Hubble Finds Carbon 60+ Buckyballs in Interstellar Space
The many forms of carbon compounds -- Each pattern of carbon atoms has its own properties and its own infrared signature. Soccer-ball shaped buckyballs (Carbon 60+) and related fullerenes are certainly some of the more interesting and elegant patterns of carbon atoms and have very different properties than diamonds, where the carbon atoms bonds in all directions, or graphite, where it occurs in sheets and flakes off easily – An important property that enables pencils to work. In chemistry labs around the world, scientists are hard at work making super strong carbon molecules and also trying to insert other atoms inside the carbon balls. The creative possibilities of how these molecules can be used are extensive but largely undeveloped. The soccer-ball shaped molecules can be made into threadlike tubes and extremely strong. Scientists are investigating whether these tubes can be made into very thin body armor that could stop a bullet or even be used in a “space elevator” cable to haul materials to and from satellites in geosynchronous orbit. (Image Credit: Pete Marenfeld, National Optical Astronomy Observatory - NOAO)
Hubble Finds Carbon 60+ Buckyballs in Interstellar Space
What makes up the tenuous gas and dust that pervades our galaxy, filling the space between stars? What kinds of complex molecules can form naturally in our universe, outside of the potentially contrived conditions of Earth-side laboratories? Where might these molecules form, and how are they distributed throughout space?
These are among the many open questions regarding the chemistry of our universe. One particular, longstanding puzzle for astronomers is the cause of what’s known as “diffuse interstellar bands” -- hundreds of broad absorption features that appear in optical to near-infrared spectra of reddened stars.
Now, from a jumble of confusing clues in Hubble observations of interstellar space, scientists have picked out evidence of a celebrity molecule – the soccer-ball shaped ionized Buckminsterfullerene molecule, or buckyballs.
The broad absorption features that appear in optical to near-infrared spectra of reddened stars are not caused by the stars themselves, so they must be due to absorption of light by the diffuse interstellar medium (ISM) between us and the stars. But the jumble of hundreds of features (and the unknown conditions under which they are produced) has made it incredibly challenging to identify the individual molecules present in the diffuse ISM.
A new study led by Martin Cordiner (NASA Goddard Space Flight Center and Catholic University of America) has used observations from the Hubble Space Telescope, thus avoiding the additional complication of absorption features from the Earth’s atmosphere, which explore these diffuse interstellar bands further. Hubble’s sightlines toward 11 stars provide confirmation of one special molecule within this jumble: the Buckminsterfullerene molecule.
The Carbon 60+ ion, formally known as Buckminsterfullerene and informally known as a “buckyball,” is an enormous molecule consisting of 60 carbon atoms arranged in a soccer-ball shape. Previously, the largest known molecules definitively detected in the diffuse interstellar medium contained no more than three atoms heavier than hydrogen, so the detection of buckyballs represents a dramatic increase in the known size limit.
Cordiner and collaborators used a novel scanning technique to obtain ultra-high signal-to-noise spectra of seven stars that are significantly reddened by obscuring ISM and four stars that are not. They then searched for absorption signals at four wavelengths (9348, 9365, 9428, and 9577 Angstroms) predicted by laboratory experiments to be associated with Buckminsterfullerene.
The authors obtained reliable detections of the three strongest of these absorption lines in the spectra toward the seven reddened stars and found no sign of this absorption in the four unobscured stars. The 9348 Angstrom absorption was not detected, which is not surprising since it was predicted to be a very weak line. The relative strengths of the three detected lines fit nicely with laboratory predictions.
The consistency of Cordiner and collaborators’ results with initial predictions provides the strongest confirmation yet of the presence of buckyballs in the diffuse ISM. This detection may help characterize other components of the diffuse ISM and lead to a better understanding of the conditions under which complex molecules exist in the extreme, low-density environment of interstellar space.
Buckyballs Were Previously Detected in the Small Magellanic Cloud
In 2010, Astronomers using NASA’s Spitzer Space Telescope detected buckyballs outside of the Milky Way galaxy for the first time. Sir Harry Kroto of Florida State University, who won a Nobel Prize for discovering buckyballs, says life may even owe its existence to these atom “cages” which resemble soccer-balls. The discovery of buckyballs in the Small Magellanic Cloud suggests that these complex molecules may be present around many stars where it was predicted they would be unlikely to form.
Letizia Stanghellini of the National Optical Astronomy Observatory in Tucson, Arizona, and her team from Europe used Spitzer telescope data to find the characteristic infrared signature of spherical fullerenes called buckyballs in four planetary nebulae. The fullerenes are created in the shells of gas and dust ejected from the dying stars at the center of these often-photogenic nebulae.
“Life on Earth has a love affair with carbon, because carbon chemistry is the chemistry of life,” Stanghellini explains. Our discovery shows that these carbon buckyballs, which have also been found in meteorites and around stars in our own galaxy, are probably quite common in all galaxies.”
“These complicated molecules were once thought to be very rare, while now they are found in the rather common objects that planetary nebulae are,” said NOAO team member Richard Shaw. “It is clear that there is a lot of fullerene out there.”
Around one star observed by the Stanghellini team, the total mass of Carbon 60+ is more than 3 times the mass of the planet Mercury. These carbon compounds are dispersed around the star and may form on small grains of dust in the material ejected from the star.
Using the Spitzer Infrared Spectrometer instrument, the team searched in dozens of planetary nebulae that were known to have hydrogen-rich shells of gas that had been ejected from the dying star. This ejected material contains carbon grains (much like soot) that had condensed farther from the star. In this cooling process, and under the exposure to ultraviolet radiation, the grains not only can form fullerenes but other carbon molecules called Polycyclic Aromatic Hydrocarbons (PAHs), which can be created on Earth in the exhaust of diesel engines.
“The four fullerene-rich planetary nebulae detected by us are within reach of the NOAO telescopes for follow-up spectroscopy,” said Shaw, “as well as the large sample of planetary nebulae we searched for fullerene. We are planning a scrupulous follow-up to determine the temperatures and composition of their hot gas flows, with the aim of determining the physical and evolutionary characteristics of the fullerene-rich objects compared to the general planetary nebula population.”
Previous work had identified dying stars called planetary nebula as possible sources of fullerenes. However it was expected that only stars with gas surrounding them that was depleted in hydrogen would be able to form fullerenes.
“We have been studying all kinds of planetary nebula from the ground and from space for 15 years, examining more than 250 nebulae in the Milky Way and beyond, in search for common trends and special features. Our Spitzer observations were designed to have the sensitivity to detect both gas and dust features, including carbon molecules such as fullerene”, said Stanghellini.
“The prevailing view has been that fullerenes cannot occur in hydrogen-rich outflows from these stars. On the contrary, we have found that these fullerenes may be fairly common in these kinds of hydrogen-rich environments” added team member Pedro Garcia-Lario of the European Space Astronomy Centre in Spain.
Buckyballs and related fullerenes such as nanotubes are certainly one of the more interesting and elegant patterns of carbon atoms, and have very different properties than diamond, where carbon bonds in all directions, or graphite, where it occurs in sheets and flakes off easily. In the chemistry lab, scientists are hard at work making super strong carbon nanotubes and in trying to insert atoms or even drugs inside the carbon balls. The creative possibilities of how these molecules can be used are extensive but largely undeveloped. The little ball-shaped molecules can be made into threadlike tubes that are visible to the eye, and extremely strong. Scientists are investigating whether these tubes can be made into very thin body armor that could stop any bullet or even be used in a “space elevator” cable to haul materials to and from geosynchronous orbit.
Scientists can verify the existence of fullerenes by looking at how they emit infrared light, since each molecule has a unique fingerprint. The infrared signature was used by the astronomers to distinguish fullerenes from other carbon compounds, such as PAHs.
The team has concluded that significant chemical reprocessing of the carbon has occurred, indicating that more complex carbon chemistry might be also occurring. This means that the fullerenes could be altered in even more unusual ways.
Although the molecules were detected using a space-based infrared telescope, many new studies of their cosmo-chemistry are being done with laboratory chambers simulating the stellar and interstellar environment and with ground-based telescopes that can yield clues on the nature of stellar ejecta.
According to team member Arturo Manchado, “We hope this discovery stimulates the cosmo-chemists to understand better the large number of ways that these compounds can be made. Most of the luminous mass in the Universe is in low-mass stars, and almost all of these stars go through the planetary nebula phase. Our results indicate that fullerenes can form under conditions which are common to essentially all Solar-like stars at the end of their lives.”
The measurements made with the Spitzer Space Telescope could only have been made from space because the infrared sky is too bright from the ground and much of the infrared radiation emitted by these fullerenes is absorbed by the Earth’s atmosphere.
Carbon-based Petroleum Reservoirs in the Horsehead Nebula?
In 2012, carbon molecules in the form of vast petroleum reservoirs were discovered in the Horsehead Nebula. Using the 30 meter telescope of the Institute for Radio Astronomy for astronomical observations in the millimeter range of wavelengths, astronomers detected the interstellar molecule C3H+ in our galaxy. It belongs to the hydrocarbon family, which is part of the major energy resources of our planet -- petroleum and natural gas. The discovery of this molecule at the heart of the famous Horsehead Nebula in the Constellation of Orion also confirms that this region is an active cosmic refinery.
The Horsehead Nebula, 1300 light years from Earth, is located in the Orion constellation. Due to its famous and easily recognizable shapes which gives the nebula its name, it is one of the most photographed objects by astronomers. But the Horsehead Nebula is also a fantastic interstellar chemistry lab, where high density gas and intense stellar light continuously interact and trigger many layered chemical reactions.
Using the 30 meter radio telescope near the Pico del Veleta in the Spanish Sierra Nevada, IRAM astronomer Jerome Pety and his team for the first time undertook a systematic survey of the chemical content of the Horsehead's mane. The international project, called "Whisper" would not have been possible without the recent technical upgrades of the telescope instruments.
"Earlier, such a comprehensive enterprise would have taken at least one year of observations. Now we could complete measurements after one week." said Arnaud Belloche at the Max Planck Institute for Radio Astronomy. This opens new possibilities to classify the different kinds of gas in the universe based on the molecules they contain.
In their survey, the scientists were able to detect 30 molecules in the region, including many small hydrocarbons, the smallest molecules that compose petroleum and natural gas. The researchers were surprised by the unexpectedly high levels of hydrocarbons.
"The nebula contains 200 times more hydrocarbons than the total amount of water on Earth," said IRAM astronomer Viviana Guzman. In addition, one of these small hydrocarbons, the propynylidyne ion (C3H+), was observed for the first time in space as part of this work. This positively charged ion is a key player in the chemical reactions which link the small hydrocarbons together.
But how do these hydrocarbons form? Pety and his colleagues proposed that they result from the fragmentation of giant carbonaceous molecules named PAHs. These giant molecules could be eroded by ultraviolet light, giving a large amount of small hydrocarbons. This mechanism would be particularly efficient in regions like the Horsehead Nebula where the interstellar gas is directly exposed to the light of a nearby massive star. "We observe the operation of a natural refinery of petroleum of gigantic size," concludes Pety.
Carbon is Everywhere – There’s Even A Dense Planet Made of Crystalline Carbon – Essentially a Diamond Planet
In 2011, researchers from Australia, Germany, Italy, the UK, and the USA first detected an unusual star called a pulsar using the CSIRO Parkes radio telescope in Australia and followed up their discovery with the Lovell radio telescope in the UK and one of the Keck telescopes in Hawaii. What they found was a very dense planet orbiting the pulsar that is made of crystalline carbon – Essentially a “Diamond Planet.”
Pulsars are small spinning stars about 20 km in diameter — the size of a small city — that emit a beam of radio waves. As the star spins and the radio beam sweeps repeatedly over Earth, radio telescopes detect a regular pattern of radio pulses.
For the newly discovered pulsar, known as PSR J1719-1438, the astronomers noticed that the arrival times of the pulses were systematically modulated. They concluded that this was due to the gravitational pull of a small companion planet, orbiting the pulsar in a binary system.
The pulsar and its planet are part of the Milky Way's plane of stars and lie 4,000 light-years away in the constellation of Serpens (the Snake). The system is about an eighth of the way towards the Galactic Center from the Earth.
The modulations in the radio pulses tell astronomers several things about the planet.
First, it orbits the pulsar in just two hours and ten minutes and the distance between the two objects is 600,000 km — a little less than the radius of our Sun.
Second, the companion must be small, less than 60,000 km (that's about five times the Earth's diameter). The planet is so close to the pulsar that, if it were any bigger, it would be ripped apart by the pulsar's gravity.
But despite its small size, the planet has slightly more mass than Jupiter.
"This high density of the planet provides a clue to its origin," said Professor Bailes.
The team thinks that the "Diamond planet" is all that remains of a once-massive star, most of whose matter was siphoned off towards the pulsar.
Pulsar J1719-1438 is a very fast-spinning pulsar — what's called a millisecond pulsar. Amazingly, it rotates more than 10,000 times per minute, has a mass of about 1.4 times that of our Sun but is only 20 km in diameter. About 70 per cent of millisecond pulsars have companions of some kind. Astronomers think it is the companion that, in its star form, transforms an old, dead pulsar into a millisecond pulsar by transferring matter and spinning it up to a very high speed. The result is a fast-spinning millisecond pulsar with a shrunken companion—most often a so-called white dwarf.
"We know of a few other systems, called ultra-compact low-mass X-ray binaries, that are likely to be evolving according to this scenario and may likely represent the progenitors of a pulsar like J1719-1438," said team member Dr Andrea Possenti, Director of the INAF-Osservatorio Astronomico di Cagliari in Italy.
But pulsar J1719-1438 and its companion are so close together that the companion can only be a very stripped-down white dwarf, one that has lost its outer layers and over 99.9 per cent of its original mass.
"This remnant is likely to be largely carbon and oxygen, because a star made of lighter elements like hydrogen and helium would be too big to fit the measured orbiting times," said Dr Michael Keith (CSIRO), one of the research team members.
The density means that this material is certain to be crystalline: that is, a large part of the star may be similar to a diamond.
"The ultimate fate of the binary is determined by the mass and orbital period of the donor star at the time of mass transfer. The rarity of millisecond pulsars with planet-mass companions means that producing such "exotic planets" is the exception rather than the rule, and requires special circumstances," said Dr Benjamin Stappers from the University of Manchester.
The team found pulsar J1719-1438 among almost 200,000 Gigabytes of data using special codes on supercomputers at Swinburne University of Technology, The University of Manchester, and the INAF-Osservatorio Astronomico di Cagliari.
The discovery was made during a systematic search for pulsars over the whole sky that also involves the 100 meter Effelsberg radio telescope of the Max Planck Institute for Radioastronomy (MPIfR) in Germany. "This is the largest and most sensitive survey of this type ever conducted. We expected to find exciting things, and it is great to see it happening. There is more to come!" said Professor Michael Kramer, Director of the MPIfR.
Professor Matthew Bailes is a member of the Center for Astrophysics and Supercomputing at Swinburne which is uniquely resourced to process the torrents of data generated by telescopes and simulations.
Other Mysterious Molecules in Space
Over the vast, empty reaches of interstellar space, countless small molecules tumble quietly though the cold vacuum. Forged in the fusion furnaces of ancient stars and ejected into space when those stars exploded, these lonely molecules account for a significant amount of all the carbon, hydrogen, silicon, and other atoms in the universe. In fact, some 20 percent of all the carbon in the universe is thought to exist as some form of interstellar molecule.
Many astronomers hypothesize that these interstellar molecules are also responsible for an observed phenomenon on Earth known as the "diffuse interstellar bands," spectrographic proof that something out there in the universe is absorbing certain distinct colors of light from stars before it reaches the Earth. But since we don't know the exact chemical composition and atomic arrangements of these mysterious molecules, it remains unproven whether they are, in fact, responsible for the diffuse interstellar bands.
In 2014, a group of scientists led by researchers at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts offered a tantalizing new possibility -- that these mysterious molecules may be silicon-capped hydrocarbons like SiC3H, SiC4H, and SiC5H… And they present data and theoretical arguments to back that hypothesis.
At the same time, the group cautions that history has shown that while many possibilities have been proposed as the source of diffuse interstellar bands, none has been proven definitively.
"There have been a number of explanations over the years, and they cover the gamut," said Michael McCarthy a senior physicist at the Harvard-Smithsonian Center for Astrophysics who led the study.
Astronomers have long known that interstellar molecules containing carbon atoms exist and that by their nature they will absorb light shining on them from stars and other luminous bodies. Because of this, a number of scientists have previously proposed that some type of interstellar molecules are the source of diffuse interstellar bands -- the hundreds of dark absorption lines seen in color spectrograms taken from Earth.
In showing nothing, these dark bands reveal everything. The missing colors correspond to photons of given wavelengths that were absorbed as they traveled through the vast reaches of space before reaching us. More than that, if these photons were filtered by falling on space-based molecules, the wavelengths reveal the exact energies it took to excite the electronic structures of those absorbing molecules in a defined way.
Armed with that information, scientists should be able to use spectroscopy to identify those interstellar molecules, by demonstrating which molecules in the laboratory have the same absorptive "fingerprints." But despite decades of effort, the identity of the molecules that account for the diffuse interstellar bands remains a mystery. Nobody has been able to reproduce the exact same absorption spectra in laboratories here on Earth.
"Not a single one has been definitively assigned to a specific molecule," said Neil Reilly, a former postdoctoral fellow at Harvard-Smithsonian Center for Astrophysics and a co-author of the new paper.
Now Reilly, McCarthy, and their colleagues are pointing to an unusual set of molecules, silicon-terminated carbon chain radicals, as a possible source of these mysterious bands.
As they report in their new paper, the team first created silicon-containing carbon chains SiC3H, SiC4H, and SiC5H in the laboratory using a jet-cooled silane-acetylene discharge. They then analyzed their spectra and carried out theoretical calculations to predict that longer chains in this family might account for some portion of the diffuse interstellar bands.
However, McCarthy cautioned that the work has not yet revealed the smoking gun source of the diffuse interstellar bands. In order to prove that these larger silicon capped hydrocarbon molecules are such a source, more work needs to be done in the laboratory to define the exact types of transitions these molecules undergo, and these would have to be directly related to astronomical observations. But the study provides a tantalizing possibility for finding the elusive source of some of the mystery absorption bands and it reveals more of the rich molecular diversity of space.
"The interstellar medium is a fascinating environment," McCarthy said. "Many of the things that are quite abundant there are really unknown on Earth."
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