Molecular cloud core model
These images show the distribution of density in the central plane of a
three-dimensional model of a molecular cloud core from which stars are
born. The model computes the cloud’s evolution over the free-fall
timescale, which is how long it would take an object to collapse under
its own gravity without any opposing forces interfering. The free-fall
time is a common metric for measuring the timescale of astrophysical
processes. In a) the free-fall time is 0.0, meaning this is the initial
configuration of the cloud, and moving on the model shows the cloud core
in various stages of collapse: b) a free-fall time of 1.40 or 66,080
years; c) a free-fall time of 1.51 or 71,272 years; and d) a free-fall
time of 1.68 or 79,296 years. Collapse takes somewhat longer than a
free-fall time in this model because of the presence of magnetic fields,
which slow the collapse process, but are not strong enough to prevent
the cloud from fragmenting into a multiple protostar system (d). For
context, the region shown in a) and b) is about 0.21 light years (or 2.0
x 1017 centimeters) across, while the region shown in c) and d) is
about 0.02 light years (or 2.0 x 1016 cm) across. Image is provided
courtesy of Alan Boss.
Washington, D.C.—New modeling studies from
Carnegie’s Alan Boss demonstrate that most of the stars we see were
formed when unstable clusters of newly formed protostars broke up. These
protostars are born out of rotating clouds of dust and gas, which act
as nurseries for star formation. Rare clusters of multiple protostars
remain stable and mature into multi-star systems. The unstable ones will
eject stars until they achieve stability and end up as single or binary
stars. The work is published in The Astrophysical Journal.
About two-thirds of all stars within 81 light years (25 parsecs) of
Earth are binary or part of multi-star systems. Younger star and
protostar populations have a higher frequency of multi-star systems than
older ones, an observation that ties in with Boss’ findings that many
single-star systems start out as binary or multi-star systems from which
stars are ejected to achieve stability.
Protostar clusters are formed when the core of a molecular cloud
collapses due to its own gravity and breaks up into pieces, a process
called fragmentation. The physical forces involved in the collapse are
subjects of great interest to scientists, because they can teach us
about the life cycles of stars and how our own Sun may have been born.
One force that affects collapse is the magnetic field that threads the
clouds, potentially stifling the fragmentation process.
Boss’ work shows that when a cloud collapses, the fragmentation
process depends on the initial strength of the magnetic field, which
acts against the gravity that causes the collapse. Above a certain
magnetic field strength, single protostars are formed, while below it,
the cloud fragments into multiple protostars. This second scenario is
evidently commonplace, given the large number of binary and multi-star
systems, although single stars can form by this mechanism as well
through ejection from a cluster.
“When we look up at the night sky,” Boss said, “the human eye is
unable to see that binary stars are the rule, rather than the exception.
These new calculations help to explain why binaries are so abundant.”
* * *
This work was partially supported by the NSF. The calculations were
performed on the Carnegie Xenia Cluster, the purchase of which was also
partially supported by the NSF.
Source: Carnagie Institution for Science
