Vast Cosmic Dust Clouds --The Incubators of Life in the Universe?

 

A new image from the Atacama Pathfinder Experiment (APEX) telescope in Chile shows a beautiful view of clouds of cosmic dust in the region of Orion, typical of dust clouds throughout the Universe that have recently been discovered to incubate the comlpex organic building blocks of life. While these dense interstellar clouds seem dark and obscured in visible-light observations, APEX’s LABOCA camera can detect the heat glow of the dust and reveal the hiding places where new stars are being formed. But one of these dark clouds is not what it seems.

In space, dense clouds of cosmic gas and dust are the birthplaces of new stars. In visible light, this dust is dark and obscuring, hiding the stars behind it. So much so that, when astronomer William Herschel observed one such cloud in the constellation of Scorpius in 1774, he thought it was a region empty of stars and is said to have exclaimed, "Truly there is a hole in the sky here!" 

 

In order to better understand star formation, astronomers need telescopes that can observe at longer wavelengths, such as the submillimetre range, in which the dark dust grains shine rather than absorb light. APEX, on the Chajnantor Plateau in the Chilean Andes, is the largest single-dish submillimetre-wavelength telescope operating in the southern hemisphere, and is ideal for astronomers studying the birth of stars in this way.

Located in the constellation of Orion (The Hunter), 1500 light-years away from Earth, the Orion Molecular Cloud Complex is the closest region of massive star formation to Earth, and contains a treasury of bright nebulae, dark clouds and young stars. The new image shows just part of this vast complex in visible light, with the APEX observations overlaid in brilliant orange tones that seem to set the dark clouds on fire. Often, the glowing knots from APEX correspond to darker patches in visible light — the tell-tale sign of a dense cloud of dust that absorbs visible light, but glows at submillimetre wavelengths, and possibly a site of star formation.

The bright patch below of the centre of the image is the nebula NGC 1999. This region — when seen in visible light — is what astronomers call a reflection nebula, where the pale blue glow of background starlight is reflected from clouds of dust. The nebula is mainly illuminated by the energetic radiation from the young star V380 Orionis lurking at its heart. In the centre of the nebula is a dark patch, which can be seen even more clearly in a well-known image from the NASA/ESA Hubble Space Telescope.  

Normally, a dark patch such as this would indicate a dense cloud of cosmic dust, obscuring the stars and nebula behind it. However, in this image we can see that the patch remains strikingly dark, even when the APEX observations are included. Thanks to these APEX observations, combined with infrared observations from other telescopes, astronomers believe that the patch is in fact a hole or cavity in the nebula, excavated by material flowing out of the star V380 Orionis. For once, it truly is a hole in the sky!

The region in this image is located about two degrees south of the large and well-known Orion Nebula (Messier 42), which can be seen at the top edge of the wider view in visible light from the Digitized Sky Survey.

Recent discoveries in vast interstellar dust clouds permeating the universe and in nebula have revealed hints of organic matter that could be created naturally by stars, according to researchers in a 2011 study at the University of Hong Kong. The discovery team  observed stars at different evolutionary phases and found that they are able to produce complex organic compounds and eject them into space, filling the voids 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. 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 identified, and remains a broad assumption. 

 

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. "Nature is much more clever than we had imagined."

"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. Their findings have overturned this assumption.

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.

"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."

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

The image at the top of the page shows the long tail of interstellar dust shines in the reflected light of stars in this view of a nebula in the constellation Corona Australis (the southern crown). In some parts the dust accumulates to form dense molecular clouds from which it is thought young stars are born.

Image credit: NASA / JPL-Caltech / SSI and David Malin.

Source: The Daily Galaxy via ESO,  nature.com and space.com