Artist's conception of the 'boiling disk' surrounding Orion Source I. A disk of hot, ionised gas surrounds the central star, blocking our view. A cool wind of gas is driven from the upper and lower surfaces of the disk and is sculpted into an hourglass shape by tangled magnetic field lines.
Credit: Bill Saxton, NRAO/AUI/NSF
SYDNEY: Astronomers have made a detailed study of a star-forming region inside the Orion Nebula, for the first time getting a close-up look at the birth of a massive star.
The results, to be published in The Astrophysical Journal early next year, help to explain the physics of massive star formation at a much finer scale than has been possible before.
Because they are rare, distant and swathed in dust, astronomers haven't been able to directly observe massive stars forming. Smaller stars, such as our Sun, form from clouds of gas and dust that resolve into rotating disks.
Collisions and collapsing clouds
Whether or not disks play a role in forming stars eight to 100 times the size of our own star has been a long-standing question, said astronomer and lead author of the study, Lynn Matthews from the Massachusetts Institute of Technology in Boston, USA. Competing models include collisions between smaller stars and material collapsing from a sphere of gas and dust.
Now Matthews and astronomers from the Harvard-Smithsonian Centre for Astrophysics, also in Boston, and other institutions have peered into the dense cloud of gas and dust of the Kleinmann-Low nebula within the larger Orion nebula.
The nebula is behind Orion's bright Trapezium star system, forming the middle 'star' in the sword of the constellation Orion. Their focus was a young protostar, 'Source I', around 1,350 light-years distant, and hidden from visible and infrared astronomy by a dense cloud of dust.
Motion of gas clumps
"What makes our new observations of Orion Source I unique is that we resolved for the first time an accretion disk surrounding a massive star and have been able to measure the motions of gas clumps within this disk over time," said Matthews.
The team used the Very Long Baseline Array (VBLA), an array of radio telescopes that stretches across 8000 km, to take an image of the region every month for two years. The images reveal the region with a precision 1,000 times sharper and more detailed than any previous study. The researchers then made a movie out of the images, which you can see here.
The movie shows clumps of gas flowing away from Source I over the two years. The location of the protostar is marked with crosshairs. Gas moving toward us (blueshifted) is colored green or blue, while gas moving away from us (redshifted) is colored yellow, orange and red.
Because they can't see the protostar itself, the researchers used bright radio sources called masers — the radio-wavelength equivalent of lasers — to track the movement of gas and dust in the system.
The movie reveals two jets of gas moving away from Source I - akin to the jets that stream from the poles of smaller stars. Material in these massive protostar jets would be surging at speeds up to 80,000 km/h. The streams of gas also appear to curve as they leave the disk, which suggests magnetic field lines are wrapping the streams of gas into a helix.
"Magnetic fields are supposed to be weak and unimportant to the birth process for massive stars," said Matthews. "But masers would not travel along gentle arcs unless they experience some sort of force - probably a magnetic force."
The data is akin to a "centrefold for geeks," commented Peter Tuthill an astrophysicist from the University of Sydney in Australia. "There is so much detail you can advance the data to the point where you can look at wiggles in the streams of gas. With such beautifully detailed pictures you can find out the exact physics of the system," he said.
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