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A stellar dust factory

STRIKING NEW OBSERVATIONS with the Atacama Large Millimetre/submillimetre Array (ALMA) telescope capture, for the first time, the remains of a recent supernova brimming with freshly formed dust. If enough of this dust makes the perilous transition into interstellar space, it could explain how many galaxies acquired their dusty, dusky appearance.

Cosmic dust consists of silicate and graphite grains – minerals also abundant on Earth. The soot from a candle is very similar to cosmic graphite dust, although the size of the grains in the soot are ten or more times bigger than typical grain sizes of cosmic graphite grains.

This image shows the remnant of Supernova 1987A

This image shows the remnant of Supernova 1987A seen in light of very different wavelengths. ALMA data (in red) shows newly formed dust in the middle of the remnant. Hubble Space Telescope (in green) and Chandra Space Observatory (in blue) data show the expanding shock wave. Credit: ALMA (ESO/NAOJ/NRAO) / A. Angelich. Visible light image: the NASA/ESA Hubble Space Telescope. X-Ray image: The NASA Chandra X-Ray Observatory

Galaxies can be remarkably dusty places and supernovae – exploded stars – are thought to be a primary source of that dust, especially in the early universe. But direct evidence of a supernova’s dust-making capabilities has been slim up to now, and could not account for the copious amount of dust detected in young, distant galaxies. But now observations with ALMA are changing that.

An international team of astronomers used ALMA to observe the glowing remains of Supernova 1987A, which is in the Large Magellanic Cloud, a dwarf galaxy orbiting the Milky Way about 160,000 light-years from Earth. SN 1987A is the closest observed supernova explosion since Johannes Kepler’s observation of a supernova inside the Milky Way in 1604. Being far in the southern sky, it is clearly visible only from the Southern Hemisphere.

The Tarantula Nebula and its surroundings

This is an image of the Tarantula Nebula and its surroundings in the Large Magellanic Cloud galaxy, taken in 1987. Supernova 1987A is the bright star just to the right of centre. Credit: ESO

“This is the first time we’ve been able to really image where the dust has formed, which is important in understanding the evolution of galaxies,” said Remy Indebetouw, an astronomer at the National Radio Astronomy Observatory (NRAO) and the University of Virginia, both in Charlottesville, USA

Astronomers predicted that as the gas cooled after the explosion, large amounts of dust would form as atoms of oxygen, carbon, and silicon bonded together in the cold central regions of the remnant. However, earlier observations of SN 1987A with infrared telescopes, made during the first 500 days after the explosion, detected only a small amount of hot dust.

With ALMA’s resolution and sensitivity, the team was able to image the far more abundant cold dust, which glows brightly in millimetre and submillimetre light. The astronomers estimate that the remnant cloud now contains about 25 percent the mass of the Sun in newly formed dust. They also found that significant amounts of carbon monoxide and silicon monoxide have formed.

Aerial view of dishes of the Atacama Large Millimetre/submillimetre Array

Aerial view of dishes of the Atacama Large Millimetre/submillimetre Array (ALMA) telescope. Credit: ALMA

“SN 1987A is a special place since it hasn’t mixed with the surrounding environment, so what we see there was made there,” said Indebetouw. “The new ALMA results, which are the first of their kind, reveal a supernova remnant chock full of material that simply did not exist a few decades ago.”

There’s more information on Supernova 1987A, including an interview with Australian astronomers, on the ABC’s web site.

Adapted from information issued by NRAO.

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Dish complex will study ‘cool’ cosmos

First eight ALMA dishes

The first eight ALMA dishes have already been pressed into service, 5,000 metres above mean sea level on the Chajnantor plateau in Chile. They are seen here in September 2010.

A GIANT NETWORK OF RADIO DISHES is taking shape high in the deserts of the Atacama Plateau in Chile. Known as the Atacama Large Millimetre/Submillimetre Array (ALMA), it will be used to study the ‘coolest’ parts of the cosmos.

When completed, ALMA will comprise 66, twelve-metre-diameter antennae, each weighing about 95 tonnes. The dishes will be electronically joined to form one single, huge telescope that picks up millimetre and submillimetre wavelengths from deep space.

These wavelengths are affected by water vapour in the atmosphere, which explains the choice of the high and dry site in the Atacama.

As each dish arrives from the manufacturer, it is moved on a special transporter from the Site Erection Facility (SEF) where it is assembled and tested, to the Operations Support Facility (OSF), where it is fitted with its extremely sensitive radio receivers and cooling systems.

Artist's impression of the finished ALMA

Artist's impression of the finished ALMA network of 66 dishes.

Both the SEF and OSF are at an elevation of 2,900 metres above mean sea level, which seems high enough. But the antennae’s final resting place is the observatory site on the Chajnantor plateau, which is at 5,000 metres elevation.

Cool cosmos

ALMA’s targets are the ‘coolest’ components of the universe…the tiny particles of interstellar dust and gas molecules from which everything—stars, planets and galaxies—formed and are still forming.

The array will be able to peer back in time to reveal some of the earliest galaxies, when the universe was only a few billion years old. It’ll also provide information on the formation of stars and planetary systems in the closer and more recent universe.

ALMA dish on a transporter vehicle

The ALMA antennae each weigh about 95 tonnes, and are moved around on giant transporter vehicles.

The dishes are state-of-the-art, with surface panels built and aligned to a precision of less than the thickness of a human hair. Theoretically, ALMA could spot a golf ball 15 kilometres away.

Conditions on the Chajnantor plateau are tough, with strong sunlight and fierce winds. None of the dishes have protective domes, and the air temperature can drop to –20 degrees Celsius.

ALMA is an international facility, being a partnership of Europe, North America and East Asia working collaboratively with the host country, Chile. Twenty-five antennae are being provided by Europe, 25 by North America and 16 by East Asia.

Story by Jonathan Nally, copyright SpaceInfo.com.au. Images courtesy ALMA and (ESO/NAOJ/NRAO), J. Guarda.

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