Discovery: Cosmic Dust Contains Organic Matter from Stars

Space.com 26 October 2011

A new look at the interstellar dust permeating the universe has revealed hints of organic matter that could be created naturally by stars, scientists say.

Researchers at the University of Hong Kong observed stars at different evolutionary phases and found that they are able to produce complex organic compounds and eject them into space, filling the regions between stars. The compounds are so complex that their chemical structures resemble the makeup of coal and petroleum, the study's lead author Sun Kwok, of the University of Hong Kong, said.

Such chemical complexity was thought to arise only from living organisms, but the results of the new study show that these organic compounds can be created in space even when no life forms are present. In fact, such complex organics could be produced naturally by stars, and at an extremely rapid pace.

"What impressed me most is that complex organics are easily formed by stars, they are everywhere in our own galaxy and in other galaxies," Kwok told SPACE.com in an email interview. "Nature is much more clever than we had imagined."

The findings of the new study were published online today (Oct. 26) by the journal Nature.

Scanning the cosmos in infrared

Kwok and his colleague Yong Zhang, also of the University of Hong Kong, studied a set of well-known but mysterious infrared emissions found in stars, interstellar space and galaxies. These phenomena, which are collectively called Unidentified Infrared Emission (UIE) features, have been known for 30 years, but the exact source of the emissions has not been pinned down, and remains a broad assumption.

"In the astronomy community, it has been commonly assumed that the UIE features are emitted by [polycyclic aromatic hydrocarbon, or PAH] molecules, which are simple, purely aromatic, molecules made of carbon and hydrogen," Kwok said. "Our paper suggests that the PAH hypothesis is not correct." [Spectacular Photos of Nebulas in Deep Space]

Kwok and Zhang analyzed data from the European Space Agency's Infrared Space Observatory and NASA's Spitzer Space Telescope to show that the Unidentified Infrared Emission features are not emitted by PAH molecules because the emissions have chemical structures that are far more complex.

"I have been suspecting this for many years," Kwok said. "Now we think we have the evidence."

The researchers observed stars at different phases of stellar evolution — first low- to medium-mass stars, then stars in the protoplanetary nebula phase, which is a short-lived episode during a star's rapid evolution, and finally stars in the planetary nebula phase, which is characterized by an expanding shell of ionized gas that is ejected by certain types of stars late in their life.

Kwok and his colleague found that characteristics of the Unidentified Infrared Emission features could not be detected in low- to medium-mass stars. But, the astronomers found that the emissions began to appear in stars in the protoplanetary nebula stage and grew stronger as the stars matured into the planetary nebula phase.

"We therefore know that these organics are being made in the circumstellar stellar environment," Kwok said.

More surprises

Another surprising thing they found was just how quickly stars were generating complex organic compounds and ejecting the dusty material into their surrounding environment.

"Since we know their dynamical and evolutionary ages of these objects (dynamical age is how fast the nebula will disperse, and evolutionary age is how fast the star is evolving), we can put constraints on the chemical time scales," Kwok said. "Since the dynamical/evolution ages are of the order of thousands of years, the appearance of the spectral features suggests that the organic compounds are made on time scales shorter than thousands of years." 

The researchers also studied emissions from exploding stars and found that these dynamic cosmic events produced dust even more rapidly.

"Their spectra changed from a pure gas spectrum to a dust spectrum on a matter of days or weeks," Kwok said. "The sudden appearance of the features suggests that organic dust can be made extremely quickly."

But, the findings throw a wrench into existing theories that posit that stars cannot produce such complex organic compounds in the near-vacuum environment of space.

"Theoretically, it is very difficult to understand because of the very low density of the circumstellar environment," Kwok said. "But, observationally, there is no doubt as we see these spectral features appearing and changing on very short time scales. This means that these organic solids are condensing directly from the gas phase."

Star dust and the early solar system

The scientists also found that the chemical structure of organic star dust is similar to complex organics found in meteorites. Since meteorites originate from space rocks that are remnants of the early solar system, the results of the study suggest that stars could have enriched the early solar system with organic compounds, Kwok said.

"It is quite possible that the organics in meteorites are remnants of star dust in the solar nebula," he explained. "The star dust [was] ejected by nearby planetary nebula[s] and survived the journey across the galaxy."

Early in the Earth's formation history, the planet was pummeled in a shower of meteorites and comets during a period known as the Late Heavy Bombardment. Since the organic compounds in meteorites are similar to what was found in stellar dust, the results of this new study show that the barrage of meteorites that fell to Earth during the Late Heavy Bombardment could have carried organic star dust.

While it may be too soon to determine whether these organic compounds played a role in kick-starting the development of life on Earth, it certainly is a possibility, Kwok said. [Extremophiles: World's Weirdest Life]

"If this is the case, life on Earth may have had an easier time getting started as these organics can serve asbasic ingredients for life," he said.

Further research in this area will be necessary, and Kwok intends to continue analyzing additional infrared observations to better pin down the chemical structure of organic star dust. He is also interested in studying more about how and why stars are able to produce complex organics as quickly as they seem to.

"Coal and kerogen are products of life and it took a long time for them to form," Kwok said. "How do stars make such complicated organics under seemingly unfavorable conditions and [do] it so rapidly?"

Astronomers discover complex organic matter in the universe

In today's issue of the journal Nature, astronomers report that organic compounds of unexpected complexity exist throughout the Universe. The results suggest that complex organic compounds are not the sole domain of life but can be made naturally by stars.

Prof. Sun Kwok and Dr. Yong Zhang of the University of Hong Kong show that an organic substance commonly found throughout the Universe contains a mixture of aromatic (ring-like) and aliphatic (chain-like) components. The compounds are so complex that their chemical structures resemble those of coal and petroleum. Since coal and oil are remnants of ancient life, this type of organic matter was thought to arise only from living organisms. The team's discovery suggests that complex organic compounds can be synthesized in space even when no life forms are present.

The researchers investigated an unsolved phenomenon: a set of infrared emissions detected in stars, interstellar space, and galaxies. These spectral signatures are known as "Unidentified Infrared Emission features". For over two decades, the most commonly accepted theory on the origin of these signatures has been that they come from simple organic molecules made of carbon andhydrogen atoms, called polycyclic aromatic hydrocarbon (PAH) molecules. From observations taken by the Infrared Space Observatory and the Spitzer Space Telescope, Kwok and Zhang showed that the astronomical spectra have features that cannot be explained by PAH molecules. Instead, the team proposes that the substances generating these infrared emissions have chemical structures that are much more complex. By analyzing spectra of star dust formed in exploding stars called novae, they show that stars are making these complex organic compounds on extremely short time scales of weeks.

Not only are stars producing this complex organic matter, they are also ejecting it into the general interstellar space, the region between stars. The work supports an earlier idea proposed by Kwok that old stars are molecular factories capable of manufacturing organic compounds. "Our work has shown that stars have no problem making complex organic compounds under near-vacuum conditions," says Kwok. "Theoretically, this is impossible, but observationally we can see it happening."

Most interestingly, this organic star dust is similar in structure to complex organic compounds found in meteorites. Since meteorites are remnants of the early Solar System, the findings raise the possibility that stars enriched the early Solar System with organic compounds. The early Earth was subjected to severe bombardments by comets and asteroids, which potentially could have carried organic star dust. Whether these delivered organic compounds played any role in the development of life on Earth remains an open question.

Complex organic matter may have been found beyond the Solar System

Chemistry World 26 October 2011

Scientists in Hong Kong believe they have found traces of organic compounds deep in interstellar space that have similar structures to coal and oil. The findings - which are based on infrared spectroscopic data - suggest that these organic compounds exist throughout the universe, and may have seeded life on Earth.

For decades astronomers have been aware of a band of mid-infrared emissions coming from interstellar space, but haven't been certain of its origin. Many believe the emissions arise from polycyclic aromatic hydrocarbons (PAH) that are excited by ultraviolet photons. Others claim that there are flaws with this hypothesis. These scientists argue that the infrared emissions exist even around cool stars where there is no stimulating ultraviolet radiation, and - perhaps more significantly - that in interstellar environments no individual PAHs have ever been identified.

Now, Sun Kwok and Yong Zhang of the University of Hong Kong have come up with a different answer to what these emissions mean. Taking data from the European Space Agency's Infrared Space Observatory and Nasa's Spitzer Space Telescope, Kwok and Zhang analysed the spectra of dust formed in exploding stars, known as novae. Rather than finding sharp, well-defined features that would be consistent with pure PAHs, the researchers discovered features hinting at non-aromatic, or aliphatic, compounds. Indeed, the researchers believe their data are most consistent with emissions from organic matter with a mixed aromatic-aliphatic structure, similar to coal and oil.

If interstellar space does harbour such complex organic matter, it would be a significant discovery. Coal- and oil-like compounds have already been spotted on meteorites - the remnants of a distant past when Earth was bombarded by comets and asteroids. Linking such compounds to deep space suggests that Earth's complex organic matter - and by extension life - might have been seeded from outside of the Solar System.

However, Aigen Li of the University of Missouri in Columbia, US, disagrees with an aspect of Kwok and Zhang's model - that the organic matter's aromatic component causes a 3.3um infrared emission feature and its aliphatic component causes a weaker, 3.4um feature. If this were true, Li says, absorption spectra from particularly dusty regions of space should also have a strong 3.3um feature and a weak 3.4um feature - but past observations have actually shown these strengths to be reversed. 'Therefore, the [two features] cannot be from the same material,' and PAHs are still a likely cause of the infrared emissions, he concludes.

Still, Kwok and Zhang are persevering. 'We hope to continue to pursue astronomical infrared spectroscopic observations to better identify the exact chemical composition of these compounds,' says Kwok.

Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features
Nature 2011 doi:10.1038/nature10542 26 October 2011

Sun Kwok1 & Yong Zhang

Unidentified infrared emission bands at wavelengths of 3–20 micrometres are widely observed in a range of environments in our Galaxy and in others1. Some features have been identified as the stretching and bending modes of aromatic compounds2, 3, and are commonly attributed to polycyclic aromatic hydrocarbon molecules4, 5. The central argument supporting this attribution is that single-photon excitation of the molecule can account for the unidentified infrared emission features observed in ‘cirrus’ clouds in the diffuse interstellar medium6. Of the more than 160 molecules identified in the circumstellar and interstellar environments, however, not one is a polycyclic aromatic hydrocarbon molecule. The detections of discrete and broad aliphatic spectral features suggest that the carrier of the unidentified infrared emission features cannot be a pure aromatic compound. Here we report an analysis of archival spectroscopic observations and demonstrate that the data are most consistent with the carriers being amorphous organic solids with a mixed aromatic–aliphatic structure. This structure is similar to that of the organic materials found in meteorites, as would be expected if the Solar System had inherited these organic materials from interstellar sources.