A faint and mysterious stream of microwaves emanating from young star systems far out in the Milky Way Galaxy could be caused by nanodiamonds about 1.5-2.2 nm in size, according to new research published in the journal Nature Astronomy.
Artist’s impression of diamond dust. Image credit: S. Dagnello.
“For decades we have puzzled over the exact source of a peculiar type of faint microwave light emanating from a number of regions across the Milky Way,” said co-author Professor Anna Scaife, an astronomer in the School of Physics and Astronomy at the University of Manchester, UK.
“Known as anomalous microwave emission (AME), this light is thought to come from energy released by rapidly spinning nanoparticles.”
“Though we knew that some type of particle is responsible for this microwave light, its precise source has been a puzzle since it was first detected nearly 20 years ago,” said lead author Dr. Jane Greaves, an astronomer at Cardiff University, UK.
Until now, the most likely culprit for this microwave emission was thought to be a class of organic molecules known as polycyclic aromatic hydrocarbons — carbon-based molecules found throughout interstellar space and recognized by the distinct, yet faint infrared (IR) light they emit.
But Professor Scaife, Dr. Greaves and their colleagues found that nanodiamonds (particularly hydrogenated nanodiamonds, those bristling with hydrogen-bearing molecules on their surfaces), which form naturally within protoplanetary disks and are found in meteorites on Earth, are the most likely source of AME light.
The astronomers used the Robert C. Byrd Green Bank Telescopes in West Virginia and the Australia Telescope Compact Array to survey 14 young stars across the Milky Way for hints of AME.
AME light was clearly seen in three of the 14 stars — V892 Tau, HD 97048, and MWC 297 — which are also the only three stars of the 14 that show the IR spectral signature of hydrogenated nanodiamonds.
Nanodiamonds likely form out of a superheated vapor of carbon atoms in highly energized star-forming regions. This is not unlike industrial methods of creating nanodiamonds on Earth.
In astronomy, nanodiamonds are special because their structure produces what is known as a ‘dipole moment’ — an arrangement of atoms that allows them to emit electromagnetic radiation when they spin.
Because these particles are so small they are able to spin exceptionally fast, emitting radiation in the microwave range rather than in the meter-wavelength range, where galactic and intergalactic radiation would probably drown it out.
“This is a cool and unexpected resolution to the puzzle of anomalous microwave radiation,” Dr. Greaves said.
“It’s even more interesting that it was obtained by looking at protoplanetary disks, shedding light on the chemical features of early solar systems, including our own.”
“This is good news for those who study polarization of the Cosmic Microwave Background, since the signal from spinning nanodiamonds would be weakly polarized at best,” said co-author Dr. Brian Mason, an astronomer at the National Radio Astronomy Observatory.
“This means that astronomers can now make better models of the foreground microwave light from our Galaxy, which must be removed to study the distant afterglow of the Big Bang.”
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J.S. Greaves et al. Anomalous microwave emission from spinning nanodiamonds around stars. Nature Astronomy, published online June 11, 2018; doi: 10.1038/s41550-018-0495-z
