Cosmic chemistry inside interstellar clouds points to galactic wind of low-energy rays
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.