How Do Multiple-Star Systems Form? VLA Study Reveals "Smoking Gun"(Forwarded)

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December 15, 2006

How Do Multiple-Star Systems Form? VLA Study Reveals "Smoking Gun"

Astronomers have used the National Science Foundation's Very Large Array
(VLA) radio telescope to image a young, multiple-star system with
unprecedented detail, yielding important clues about how such systems are
formed. Most Sun-sized or larger stars in the Universe are not single,
like our Sun, but are members of multiple-star systems. Astronomers have
been divided on how such systems can form, producing competing theoretical
models for this process.

The new VLA study produced a "smoking gun" supporting one of the competing
models, said Jeremy Lim, of the Institute of Astronomy & Astrophysics,
Academia Sinica, in Taipei, Taiwan, whose study, done with Shigehisa
Takakuwa of the National Astronomical Observatory of Japan, is published
in the December 10 issue of the Astrophysical Journal.

Ironically, their discovery of a third, previously-unknown, young star in
the system may support a second theoretical model. "There may be more than
one way to make a multiple-star system," Lim explained.

The astronomers observed an object called L1551 IRS5, young, still-forming
protostars enshrouded in a cloud of gas and dust, some 450 light-years
from Earth in the direction of the constellation Taurus. Invisible to
optical telescopes because of the gas and dust, this object was discovered
in 1976 by astronomers using infrared telescopes. A VLA study in 1998
showed two young stars orbiting each other, each surrounded by a disk of
dust that may, in time, congeal into a system of planets.

Lim and Takakuwa re-examined the system, using improved technical
capabilities that greatly boosted the quality of their images. "In the
earlier VLA study, only half of the VLA's 27 antennas had receivers that
could collect the radio waves, at a frequency of 43 GigaHertz (GHz),
coming from the dusty disks. When we re-observed this system, all the
antennas could provide data for us. In addition, we improved the level of
detail by using the Pie Town, NM, antenna of the Very Long Baseline Array,
as part of an expanded system," Lim said. The implementation and
improvement of the 43 GHz receiving system was a collaborative program
among the German Max Planck Institute, the Mexican National Autonomous
University, and the U.S. National Radio Astronomy Observatory.

Two popular theoretical models for the formation of multiple-star systems
are, first, that the two protostars and their surrounding dusty disks
fragment from a larger parent disk, and, second, that the protostars form
independently and then one captures the other into a mutual orbit.

"Our new study shows that the disks of the two main protostars are aligned
with each other, and also are aligned with the larger, surrounding disk.
In addition, their orbital motion resembles the rotation of the larger
disk. This is a 'smoking gun' supporting the fragmentation model," Lim
said.

However, the new study also revealed a third young star with a dust disk.
"The disk of this one is misaligned with those of the other two, so it may
be the result of either fragmentation or capture," Takakuwa said.

The misalignment of the third disk could have come through gravitational
interactions with the other two, larger, protostars, the scientists said.
They plan further observations to try to resolve the question.

"We have a very firm indication that two of these protostars and their
dust disks formed from the same, larger disk-like cloud, then broke out
from it in a fragmentation process. That strongly supports one theoretical
model for how multiple-star systems are formed. The misalignment of the
third protostar and its disk leaves open the possibility that it could
have formed elsewhere and been captured, and we'll continue to work on
reconstructing the history of this fascinating system," Lim summarized.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.

[NOTE: Images supporting this release are available at
http://www.nrao.edu/pr/2006/multidis...hicspage.shtml ]