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Distant Stars, Diamond Dust, And A Mysterious Microwave Light
Sparkling newborn sun-like stars are born surrounded by a swirling, whirling disk of gas and dust that astronomers call protoplanetary accretion disks, and these encircling disks contain the precious ingredients from which the baby star's family of planets and other objects ultimately emerge. Indeed, a protoplanetary accretion disk can be thought of as an accretion disk for the baby star itself, because gases and other material may be tumbling down from the inner edge of the disk onto the surface of the hungry young star. For decades, astronomers have been trying to discover the source of a mysterious type of faint microwave light flowing out from a number of regions across our Milky Way Galaxy. These strange emissions of microwave light, called anomalous microwave emission (AME) emanate from the energy liberated by rapidly spinning nanoparticles, which are tiny tidbits of matter that are so small they cannot be detected by ordinary microscopes. In June 2018, a team of astronomers announced their findings that some of the tiniest diamonds in the Cosmos--tidbits of crystalline carbon hundreds of thousands of times smaller than a grain of sand--have been detected swirling around a trio of newborn stellar systems in our Galaxy. These microscopic gemstones are neither as precious nor as rare as diamonds on Earth. However, they are a treasure for the astronomers who identified them as the source of the mysterious cosmic microwave "glow" flowing out from several protoplanetary accretion disks in our Galaxy.
To get an idea of how tiny nanoparticles are, the period on an average printed page is about 500,000 nanometers 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," explained Dr. Jane Greaves in a June 11, 2018 Green Bank Observatory Press Release. Dr. Greaves is an astronomer at Cardiff University in Wales and lead author on a paper announcing this result published in Nature Astronomy. The Robert C. Byrd Green Bank Telescope is located in Green Bank, West Virginia, and it is the largest completely steerable radio telescope in the world. The Green Bank site was part of the National Radio Astronomy Observatory (NRAO) until September 30, 2018.
Until this study, astronomers considered polycyclic aromatic hydrocarbons (PAHs) to be the most likely culprit behind this mysterious microwave emission. PAHs represent a class of carbon-based molecules found throughout the space between stars. These molecules can be identified by the distinct--yet faint-- infrared (IR) light that they send out into space. Nanodiamonds--especially hydrogenated nanodiamonds (those richly-endowed with hydrogen-bearing molecules on their surfaces)--also naturally emit in the infrared portion of the electromagnetic spectrum, but at a different wavelength.
Swirling, Whirling Protoplanetary Accretion Disks
Baby stars, called protostars, are mainly born within the secretive, ruffling folds of one of the numerous giant, cold, dark molecular clouds that haunt our Milky Way Galaxy like lovely phantoms. These frigid, enormous clouds are primarily composed of molecular hydrogen. When a blob cradled within a molecular cloud manages to reach a critical size, mass, or density, it starts to collapse under its own powerful gravity. As the collapsing blob, called a solar nebula, becomes more and more dense under the relentless pull of gravity, the random motions of gas, that were originally present in the natal cloud, start to average out in favor of the direction of the solar nebula's angular momentum. Conservation of angular momentum causes the rotation to increase while, at the same time, the radius of the nebula decreases. This makes the cloud flatten out into a pancake-like shape. Imagine the way that a glob of pizza dough flattens out, and then takes the shape of a disk. The initial collapse takes about 100,000 years. After that amount of time has passed, the star attains a surface temperature that is similar to that of a main-sequence (hydrogen-burning) star of the same mass, and is now visible.
This is how a baby sun-like star becomes a type of stellar toddler called a T Tauri. What is left of the gas and dust of the natal cloud, after it has formed at the center of a dense blob, goes into the formation of the protoplanetary accretion disk from which planets, moons, and smaller objects emerge. In their earliest stages, accretion disks are both searing-hot and extremely massive, and they can linger around their young star for as long as ten million years before they vanish--perhaps blown away by the ferocious T Tauri wind or, alternatively, simply ceasing to emit radiation after accretion has come to an end. The most ancient protoplanetary disk known is about 25 million years old.
Protoplanetary accretion disks have been discovered circling several young stars in our Milky Way. Recent observations conducted by the Hubble Space Telescope (HST) have unveiled proplyds and planetary disks forming within the Orion Nebula. A proplyd is a syllabic abbreviation of an ionized protoplanetary disk. Proplyds are externally illuminated photoevaporating disks circling young stars. There are 180 proplyds inhabiting the Orion Nebula alone.
Protoplanetary disks are thin structures, having a typical vertical height considerably smaller than the radius, as well as a typical mass much smaller than the central young star.
The mass of a typical protoplanetary disk is mostly composed of gas. However, dust motes also play a starring role in a disk's evolution. Dust motes shield the mid-plane of the disk from energetic radiation coming from interstellar space that forms a "dead zone" in which the magnetorotational instability (MRI) no longer operates.
Some astronomers propose that these disks are made up of a turbulent envelope of plasma. This is also termed the "active zone", that contains an extensive region of quiescent gas (the "dead zone"). The "dead zone" is situated at the mid-plane, and it can slow down the rush of matter through the disk which prevents achieving a steady state.
Sparkling T Tauri stellar tots sport large diameters that are several times greater than that of our Sun. However, they are still shrinking. Unlike human children, T Tauris shrink as they grow up. By the time the stellar toddler has reached this stage of early development, less volatile materials have started to condense close to the center of the encircling disk, creating very fine and sticky dust grains. The delicate dust motes contain crystalline silicates.
The sticky, tiny grains of dust collide with one another and then merge together within the dense protoplanetary accretion disk environment. As a result, increasingly larger, and larger, and larger objects form--from pebble size, to boulder size, to mountain size, to moon size, to planet size. These growing objects eventually become what are termed planetesimals--the primordial building blocks of planets. Planetesimals can reach impressive sizes of 1 kilometer across, or even larger, and they represent an enormous population within a young accretion disk, swirling around their sparkling stellar toddler. They can also linger around their star long enough for some of them to still be present billions of years after a mature planetary system has emerged. In our Solar System, the asteroids are the relic rocky and metallic planetesimals that went into the formation of the quartet of inner major planets: Mercury, Venus, Earth, and Mars. In contrast, the comets are the icy, dusty leftovers of the frozen planetesimals that contributed to the emergence of the four giant gaseous major planets of our Solar System's outer regions: Jupiter, Saturn, Uranus, and Neptune.
Some of the many moons of Jupiter, Saturn, and Uranus are thought to have formed from smaller, circumplanetary analogs of the protoplanetary accretion disks. Tens of millions of years after the birth of our 4.56 billion year old Solar System, the inner few astronomical units (AU) of our Solar System probably hosted dozens of moon-to Mars-sized bodies that were accreting and consolidating into the quartet of inner, solid terrestrial planets. One AU is equivalent to the average separation between Earth and Sun, which is about 93,000,000 miles.
Earth's own large and bewitching Moon is believed to have been born after a Mars-sized protoplanet, named Theia, obliquely impacted the proto-Earth about 30 million years after our Solar System's formation. Imagine what would happen if Mars impacted Earth, in order to understand the magnitude of the catastrophic event that likely formed Earth's lunar companion.
AME In The Sky With Diamonds
A series of observations by astronomers using the National Science Foundation's Green Bank Telescope (GBT) in Green Bank, West Virginia, and the Australia Telescope Compact Array (ATCA) have--for the first time--detected a trio of clear sources of the mysterious AME light: the protoplanetary disks surrounding the young stars known as V892 Tau, HD 97048, and MWC 297. The GBT watched V892 Tau and the ATCA observed the other two systems.
"This is the first clear detection of anomalous microwave emission coming from protoplanetary disks," explained Dr. David Frayer in a June 11, 2018 Green Bank Observatory Press Release. Dr. Frayer is a coauthor on the paper and an astronomer with the Green Bank Observatory.
The team of astronomers also explained that the infrared light streaming out from these systems matches the unique signature of nanodiamonds. Other protoplanetary accretion disks throughout entire Galaxy, however, have the clear infrared signature of PAHs but show no signs of the AME light.
This observation strongly indicates that PAHs are not the mysterious source of anomalous microwave radiation, as many astronomers had previously proposed. Instead, hydrogenated nanodiamonds, which form naturally within protoplanetary disks and are seen in meteorites on Earth are the most probable origin of AME light in our Galaxy.
"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," Dr. Greaves commented in the June 11, 2018 Green Bank Observatory Press Release. Based on their observations, the astronomers estimate that up to 1 to 2 percent of the total carbon in these protoplanetary disks has contributed to the formation of nanodiamonds.
Over the past several decades, evidence for nanodiamonds within protoplanetary accretion disks has grown. However, this study represents the first clear connection between nanodiamonds and AME in any setting.
Statistical models also strongly suggest the theory that nanodiamonds are very abundant around newborn stars and are responsible for the anomalous microwave emission detected there. "There is a one in 10,000 chance, or less, that this connection is due to chance," commented Dr. Frayer in the June 11, 2018 Green Bank Observatory Press Release.
For their study, the astronomers used the GBT and ATCA to survey 14 youthful stars across our Galaxy, searching for hints of the anomalous microwave emission. AME was clearly observed in 3 of the 14 stars, which also proved to be the only three stars out of the 14 that display the IR spectral signature of hydrogenated nanodiamonds. "In fact, these are so rare no other stars have the confirmed infrared imprint," commented Dr. Greaves in the Green Bank Observatory Press Release.
This discovery has some intriguing implications for the study of cosmology and the hunt for evidence that our Universe began with a period of inflation--the faster-than-the-speed-of-light exponential expansion of Space. Although there is no known signal that can travel faster than light in a vacuum, Space itself can exceed this otherwide universal speed limit. If, indeed, immediately after the Big Bang birth of the Universe almost 14 billion years ago, it expanded at a pace that vastly exceeded the speed of light, a trace of that period of inflation should be observed in a peculiar polarization of the cosmic microwave background (CMB) radiation. The CMB is the relic radiation of the Big Bang itself. Even though this signature of polarization has yet to be conclusively observed, the work of Dr. Greaves and her colleagues provides some hope that one day it could be.
"This is good news for those who study polarization of the cosmic microwave background, since the spinning nanodiamonds would be weakly polarized at best. 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," Dr. Brian Mason explained in the June 11, 2018 Green Bank Observatory Press Release. Dr. Mason is an astronomer at the NRAO and coauthor on the paper.
Nanodiamonds probably form out of a superheated vapor of carbon atoms in highly energized star-birthing regions. This is similar to industrial methods of creating nanodiamonds on Earth.
In astronomy, nanodiamonds play a special role because their structure produces what is called a "dipole moment". This is an arrangement of atoms that permits them to emit electromagnetic radiation when they spin. Because these diamonds are so small--smaller than normal dust motes swirling within a protoplanetary accretion disk--they are able to spin exceptionally fast, and emit radiation in the microwave range instead of the meter-wavelength range, where Galactic and intergalactic radiation would likely drown it out.
"This is a cool and unexpected resolution to the puzzle of anomalous microwave radiation. It's even more interesting that it was obtained by looking at protoplanetary disks, shedding light on the chemical feature of early solar systems, including our own," Dr. Greaves commented to the press on June 11, 2018.
Future centimeter wave instruments, such as the planned Band 1 receivers on ALMA and the Next Generation Very Large Array, will be able to study this phenomenon in much greater detail. Now that there is a physical model and, for the first time, a clear spectral signature, astronomers can look forward to a great improvement in their scientific understanding of this mystery.
Study coauthor, Dr. Anna Scaife from Manchester University (UK) commented in the June 11, 2018 Green Bank Press Release that "It is an exciting result. It's not often you find yourself putting new words to famous tunes, but 'AME in the Sky With Diamonds' seems a thoughtful way of summarizing our research."
