July 19, 2001
VLBA Provides Best Detail Yet of Star-Forming Cloud's Magnetic Field
Astronomers have used the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope to do a very detailed map of the magnetic field within a star-forming cloud, an achievement that will help scientists unravel the mysterious first steps of the stellar birth process.
"This study provides new and important data needed by theorists to understand how magnetic fields affect the early stages of star formation," said Anuj Sarma, an astronomer at the University of Illinois at Urbana-Champaign. Sarma worked with Thomas Troland of the University of Kentucky and Jonathan Romney of the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico. Their research results were published in the Astrophysical Journal Letters.
Stars are formed when gas in giant interstellar clouds collapses gravitationally. Magnetic fields are believed to support such gas clouds, helping them resist gravitational collapse, so the beginning stages of star formation arise from a complex interplay of the magnetic fields and gravity that is not yet well understood.
"In order to understand how star formation gets started, we need to know in detail the structure of the magnetic fields in a star-forming cloud," Sarma said. "Our observations with the VLBA have provided one more big step in this direction," he added.
The astronomers studied a cloud of molecular gas more than 5,000 light- years from Earth in a spiral arm of our own Milky Way Galaxy. The cloud, known as W3 IRS5, contains seven newly-formed stars. In addition, it contains a number of regions, somewhat smaller than the diameter of Earth's orbit, in which water vapor molecules act to amplify, or strengthen, radio emission. Such regions, called masers, are a radio- wave parallel to lasers, which amplify light.
The scientists used the VLBA to make a detailed study of the radio waves coming from these maser regions in the gas cloud. They detected a phenomenon called the Zeeman effect, in which a very precise frequency emitted by atoms or molecules, called a spectral line, is split into two by a magnetic field. Analyzing this effect allowed the astronomers to measure the strength of the magnetic field at the locations of the maser regions.
"The bright, amplified radio emission coming from these water masers allowed us to measure the magnetic-field strength," Troland said.
The Expanded Very Large Array (EVLA) and the Atacama Large Millimeter Array (ALMA), two instruments under development by the NRAO, will provide improved sensitivity to faint radio emissions, and allow even more detailed studies of the magnetic fields in star-forming regions.