In this artist's conception, a widely-separated pair of young, still-forming stars is in the background, forming by fragmentation of the material in the larger cloud in which they are born. In the foreground, companions in a multiple-star system are forming through fragmentation of a dusty disk that surrounds the original young star. Credit: Bill Saxton, NRAO/AUI/NSF. Hi-res image
A young double-star system in the Perseus Molecular Cloud, imaged with the VLA. This pair would fit within the orbit of Neptune in our Solar System.
Credit: Tobin, et al., NRAO/AUI/NSF. Hi-res image
A young triple-star system in the Perseus Molecular Cloud, imaged with the VLA.
Credit: Tobin et al., NRAO/AUI/NSF. Hi-res image
Disks
of material surrounding young stars in the Perseus Molecular Cloud,
imaged with the VLA. Arrows indicate the direction of outflows from the
young systems. Credit: Segura-Cox, et al., NRAO/AUI/NSF. Hi-res image
A detailed study of young stars and their surroundings has produced
dramatic new evidence about how multiple-star systems form and how the
dusty disks that are the raw material for planets grow around young
stars. Teams of scientists used the National Science Foundation's Karl
G. Jansky Very Large Array (VLA) radio telescope to study nearly 100
newborn stars in a cloud of gas and dust about 750 light-years from
Earth, in which new stars are forming.
Images made from the study
showed unprecedented detail of a number of the young stars, and are
helping astronomers resolve important questions about how stars, binary
stars, and planets get their starts. The astronomers presented their
results to the American Astronomical Society's meeting in Kissimmee,
Florida.
Looking at young multiple-star systems, one team
concluded that two different formation mechanisms may be at work to
produce such systems. They noted that the systems they studied fall into
two distinct types, based on the distance between the stars in the
system. The closer systems have stars separated by about 75 times the
Sun-Earth distance, and another group has its stars separated by about
3,000 times the Sun-Earth distance. They also found that more than half
of the youngest stars they studied are in multiple systems, suggesting
that star formation tends to produce multiples rather than single stars.
"Several
different processes have been suggested for how multiple-star systems
form, and our results indicate that the separation between stars may
tell us which of these processes is responsible for a particular
system," said John Tobin, of Leiden Observatory in the Netherlands.
Stars
form in giant clouds of gas and dust, when tenuous material in such
clouds collapses gravitationally into cores that then begin to draw
additional material inward. Infalling material forms a rotating disk
around the young star. Eventually, the young star gathers enough mass to
create the temperatures and pressures at its center that will trigger
thermonuclear reactions. The rotating disk around the star provides the
material from which planets may form.
The researchers concluded
that the more widely-separated multiple-star systems form through
turbulent fragmentation of the larger cloud, while the closer systems
are the result of fragmentation within the disk of material orbiting the
original protostar. They also found that somewhat older systems have
fewer widely-separated companions than the youngest group of protostars.
This, they said, suggests that perhaps some young stars that form as
widely-separated systems are not gravitationally bound and simply drift
apart over time.
Another team, led by Dominique Segura-Cox, of
the University of Illinois, found that the dusty disks around some of
the protostars are larger than some theoretical models predict. These
disks are essential to the formation of planets, some binary companions,
and the young star's ability to draw in additional material. Despite
their central role in these processes, however, their formation
mechanisms have been debated among astronomers.
As material falls
inward toward a young star, it pulls magnetic fields along with it.
Theorists suggested that these fields, which become stronger as they are
concentrated closer to the star, could be aligned so that they
drastically slow the disk's rotation, limiting the size of the disk.
Theoretical models predicted that this effect, called magnetic braking,
would limit the disks to a radius about 10 times the Earth-Sun distance,
or slightly more than the distance from the Sun to Saturn.
"We
found disks with radii that are at least 15-30 times the Earth-Sun
distance, significantly larger than the magnetic-braking model would
allow," Segura-Cox said. "This is a lower limit, and the disks may
actually be larger. Studies of other systems have indicated that disks
are larger when observed at radio frequencies different than the ones we
used in this project," she added.
One explanation for the larger
disk sizes may be that, in some systems, the magnetic field and the
rotation axis of the star are misaligned, a configuration that reduces
the magnetic-braking effect. Evidence for this has been seen in some
objects, the researchers said.
In another study published last December,
a team using data from the same project found that the material falling
toward one protostar is twisting the magnetic field lines and changing
their configuration as it drags them inward. That study, which measured
the magnetic-field alignments near the star, indicates one mechanism for
minimizing the magnetic-braking effect.
"These observations of
disks around such young stars suggests that all the elements needed for
planet formation are present very early in the life of a star. Plus, it
is probable that there are already centimeter-sized particles in these
young disks, meaning that the growth of solids progresses rapidly,"
Tobin said.
The images for this work came from a project called
the VLA Nascent Disk and Multiplicity (VANDAM) Survey. This survey used
264 hours of VLA observing time from 2013 to 2015 to study protostars in
the Perseus Molecular Cloud, about 750 light-years distant. The Perseus
Molecular Cloud, containing as much material as 10,000 suns, is one of
the closest regions where low- to intermediate-mass stars are actively
forming, and thus serves as a valuable "laboratory" for astronomers
seeking to understand star formation.
"This survey sampled the
largest number of young stars, and revealed fainter objects than we
could study previously, and did so in greater detail. The information it
provided has dramatically improved our knowledge," Tobin said.
"The
disks we studied are difficult to observe as they are obscured by the
cloud in which they are forming, but these new VLA observations reveal
the disks and provide critical data into their formation mechanism,”
Segura-Cox said.
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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