Magnetic fields crucial to star formation, astronomer says

September 07, 1999

Observations by a University of Illinois astronomer have shown that magnetic fields are a critical component controlling when and how stars form.

"Understanding the physics governing the structure and evolution of dense interstellar clouds is a necessary part of understanding the fundamental astrophysical process of star formation," said Richard Crutcher, a professor of astronomy at the U. of I. "Theoretical studies have suggested that magnetic fields play a vital role in the evolution of interstellar clouds and in the formation of stars, but those studies needed to be compared with observational data."

Two basic problems have persisted in our understanding of star formation, Crutcher said. First, in a fully formed star, the outward pressure of thermonuclear reactions in the core will balance the inward pull of gravity. In a molecular cloud, however, some other force must be supporting the cloud against its own gravity. Otherwise, all the clouds would have collapsed into stars long ago.

The second problem involves transferring excess angular momentum from a developing star. As a molecular cloud coalesces into stars, the material rotates faster and faster--like a spinning ice skater who tucks in her arms. Unless the excess angular momentum is removed, the star will fly apart.

"Theorists have performed extensive simulations that show how an interstellar cloud might collapse in the presence of a magnetic field," Crutcher said. "In those studies, the researchers could prevent the clouds from quickly collapsing and forming stars, and they could get rid of the extra angular momentum, by making the magnetic fields sufficiently strong."

To test theory against data, Crutcher measured the strengths of magnetic fields in 27 interstellar clouds of varying molecular density. By comparing each cloud's magnetic energy with its gravitational energy, he found that magnetic fields were strong enough to control the rate of collapse and to assist in the star-formation process by providing a means of shedding excess angular momentum.

"The magnetic field strength-- which does indeed scale with the square root of the gas density as theory predicts-- is nearly large enough to keep the cloud from collapsing," Crutcher said. "The gravitational energy is still about twice as strong as the static magnetic energy, but the magnetic field also supports the cloud indirectly by allowing magnetic turbulence and waves to be present."

The turbulence and waves supply an additional force that opposes the pull of gravity and provide a mechanism for transferring angular momentum from the developing star into the surrounding envelope of gas and dust.

"By flinging a small amount of matter outward along the magnetic field lines, the magnetic waves can remove a huge amount of angular momentum, making star formation possible," said Crutcher, who reported his findings in the August issue of the Astrophysical Journal.

University of Illinois at Urbana-Champaign

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