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Media Relations
University of California-Berkeley

Media Contacts:

Robert Sanders
(510) 643-6998, (510) 642-3734
rls{at}pa.urel.berkeley.edu
  
Additional Resources:
  
Ben McCall
(510) 642-1047
bjmccall{at}astron.berkeley.edu

FOR IMMEDIATE RELEASE: Friday, April 04, 2003 

Part 2 of 2

For so much H3+ to be present, McCall said, astronomers
must be wrong about either how rapidly H3+ is produced
or how quickly it is destroyed. Either high-speed
electrons in the cloud don't destroy H3+ as easily as
people thought, or more H3+ is produced from cosmic
rays than astronomers suspect. Cosmic rays generate
H3+ when they hit hydrogen molecules, ionizing them
and catalyzing their reaction with other hydrogen
molecules. 

The big unknown was the rate at which electrons
destroy cold H3+. All reaction rates had been measured
in relatively hot H3+ -- hundreds of degrees above
absolute zero Kelvin, or more than 100 degrees
Celsius -- while the temperature of the interstellar
medium is about 30-100 degrees above absolute zero.
Different experimental measurements also differed by
a factor of 10,000. 

To measure the reaction rate at a temperature closer
to that found in the interstellar medium, McCall
teamed up with UC Berkeley chemist Richard Saykally,
who has pioneered the use of novel infrared laser
technologies for the study of ionized molecules in
the laboratory. The group combined its new method of
infrared cavity ringdown spectroscopy with supersonic
cooling to take H3+ ions down to the ultra-low
temperatures found in interstellar space. The
supersonic cooling technique cools a gas by letting it
expand quickly through a pinhole into a vacuum, much
the way air cools as it escapes through a pinhole in
a tire. 

Using this technique, McCall and Saykally group member
Alex Huneycutt cooled H3+ to about 20-60 Kelvin, a
temperature at which the molecule is only found in its
two lowest energy levels. They carried this supersonic
beam source for making cold H3+ to the Manne Siegbahn
Laboratory in Stockholm, Sweden, where they placed it
in the collision path of a beam of electrons from the
CRYRING ion storage ring. CRYRING is a rare facility
able to accelerate molecular ions like H3+, store
them for a long period of time, and then superimpose
the molecular beam with a beam of very cold electrons. 

Their measurement, the first time anyone has looked
at the electron destruction rate of cold H3+, showed
that electrons destroy cold H3+ ions about 40 percent
less efficiently than they destroy hotter H3+. Though
significant, this discrepancy does not explain the
greater-than-expected abundance of H3+ in diffuse
clouds. 

With this precision measurement in hand, however,
McCall and his colleagues turned their attention to
the well-studied diffuse cloud in the direction of
Zeta Persei, in hopes of pinning down the reason for
such high H3+ abundances. Using the United Kingdom
Infrared Telescope in Hawaii, McCall and Geballe
measured for the first time the amount of H3+ present
in the diffuse cloud toward Zeta Persei and were
able to calculate the cosmic ray ionization rate
generating H3+, which turned out to be 40 times
higher than previously assumed. 

McCall and his team speculate that this can only be
true if there are lots of low-energy cosmic rays
permeating the cloud and reacting with molecular
hydrogen to create H3+. Such low energy cosmic rays
had been proposed once before, but experiments
seemed to rule them out. Cosmic rays are thought
to be produced in the shock fronts generated by
supernova explosions. 

This novel interpretation would have implications for
the physics and chemistry inside interstellar clouds,
implying, for example, more abundant oxygen compounds
like OH. It also implies much greater heating of
clouds by cosmic rays, and a higher rate of production
of complex molecules. 

McCall plans to continue his studies of interstellar
H3+ to prove or disprove what he calls his "heretical"
assertion. 

"This is the very beginning of studies like these,"
McCall said. "We will re-measure Zeta Persei and look
at other clouds to determine if there really are 40
times more cosmic rays pervading the galaxy than we
think there are." 

The UC Berkeley component of the work was funded by
the Air Force Office of Scientific Research, the
National Science Foundation and NASA.

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