Astronomers Describe Predicted Present Day Distribution of ElusiveFirst Stars (Forwarded)

University of California-Santa Barbara

CONTACTS:
Gail Gallessich, 805-893-7220

FEATURED RESEARCHERS:
Evan Scannapieco, 415-425-7073
Daisuke Kawata, 626-304-0232
Brad Gibson, 44 (0) 1772 893585

December 11, 2006

Using Enormous Computer Simulations, Astronomers Describe Predicted
Present Day Distribution of Elusive First Stars

Santa Barbara, CA -- With the help of enormous computer simulations,
astronomers have now shown that the first generation of stars -- which
have never been observed by scientists -- should be distributed evenly
throughout our galaxy, deepening the long-standing mystery about these
missing stellar ancestors. The results are published in this week's issue
of the Astrophysical Journal.

The problem is that despite years of looking, no one has ever found any of
these stars. "Many astronomers thought this was because the stars without
heavy elements were hidden from us," said Evan Scannapieco, first author
and a postdoctoral fellow at the Kavli Institute for Theoretical Physics
at the University of California, Santa Barbara. "Because our galaxy formed
from the inside out, the idea was that these very old stars would all be
near the center. But the center of Milky Way is extremely crowded with
dust and newer stars, making it very hard to detect individual old stars
in this environment."

This earliest generation of stars should look very different from
later-forming stars like the Sun; yet so far, no one has detected a
survivor from this primordial population. One of the long-standing
explanations for this discrepancy was that these stars might all be
contained in regions near the center of the Milky Way, where they are very
hard to observe. The results of the new study make that explanation
unlikely.

Oxygen, carbon, and most of the elements we encounter every day on Earth
were made in stars, rather than during the Big Bang. "But these heavy
elements are made in the centers of stars and remain buried under the gas
at the surface until the stars die and explode, so what you see when you
look at a star are the elements that were present when it was born," said
Brad K. Gibson, co-author and chair of the Department of Theoretical
Astrophysics at the University of Central Lancashire in Britain. "This
means that the stars still living from this first generation should
continue to show no heavy elements."

Carrying out a detailed simulation of the formation of the Milky Way, the
research group constructed not only the history of where stars formed over
time, but the chemical composition of the gas out of which these stars
formed. "We found that while the very oldest stars all end up near the
center of the Milky Way, it takes a long time for heavy elements to enrich
the gas that is further out," said co-author Daisuke Kawata, a scientist
with the Carnegie Observatories in Pasadena, California. "This means that
while the oldest stars all end up near the center of the galaxy, plenty of
stars that contain only primordial elements are formed at later times
throughout the galaxy. These primordial stars should be everywhere."

Because the stars forming in the Milky Way suburbs are easily detectable
with present day telescopes, there must be some other reason that the
remnants of this primordial generation didn't survive. "It could be that
they were all high-mass stars, which would not have lived long enough to
still be around, or there could be another twist to the story that we
haven't yet figured out," said co-investigator Chris Brook, a scientist
with the University of Washington. "Whatever the answer, it's clear that
studies of the outskirts of our galaxy will have lots more to tell us
about this remarkable, missing generation."

Other scientists involved in the research were Andrea Ferrara of the
International School for Advanced Studies in Trieste, Italy and Rafaella
Schneider of the Arctri Observatory in Florence, Italy.

IMAGE CAPTION:
[http://www.ia.ucsb.edu/pa/image.aspx...532&Position=1 (480KB)]
Distribution of the oldest stars in our galaxy (upper panel) compared with
the full population of primordial stars (lower panel). While the oldest
stars are all located near the center, where they are very difficult to
detect, later forming stars made of gas without heavy elements should be
located throughout the galaxy. The fact that no such stars have ever been
detected places important scientific constraints on the properties and
formation of these stars.