Tiny diamonds are littered throughout the cosmos in the form of bits of crystalline carbon, a fraction the size of a grain of sand. New research from the National Science Foundation’s Green Bank Telescope (GBT) in West Virginia, United States has detected these diamonds around three young star systems within our Milky Way. These ‘microscopic gemstones’ are exciting to astronomers as they have been identified as a source of a mysterious cosmic microwave ‘glow’ emanating from different protoplanetary discs in our galaxy.
There is an unusual faint microwave light emitted from different parts of the Milky Way that has perpetually perplexed astronomers. This is best known as Anomalous Microwave Emission (AME) and is thought to be light emitted from rapidly spinning nanoparticles – bits of matter so small that even ordinary microscopes cannot detect them. To put this into context, the period on an average printed page is approximately 500,000 nanometres across.
“Though we know 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,” says Jane Greaves, an astronomer at Cardiff University in Wales.
Until these recent results, the most likely source for this microwave emission was thought to be a class of organic molecules known as Polycyclic Aromatic Hydrocarbons (PAHs). These are carbon-based molecules found in interstellar space and recognisable from its faint infrared light. Nanodiamonds, in particular hydrogenated nanodiamonds, have hydrogen-bearing molecules on the surface that naturally emit light in the infrared section of the spectrum, but at a different wavelength.
For the first time, observations made by the GBT and the Australia Telescope Compact Array (ATCA) have focused on three clear sources of AME light. The three infant stars were V892 Tau, HD 97048 and MWC 297, with GBT observing V892 Tau and the ATCA observing the other two.
“This is the first clear detection of anomalous microwave emission coming from protoplanetary discs,” says David Frayer, an astronomer with the Green Bank Observatory. Astronomers also noted that the infrared light coming from these systems matches the unique signature of nanodiamonds. Other protoplanetary discs throughout the galaxy have a clear infrared signature of PAHs, yet they show no signs of the AME light.
These results strongly suggest that PAHs are not the mysterious source of the elusive microwave radiation, as astronomers once thought. Instead, hydrogenated nanodiamonds are the most likely source of AME light in our galaxy, and appear to form naturally in such environments.
“In a Sherlock Holmes-like method of eliminating all other causes, we can confidently say the best candidate capable of producing this microwave glow is the presence of nanodiamonds around these newly formed stars,” says Greaves. Astronomers estimate that up to one to two per cent of the total carbon in these protoplanetary discs constitute nanodiamonds.
Evidence for nanodiamonds being present in protoplanetary discs has become more prominent in recent years. However, this is the first clear connection between nanodiamonds and AME. “There is a one in 10,000 chance, or less, that this connection is due to chance,” says Frayer.
In this research, astronomers surveyed 14 young stars across the Milky Way in the search for anomalous microwave emission. AME was found in the three aforementioned stars out of the 14 surveyed, and these were the only ones to show the infrared spectra signature of hydrogenated nanodiamonds. “In fact, these are so rare,” notes Greaves, “no other young stars have the confirmed infrared imprint.”
“This is good news for those who study polarisation of the cosmic microwave background, since the signal from spinning nanodiamonds would be weakly polarised at best,” says 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.”
Future observations will be made using centimetre-wave instruments, like the planned Band 1 receivers on ALMA and the Next Generation Very Large Array. Along with improved models and a better understanding of the spectral signatures, astronomers expect this area of astronomy will become clearer with time.
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