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Massive Star Forms by Absorption, Not Collision

massive star



M. BELTR¿N AND L. CARBONARO
Astronomers have captured the strongest evidence yet that the growth of high-mass stars occurs by the rapid absorption of hot gas and not by the collision of several smaller stars. Researchers observed a young high-mass star that seems to be pulling in a rotating disk of gas and spraying some of that gas outward in jets, as modeling predicts.

Researchers believe that low-mass stars such as the sun start to grow by dragging gas from their surroundings around them in a ball, which later flattens into a disk. If the same process of accretion is responsible for growing stars of 10 times the sun's mass or more, then the incoming ball of gas has to crumple much faster into a rotating donut or disk to release the enormous buildup of radiation in the still-forming star. The radiation could then escape perpendicular to the disk, taking some gas with it [see image above]. Despite identifying rotating gaseous disks around several massive young stars, researchers had never found a star exhibiting all three characteristics of the process--rotation, ejected gas and infalling gas.

A group of astronomers had already determined that a young star of about 20 solar masses, G24 A1, has a gaseous torus and outflowing gas. To complete the trifecta the team directed the Very Large Array at the National Radio Astronomy Observatory in Socorro, N.M., to tune in on the star's ammonia, a marker for the densest material encircling it. Judging from a shift in the wavelength of radiation absorbed by the ammonia compared with that absorbed by the molecule at rest, the team concluded that the star's dense material is in motion, which strongly indicates that gas is rotating and falling into the star like water circling a drain, the group reports in the September 28 Nature. "We have detected all the elements that one would expect," says astronomer Maria Beltr¿n of the University of Barcelona, the report's first author.

Some researchers have proposed that massive stars should often form by the collision of smaller stars, like water droplets fusing. "If you think massive stars form by collision you wouldn't expect to see a nice clean disk with jets coming out of it," says star-formation theorist Mark Krumholz of Princeton University. "It is pushing up the regime in mass range where accretion works," agrees observational astrophysicist John Bally of the University of Colorado at Boulder, but "it still leaves quite open in my mind what happens in larger stars," which are bright but very rare.

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