculty.

"I would not have continued this if I had not heard Maitrayee's speech and then had our follow-up discussion," he said. This led him to deepen the details of the nova eruptions in general and what the presolar beans could say about these explosions that threw them into space.

A problem is quickly posed. "After talking with her," says Starrfield, "I discovered that our initial way of solving the problem was not in keeping with the astronomical observations nor with its results.

"Then I had to find a way around that."

He turned to multidimensional studies of classical nova explosions and developed a completely new calculation method.

There are two major compositional classes of nova, Starrfield said. "One is the oxygen-neon class I've been working on for 20 years.Other is the carbon-oxygen class that I did not pay so much attention to." The class designations for novae come from the elements seen in their spectra.

"The carbon-oxygen type produces a lot of dust as part of the explosion itself," Starrfield said. "The idea is that the explosion of nova spills out in the carbon dwarf core of the white dwarf, bringing all these elements improved and enriched in a region with high temperatures."

This, he says, can cause a much bigger explosion, adding, "It's really messy.This eliminates dust in tendrils, sheets, jets, drops and clumps."

Starrfield's calculations predicted 35 isotopes, including those of carbon, nitrogen, silicon, sulfur, and aluminum, which would be created by nova carbon-oxygen explosions.

It turned out that it was absolutely necessary for the simulations to work with the right proportion of core and added material of white dwarf. Bose and Starrfield then compared the predictions to published compositions of silicon carbide grains.

This led to a somewhat surprising conclusion. Bose said: "We found that only five of the approximately 30 grains could come from novae."

While this may sound disappointing, the scientists were actually thrilled. Bose said: "We must now explain the composition of grains that do not come from explosions of nova. This means that there is a completely new star source or sources to discover."

Then, looking at the situation as a whole, she added, "We also found that astronomical observations, computer simulations and high-precision laboratory measurements of starburst grains were needed to understand the 39, evolution of the stars.This is exactly the kind of interdisciplinary science that the school excels at. "

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The dust grains could be remnants of stellar explosions billions of years ago

More information:
Astrophysical Journal (2019). DOI: 10.3847 / 1538-4357, https://iopscience.iop.org/article/10.3847/1538-4357/aafc2f/meta