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Mega-array reveals birthplace of giant stars

Early results from Atacama telescope signal the opening of a scientific frontier.

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R. Durán/ESO/NAOJ/NRAO/ALMA

The Atacama Large Millimeter/submillimeter Array (ALMA), set on a 5,000-metre-high plateau in Chile, will ultimately consist of 66 dish antennas.

Astronomers have punched through a brick wall. They have seen into the heart of a massive cloud of cold dust and gas near the Galactic Centre that, because it blocks visible light, had been dubbed the ‘brick’. Viewed in millimetre waves (which have wavelengths between those of microwaves and radio waves), the faintly glowing dust reveals knots of gas — the embryos of stars, seen in such detail that they could show how the Galaxy’s most massive stars are born.

The images testify to the penetrating vision of astronomy’s newest global megaproject, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. “Up till now we’ve just been looking at big blobs,” says Jill Rathborne of the Commonwealth Scientific and Industrial Research Organization’s Astronomy and Space Science division in Sydney, Australia. She likens the new images to seeing the details of a tree. “Now we’re looking at the limbs, the branches, the flowers and the roots.”

Rathborne presented her team’s images, produced with just six hours of observing time, on 12 December here at the first ALMA science conference. The US$1.4-billion array, perched on a 5,000-metre-high plateau near the Chilean–Bolivian border, is due to have all 66 of its dish antennas completed by the end of 2013. At that point, ALMA will be able to separate its movable antennas by as much as 16 kilometres, which will allow its signals to be integrated into images of the cold Universe that will have exquisite spatial and spectral resolution. But even at partial strength — Rathborne’s work was done with 25 antennas — ALMA has reached a scientific frontier. That is a relief to director Thijs de Graauw. “We have been working 30 years to get here,” he says.

The array — an international collaboration involving North America, Europe, Taiwan and Japan — is unveiling cool, dusty parts of the Universe that are invisible at other wavelengths. ALMA astronomers looking outside the Milky Way are detecting bubbles of gas being expelled from distant galaxies — presumably propelled by jets shooting out from supermassive black holes at the galaxies’ centres. The loss of gas in these bubbles is thought to put a brake on galaxy growth.

In our own Galaxy, ALMA is training its antennas on disks of gas and dust around young stars — material that eventually co­alesces into planets. Jes Jørgensen of the Niels Bohr Institute at the University of Copenhagen presented the first discovery of glycolaldehyde around a protostar. This organic molecule is an essential building block of ribose, a component of RNA. “We have to get organic molecules from somewhere,” says Neal Evans, an ALMA board member from the University of Texas at Austin. The ALMA find is a hint that they formed at the same time as the planets.

JILL RATHBORNE / CSIRO / JCMT / ALMA

ALMA sees star-forming gas clumps in the ‘brick’.

But with the array not yet at full strength, many of the main conference results came from large, nearby objects — in particular, the giant clumps of gas within our Galaxy that become clusters of stars. Astronomers especially want to identify the elusive conditions that lead to the growth of high-mass stars — those 8–150 times heavier than the Sun. Such behemoths are rare, but are thought to have been the Universe’s first stars and to have had a major effect on cosmic history. Their intense ultraviolet light would have ionized the hydrogen of interstellar space, for example, and they would have quickly exploded as super­novae, seeding the Universe with elements heavier than hydrogen and helium.

Yet astronomers have never witnessed a high-mass star being born, and hotly debate how they form. The turbulent-core theory posits that they originate in an exceptionally large knot of dense gas — a ‘core’ — in which turbulent winds boost the internal pressure. That would allow the core to grow bigger than it ordinarily would before igniting by thermonuclear fusion and producing an outgoing ‘stellar wind’ of mater­ial that blows away the remaining gas and cuts off growth. A rival theory, known as competitive accretion, suggests that the cores break up within the gas cloud, forming small protostars that later become giants by eating up material elsewhere in the cloud before their stellar winds can blow the gas away.

Advocates of both theories are looking to natural laboratories such as the brick for answers. Rathborne has identified some 50 possible cores within the cloud, which contains material with an overall mass of 100,000  Suns. But the images are not quite sharp enough to show whether the cores are large and bounded — as the turbulent-core theory suggests — or are seeds in the process of breaking up into smaller cores, which competitive accretion calls for. That detail could start coming in January with the next cycle of ALMA experiments, which will be conducted with at least 32 antennas. Whatever they show, Rathborne says, the region is a lode that astronomers will be mining for quite some time. Bricks like this, she says, “are made of gold”.

Journal name:
Nature
Volume:
492,
Pages:
319–320
Date published:
()
DOI:
doi:10.1038/492319a

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