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Unraveling the mystery of massive star birth

Scientists' observations show that formation works the same for all stars, regardless of mass.
Provided by ESO, Garching, Germany
Baby star
The disk around IRAS 13481-6124.
ESO/S. Kraus
July 14, 2010
Astronomers have obtained the first image of a dusty disk closely encircling a massive baby star, providing direct evidence that massive stars form in the same way as smaller stars.

"Our observations show a disk surrounding a young, massive star, which is now fully formed," said Stefan Kraus from the University of Michigan.

The team of astronomers looked at an object named IRAS 13481-6124. About 20 times the mass of our Sun and 5 times its radius, the young, central star, which is still surrounded by its prenatal cocoon, is located in the constellation Centaurus and lies about 10,000 light-years away.

From archival images obtained by the NASA Spitzer Space Telescope as well as from observations done with the APEX 12-meter submillimeter telescope, astronomers discovered the presence of a jet.

"Such jets are commonly observed around young, low-mass stars and generally indicate the presence of a disk," said Kraus.

Circumstellar disks are essential in the formation process of low-mass stars such as our Sun. However, it is not known whether such disks are also present during the formation of stars more massive than about 10 solar masses where the strong light emitted might prevent mass falling onto the star. For instance, it has been proposed that massive stars might form when smaller stars merge.

In order to discover and understand the properties of this disk, astronomers employed the European Southern Observatory's (ESO) Very Large Telescope Interferometer (VLTI). By combining light from three of the VLTI's 6-foot (1.8 meters) Auxiliary Telescopes with the Astronomical Multi-BEam combiner (AMBER) instrument, this facility allows astronomers to see details equivalent to those a telescope with a mirror of 280 feet (85 meters) in diameter would see. The resulting resolution is about 2.4 milliarcseconds, which is equivalent to picking out the head of a screw on the International Space Station, or more than 10 times the resolution possible with current visible-light telescopes in space.

With this unique capability, complemented by observations done with another of ESO's telescopes, the 11.7-foot (3.58 meters) New Technology Telescope at La Silla, Kraus and colleagues were able to detect a disk around IRAS 13481-6124.

"This is the first time we could image the inner regions of the disk around a massive young star," said Kraus. "Our observations show that formation works the same for all stars, regardless of mass."

The astronomers conclude that the system is about 60,000 years old, and the star has reached its final mass. Because of the intense light of the star — 30,000 times more luminous than our Sun — the disk will soon start to evaporate. The flared disk extends to about 130 times the Earth-Sun distance, or 130 astronomical units (AU), and has a mass similar to that of the star, roughly 20 times the Sun. In addition, the inner parts of the disk are shown to be devoid of dust.

"Further observations with the Atacama Large Millimeter/submillimeter Array (ALMA), currently being constructed in Chile, could provide much information on these inner parts, and allow us to better understand how baby massive stars became heavy," said Kraus.
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