Berkeley – A bit of Earth-bound chemistry has led scientists at the
University of California, Berkeley, to conclude that there is an
unsuspected wind of low-energy cosmic ray particles blowing through
the galaxy.

The cosmic rays aren’t energetic enough to make headway against the
solar wind to reach Earth, but they appear to have a big impact on
the chemistry within tenuous clouds of gas between stars, so-called
diffuse interstellar clouds.

“This implies a new population of cosmic rays not energetic enough to
make it into dense clouds but able to penetrate and play a major role
in diffuse clouds,” said astrophysicist and chemist Benjamin J.
McCall, a Miller Post-doctoral Fellow in the departments of chemistry
and astronomy at UC Berkeley.

Unlike dense clouds, which look black and empty because the dust and
gas block the light of stars forming inside, diffuse clouds are
invisible, betrayed only by the reddening of stars whose light passes
through them.

McCall and his colleagues estimate a low-energy cosmic ray flux 40
times greater than standard estimates, which are based on
observations of dense clouds.

The finding, reported in the April 3 issue of the journal Nature,
implies that cosmic rays are a more significant source of heating and
ionization in diffuse interstellar gas clouds than generally
recognized, reviving a theory proposed some 30 years ago. The greater
ionization also implies more abundant production of complex molecules
than previously thought.

“It would be a major development if it is true,” said Carl Heiles, a
UC Berkeley professor of astronomy who studies interstellar magnetic
fields. “I think it’s plausible, because there are indications of
increased heating in low-density gas.”

McCall’s colleagues include Richard J. Saykally, UC Berkeley
professor of chemistry, and a member of his group, Alex Huneycutt;
astronomer Thomas R. Geballe of the Gemini Observatory in Hawaii; and
a team of physicists based at the CRYRING in Sweden, led by Mats
Larsson of Stockholm University.

Though McCall’s interpretation is not accepted by all astronomers,
the findings clearly point to something wrong with current
understanding of the chemistry inside the diffuse clouds that dot the
galaxy.

“Interstellar chemistry is very important in that it helps determine
certain properties of the galaxy, in particular the intensity of the
low-energy part of the cosmic ray spectrum,” said Al Glassgold,
professor emeritus of physics at New York University and now adjunct
professor of astronomy at UC Berkeley. “Ben’s straightforward
interpretation … presents all kinds of problems understanding
what’s going on with the diffuse, and more generally, the entire
interstellar medium. It’s a result that shakes up what people thought
they had been understanding reasonably well now for about two
decades.”

Despite the cold, rarified gas and dust in diffuse clouds – a mere
100-300 particles per cubic centimeter – chemical reactions are
ongoing, sparked by the ATP of the cosmos, H3+. A highly reactive
molecule, H3+ is composed of three hydrogen atoms linked in the form
of an equilateral triangle – three bare protons enveloped in a cloud
of two electrons. H3+ readily donates an “extra” proton to other
atoms or molecules, leaving behind a hydrogen molecule, H2, the main
component of molecular clouds. The molecule that accepts the proton
is then activated, itching to start another chemical reaction.

“H3+ acts like a strong acid,” McCall said. “It’s very happy to give
up one of its protons to any molecule it runs into.”

The reaction breeds a cascade of other reactions, producing many
types of organic molecules, from the simple ones like water, carbon
monoxide and hydroxyl radicals (OH) to complex hydrocarbons.

“H3+ begins this whole sequence of ion-molecule chemistry that is
fundamental for our understanding of what’s going on in diffuse
clouds as well as dense molecular clouds,” said UC Berkeley professor
and chair of physics Chris McKee, a theoretical astrophysicist who
models the interior of interstellar clouds.

Many people suspect that hydrocarbons interacting on the surface of
dust grains could have given rise to the organic molecules essential
for the origin of life.

H3+ was first detected in dense molecular clouds seven years ago, and
its abundance fits fairly well with the chemistry of other molecules
in such clouds. Astronomers thought H3+ would be undetectable in
diffuse clouds, however. To their astonishment, in 1997, H3+ was
detected in diffuse clouds as a slight dip in a characteristic
wavelength of starlight passing through a cloud. This absorption line
is a telltale sign that photons are exciting vibrations in H3+
molecules.

“It was quite surprising to see H3+ at all in a diffuse cloud – there
was 100 times more H3+ there than we would expect,” McCall said. “It
made no sense at all.”

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.

The United Kingdom Infrared Telescope (UKIRT) is the world’s largest
telescope dedicated solely to infrared astronomy. The 3.8-metre
telescope is sited near the summit of Mauna Kea, Hawaii, at an
altitude of 4194 meters above sea level. It is operated by the Joint
Astronomy Centre in Hilo, Hawaii, on behalf of the UK Particle
Physics and Astronomy Research Council.

NOTE: Ben McCall can be reached at (510) 642-1047 or
bjmccall@astron.berkeley.edu.

For questions about UKIRT, contact Dr. Douglas Pierce-Price, Science
Outreach Specialist at the Joint Astronomy Centre in Hawaii:
outreach@jach.hawaii.edu; (808) 969 6524. Further information is on
their Web site, http://outreach.jach.hawaii.edu/