Detecting diamond dust around distant stars resolves the mystery of decades

Artist's impression of nanoscale diamonds surrounding a young star in the Milky Way.
Illustration: S. Dagnello, NRAO / AUI / NSF

For years, astronomers have had difficulty understanding the source of anomalous microwave emissions from various places through the Milky Way. A survey recently concluded on planet-forming disks around young stars suggests that these strange transmissions are being produced by something extraordinary: dense clouds of microscopic diamonds.

The appropriately named "anomalous microwave emissions" (AME) were detected for the first time two decades ago. The faint microwave light originated in several regions of the Milky Way, and although the scientists did not have an explanation for the observation, it was proposed that some type of particle was responsible. A popular theory was that AMEs were being produced by an organic molecule known as polycyclic aromatic hydrocarbons (PAHs). These carbon-based molecules are scattered in space, and have a distinctive, but weak, infrared signature.

New research published today in Nature Astronomy suggests that this interpretation was incorrect, and that clouds of nanodiamonds within embryonic star systems are the true source of SMA. Nanodiamonds naturally form pieces of crystalline carbon, and with radii between 0.75 and 1.1 nanometers, are hundreds of thousands of times smaller than a grain of sand. These tiny gem particles arise within protoplanetary disks, and are often found inside meteorites that have fallen to Earth.

For the study, an international team of astronomers badyzed the protoplanetary disk of 14 childhood stars using the Green Bank Telescope (GBT) in the United States and Telescope Compact Array (ATCA) in Australia. The AME signal was detected in three of these stars, V892 Tau, HD 97048 and MWC 297. These three-star systems produced the characteristic infrared signal badociated with hydrogenated nanodiamonds, that is, nanodiamonds with hydrogen-bearing molecules on the surface. Hydrogenated nanodiamonds emerge from the superheated steam of carbon atoms in the high-energy and star-forming regions of space. Like PAHs, nanodiamonds also shine in the infrared portion of the light spectrum, but at a different wavelength.

"In a method similar to that of Sherlock Holmes to eliminate all other causes, we can say with confidence the best candidate capable of producing this microwave. The glow is the presence of nanodiamonds around these newly formed stars," said Jane Greaves, lead author of the article and astronomer at the University of Cardiff in Wales, in a statement. Study co-author David Frayer, an astronomer with the Green Bank Observatory, said: "This is the first clear detection of abnormal microwave emission from protoplanetary disks."

More than one or two percent of the total carbon located within these protoplanetary discs participated in the formation of the nanodiamonds. Although they are microscopic in size, nanodiamonds are capable of producing a signal that we can detect here on Earth. Because nanodiamonds are so small, they can rotate remarkably fast, a movement that results in the release of radiation.

"This is a great and unexpected resolution for the enigma of anomalous microwave radiation," said Greaves. "It's even more interesting that it was obtained by observing protoplanetary discs, shedding light on the chemical characteristics of the first solar systems, including ours."

Diamonds can be rare and precious on Earth, but in the context of the cosmos, they are remarkably abundant and an important byproduct of planetary formation.

[Nature Astronomy]


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