SETI bioastro: NEW DISCOVERY AN IMPORTANT LINK IN UNDERSTANDING THE LAST EVOLUTIONARY STAGES OF LOW-MASS STARS

From: Larry Klaes (lklaes@bbn.com)
Date: Fri Apr 14 2000 - 15:24:23 PDT


X-Mailer: Mozilla 4.72 [en] (Win98; I)
X-Accept-Language: en
Approved-By: "H. Alan Montgomery" <fhd@TCA.NET>
Newsgroups: sci.space.news
Date: Fri, 14 Apr 2000 16:17:33 -0500
Reply-To: News about Space from SEDS <SEDSNEWS@LISTSERV.TAMU.EDU>
Sender: News about Space from SEDS <SEDSNEWS@LISTSERV.TAMU.EDU>
From: "H. Alan Montgomery" <fhd@tca.net>
Subject: New discovery an important link in understanding the last
              evolutionary
To: SEDSNEWS@LISTSERV.TAMU.EDU

-------- Original Message --------
Subject: New discovery an important link in understanding the last
evolutionary
Date: Fri, 14 Apr 2000 11:53:25 -0400
From: Andrew Yee <ayee@nova.astro.utoronto.ca>
Organization: UTCC Campus Access
To: SEDSNEWS@LISTSERV.TAMU.EDU

------------------
University Communications
News Bureau
University of Georgia

WRITER: Phil Williams, 706/542-8501, pwilliam@franklin.uga.edu
CONTACT: Michael Duncan, 706/542-1998

** EDITORS/WRITERS: EMBARGOED UNTIL 2 P.M. ET, THURSDAY, APRIL 13, 2000
**

NEW DISCOVERY AN IMPORTANT LINK IN UNDERSTANDING THE LAST EVOLUTIONARY
STAGES OF LOW-MASS STARS

ATHENS, Ga. -- When low-mass stars called red supergiants die, they fade
away on a wimpy wind -- or so scientists have thought. New research,
just
published, suggests that the exact opposite may be true. These stars, in
fact,
may die with a bang and not with a whimper.

The study, published today in the journal Science, may lead researchers
to a
new understanding of red supergiants, which are studied to resolve
issues in
nucleosynthesis, stellar structure and the evolution of stars.

"This discovery was really a gigantic surprise," said Michael Duncan, a
research professor of chemistry at the University of Georgia. "One of
the
beauties of doing fundamental science is that you never quite know where
it may lead."

Other authors of the paper published in Science are Gerard Meijer, Gert
von
Helden and Deniz van Heijnsbergen of the FOM Institute for Plasma
Physics
in Nieuwegein, A.G.G.M. Tielens of the University of Groningen and S.
Hony
and L.B.F.M. Waters of the University of Amsterdam, all in the
Netherlands.

During their death throes, low mass stars turn into red supergiants,
which
are more properly called asymptotic giant branch stars or AGBs. Actually
a
stage of development rather than a specific kind of star, the AGB phase
is a
relatively short stage during which low-mass stars become their
brightest
but experience heavy mass loss that leads them rapidly to the planetary-
nebula phase and a final cooling to white dwarfs. (White dwarfs are
extremely hot, Earth-sized objects that fade and cool for billions of
years
until they become black, cold cinders.)

Scientists have been studying AGB stars for a long time, but research
has
been accelerated recently through use of the Hubble Space Telescope and
the European Space Agency's Infrared Space Satellite.

Duncan's involvement in the discovery was the kind of scientific
serendipity
that often leads to unexpected breakthroughs. His area, the study of
gas-phase
metal clusters, has lately taken a huge step forward due to a
collaboration
with Gerard Meijer, whom he met at a scientific meeting at Ohio State
University in 1998, and Meijer's colleagues in The Netherlands.

"He was talking about the free-electron laser called FELIX
[Free-Electron
Laser for Infrared Excitation] that had been built at the FOM Institute,
and
I happened to ask him if it had ever been used to study gas-phase metal
clusters," said Duncan. "From that, our collaboration was born."

There are probably no more than 20 free-electron lasers in the world,
and
only five in the U.S. (Priorities for use at the U.S. machines is
largely for
medical science or industrial applications.) FELIX is the only one
optimized
for measuring infrared signals or "spectra" of chemicals, and seemed a
perfect match for the metal-cluster experiments.

The study of metal clusters has been around only for about two decades.
Duncan helped initiate the field because of a laboratory accident when
he was a graduate student in chemistry at Rice University with Richard
Smalley. Duncan and a fellow graduate student were working on a
molecular
beam experiment when an accidentally misaligned laser vaporized part of
the apparatus.

Not realizing what had happened, they looked at a diagnostic tool called
a
mass spectrometer and saw a signal obviously associated with metallic
compounds. Only after studying the problem did they realize what had
happened. That accident, however, lead to a new way to produce large,
regular molecules called metal clusters -- most of which exist only for
milliseconds and are thus devilishly hard to study.

(The Smalley group later used the same equipment and repeated the
experiments on carbon and discovered a form of the element called
carbon-60. Shaped in panels like the geodesic dome invented by architect
Buckminster Fuller, the C60 forms were named "buckyballs." The team that
discovered them was award the Nobel Prize for Chemistry in 1996.)

"After meeting Meijer, we realized his team had the free-electron laser,
and I had the pulsed molecular beam machine and experience working with
metal clusters, and we needed to find a way to make them work together,"
said Duncan. Luckily, Meijer received at about that time a large grant
from
the Dutch government, and so the team in the Netherlands was able to
construct the molecular beam machine that Duncan had been using to study
metallic clusters and mate it with the free-electron laser. Gert von
Helden
and Deniz van Heijnsbergen.oversaw the actual construction of the beam
machine.

The result was a machine that could detect the infrared spectra of
gas-phase
metals and thus give important clues to how they are structured. The new
apparatus worked beautifully, and when Duncan visited the lab last
summer,
the team achieved the first direct infrared spectra of these clusters
ever
done. Their work was published in the journal Physical Review Letters
(PRL, 83, 4983, 1999).

These spectra in themselves will likely open a new era in the study of
how
gas-phase metals are structured, but a chance meeting with other Dutch
scientists at the FOM Institute initiated a startling discovery that led
the
research from the lab to the stars.

"These astronomers were visiting the FELIX lab and hearing about work on
polyaromatic hydrocarbons, which are important in the composition of
interstellar space," said Duncan. "It just so happened that our work on
gas-phase metals was on a machine nearby, and they asked what it was.
Meijer and von Helden showed them the machine and the spectra we had.
That's when their jaws dropped."

The astronomers, led by Alexander Tielens of the University of
Groningen,
realized immediately that the infrared spectra that the group had
elicited
from their study of titanium carbide nanocrystals corresponded almost
exactly to spectra of unknown origin seen again and again in AGB stars.
The discovery created a problem, however.

Meteorites containing micrometer-sized graphite grains with embedded
titanium carbide (TiC) grains have been discovered on Earth. Isotopic
analysis has identified AGB stars as the birthplace of these grains,
though
there had been no direct link. Astronomers believe that as AGB stars
begin
to die, newly synthesized elements such as TiC are mixed to the surface
where they spread over the galaxy in a wind, most often in the form of
stardust.

The problem lies in the fact that the abundance of titanium in low-mass
stars is so low that "high densities are required just to get a high
enough
collision gains to grow to the sizes observed in graphite stardust." For
some 20 years, scientists have thought that a so-called "superwind"
phase
takes place when these stars exhibit a dramatic loss of mass. But the
superwind phase, despite its name, has been considered a relatively
modest
event in which the star's remaining stellar envelope is blown away.

The identification of the infrared spectra around AGB stars as gas-phase
titanium carbide, however, changes that picture. Because of the low
amounts of titanium in the stars and the apparent large amount in the
ejecta, the event creating them must be cause by something that releases
tremendous energy over a relatively short period of time. Or as the
authors
write, "The TiC identification suggests that rather than with a wimpy
wind,
low-mass stars end their lives with (almost) a bang."

Studies of the infrared spectra of AGB stars is just beginning to take
off.
A conference in France in 1998 reported what it called the "first mature
results" of this research. The new study should add fuel to the fires of
speculation about how stars are formed, how they live -- and ultimately,
how they die.

   ##

Writers/Editors: Duncan will be out of the country for 10 days starting
April
14, but he will respond to requests for interviews. Please leave a
message
with his secretary Mamie Watson, at 706/542-1749, and he will return
your
call.

---
Andrew Yee
ayee@nova.astro.utoronto.ca



This archive was generated by hypermail 2b30 : Wed Mar 28 2001 - 16:07:53 PST