For Immediate Release: December 15, 2006

Contact:

Dave Finley, Public Information Officer
1-505-835-7302
dfinley@nrao.edu

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.

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NOTE: This release, with graphics and VLA images, is available
on the NRAO Web site, at:

http://www.nrao.edu/pr/2006/multidisk/

For More Information:

Jeremy Lim
Associate Research Fellow
Institute of Astronomy & Astrophysics, Academia Sinica (ASIAA)

Phone: 886-2-3365 2200 x727
FAX: 886-2-2367 7849
jlim@asiaa.sinica.edu.tw