Astrophysicists put kibosh on alternative theory of star formation(Forwarded)

Media Relations
University of California-Berkeley

16 November 2005

Astrophysicists put kibosh on alternative theory of star formation
By Robert Sanders, Media Relations

BERKELEY -- Astrophysicists at the University of California, Berkeley, and
Lawrence Livermore National Laboratory (LLNL) have exploded one of two
competing theories about how stars form inside immense clouds of
interstellar gas.

Using supercomputer simulations that take into account the turbulence
within a cloud collapsing to form a star, the researchers conclude that
the "competitive accretion" model cannot explain what astronomers observe
of star-forming regions studied to date.

That model, which is less than 10 years old and is championed by some
British astronomers, predicts that interstellar hydrogen clouds develop
clumps in which several small cores -- the seeds of future stars -- form.
These cores, less than a light year across, collapse under their own
gravity and compete for gas in the surrounding clump, often gaining 10 to
100 times their original mass from the clump.

The alternative model, often termed the "gravitational collapse and
fragmentation" theory, also presumes that clouds develop clumps in which
proto-stellar cores form. But in this theory, the cores are large and,
though they may fragment into smaller pieces to form binary or multiple
star systems, contain nearly all the mass they ever will.

"In competitive accretion, the cores are seeds that grow to become stars;
in our picture, the cores turn into the stars," explained Chris McKee,
professor of physics and of astronomy at UC Berkeley. "The observations to
date, which focus primarily on regions of low-mass star formation, like
the sun, are consistent with our model and inconsistent with theirs."

"Competitive accretion is the big theory of star formation in Europe, and
we now think it's a dead theory," added Richard Klein, an adjunct
professor of astronomy at UC Berkeley and a researcher at LLNL.

Mark R. Krumholz, now a post-doctoral fellow at Princeton University,
McKee and Klein report their findings in the Nov. 17 issue of Nature.

Both theories try to explain how stars form in cold clouds of molecular
hydrogen, perhaps 100 light years across and containing 100,000 times the
mass of our sun. Such clouds have been photographed in brilliant color by
the Hubble and Spitzer space telescopes, yet the dynamics of a cloud's
collapse into one or many stars is far from clear. A theory of star
formation is critical to understanding how galaxies and clusters of
galaxies form, McKee said.

"Star formation is a very rich problem, involving questions such as how
stars like the sun formed, why a very large number of stars are in binary
star systems, and how stars ten to a hundred times the mass of the sun
form," he said. "The more massive stars are important because, when they
explode in a supernova, they produce most of the heavy elements we see in
the material around us."

The competitive accretion model was hatched in the late 1990s in response
to problems with the gravitational collapse model, which seemed to have
trouble explaining how large stars form. In particular, the theory
couldn't explain why the intense radiation from a large protostar doesn't
just blow off the star's outer layers and prevent it from growing larger,
even though astronomers have discovered stars that are 100 times the mass
of the sun.

While theorists, among them McKee, Klein and Krumholz, have advanced the
gravitational collapse theory farther toward explaining this problem, the
competitive accretion theory has come increasingly into conflict with
observations. For example, the accretion theory predicts that brown
dwarfs, which are failed stars, are thrown out of clumps and lose their
encircling disks of gas and dust. In the past year, however, numerous
brown dwarfs have been found with planetary disks.

"Competitive accretion theorists have ignored these observations," Klein
said. "The ultimate test of any theory is how well it agrees with
observation, and here the gravitational collapse theory appears to be the
clear winner."

The model used by Krumholz, McKee and Klein is a supercomputer simulation
of the complicated dynamics of gas inside a swirling, turbulent cloud of
molecular hydrogen as it accretes onto a star. Theirs is the first study
of the effects of turbulence on the rate at which a star accretes matter
as it moves through a gas cloud, and it demolishes the "competitive
accretion" theory.

Employing 256 parallel processors at the San Diego Supercomputer Center at
UC San Diego, they ran their model for nearly two weeks to show that it
accurately represented star formation dynamics.

"For six months, we worked on very, very detailed, high-resolution
simulations to develop that theory," Klein said. "Then, having that theory
in hand, we applied it to star forming regions with the properties that
one could glean from a star forming region."

The models, which also were run on supercomputers at Lawrence Berkeley
National Laboratory and LLNL, showed that turbulence in the core and
surrounding clump would prevent accretion from adding much mass to a
protostar.

"We have shown that, because of turbulence, a star cannot efficiently
accrete much more mass from the surrounding clump," Klein said. "In our
theory, once a core collapses and fragments, that star basically has all
the mass it is ever going to have. If it was born in a low-mass core, it
will end up being a low-mass star. If it's born in a high mass core, it
may become a high-mass star."

McKee noted that the researchers' supercomputer simulation indicates
competitive accretion may work well for small clouds with very little
turbulence, but these rarely, if ever, occur and have not been observed to
date. Real star formation regions have much more turbulence than assumed
in the accretion model, and the turbulence does not quickly decay, as that
model presumes. Some unknown processes, perhaps matter flowing out of
protostars, keep the gases roiled up so that the core does not collapse
quickly.

"Turbulence opposes gravity; without it, a molecular cloud would collapse
far more rapidly than observed," Klein said. "Both theories assume
turbulence is there. The key is (that) there are processes going on as
stars begin to form that keep turbulence alive and prevent it from
decaying. The competitive accretion model doesn't have any way to put this
into the calculations, which means they're not modeling real star forming
regions."

Klein, McKee and Krumholz continue to refine their model to explain how
radiation from large protostars escapes without blowing away all the
infalling gas. For example, they have shown that some of the radiation can
escape through cavities created by the jets observed to come out the poles
of many stars in formation. Many predictions of the theory may be answered
by new and larger telescopes now under construction, in particular the
sensitive, high-resolution ALMA telescope being constructed in Chile by a
consortium of United States, European and Japanese astronomers, McKee
said.

The work was supported by the National Aeronautics and Space
Administration, the National Science Foundation and the Department of
Energy.

IMAGE CAPTION:
[http://www.berkeley.edu/news/media/r...es/accrete.jpg
(20KB)]
A slice through a 3-D simulation of a turbulent clump of molecular
hydrogen, with the densest areas shown in red. The zoom-in shows a
protostar accreting gas and creating a dense wake behind it. The
simulation shows that a protostar, once formed, cannot accrete much more
gas from the surrounding clump, contradicting the competitive accretion
theory. (Credit: Mark Krumholz)