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)