Using the new capabilities of
the upgraded Karl G. Jansky Very Large Array (VLA), scientists have discovered
previously-unseen binary companions to a pair of very young protostars. The
discovery gives strong support for one of the competing explanations for how
double-star systems form. Astronomers know that about half of all Sun-like
stars are members of double or multiple-star systems, but have debated over how
such systems are formed. "The only way to resolve the debate is to observe
very young stellar systems and catch them in the act of formation," said
John Tobin, of the National Radio Astronomy Observatory (NRAO). "That's
what we've done with the stars we observed, and we got valuable new clues from
them," he added. Their new clues support the idea that double-star systems
form when a disk of gas and dust whirling around one young star fragments,
forming another new star in orbit with the first. Young stars that still are
gathering matter from their surroundings form such disks, along with jet-like
outflows rapidly propelling material in narrow beams perpendicular to the disk.
When Tobin and an
international team of astronomers studied gas-enshrouded young stars roughly
1,000 light-years from Earth, they found that two had previously-unseen
companions in the plane where their disks would be expected, perpendicular to
the direction of the outflows from the systems. One of the systems also clearly
had a disk surrounding both young stars.
"This fits the
theoretical model of companions forming from fragmentation in the disk,"
Tobin said. "This configuration would not be required by alternative
explanations," he added.
The new observations add to a
growing body of evidence supporting the disk-fragmentation idea. In 2006, a
different VLA observing team found an orbiting pair of young stars, each of
which was surrounded by a disk of material. The two disks, they found, were
aligned with each other in the same plane. Last year, Tobin and his colleagues
found a large circumstellar disk forming around a protostar in the initial
phases of star formation. This showed that disks are present early in the star
formation process, a necessity for binary pairs to form through disk
fragmentation. "Our new findings, combined with the earlier data, make
disk fragmentation the strongest explanation for how close multiple star
systems are formed," said Leslie Looney of NRAO and the University of
Illinois.
"The increased
sensitivity of the VLA, produced by a decade-long upgrade project completed in
2012, made the new discovery possible," Claire Chandler of NRAO said. The
new capability was particularly valuable at the VLA's highest frequency band,
from 40-50 GHz, where dust in the disks surrounding young stars emits radio
waves. The astronomers observed the young stars during 2012 with the VLA and
with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) in
California.
Tobin, Chandler, and Looney
were part of a research team of astronomers from the U.S., Mexico, and the
Netherlands. The scientists published their findings in the Astrophysical
Journal. The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.
Credit: nrao.edu
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