A Cloud Of Water In Interstellar Space

April 09, 1998

ITHACA, N.Y. -- A team of U.S. astronomers, led by Cornell University astrophysicist Martin Harwit, has discovered a massive concentration of water vapor within a cloud of interstellar gas close to the Orion nebula. The amount of water measured is so high -- enough to fill the Earth's oceans 60 times a day -- that the researchers believe it provides an important clue to the origin of water in the solar system.

The amount of water vapor measured in Orion is 20 times larger than that observed in other interstellar gas clouds in our galaxy, the Milky Way. The discovery was made within the Orion molecular cloud, a giant interstellar gas cloud, a trillion miles across, composed primarily of hydrogen molecules.

The measurements were made with the long-wavelength spectrometer aboard the Infrared Space Observatory (ISO) launched in November 1995 by the European Space Agency with the participation of NASA. The observations were made in October 1997 and are reported today (April 20) in the Astrophysical Journal Letters.

Looking in the far-infrared region of the electromagnetic spectrum, the astronomers observed the characteristic signature of emission by water vapor. "The interstellar gas cloud that we observed is being pummeled by shock waves that compress and heat the gas," says Harwit, who is a Cornell professor emeritus, an ISO mission scientist and lead author on the research report. "These shock waves are the result of the violent early stages of star birth in which a young star spews out gas that slams into its surroundings at high speed. The heated water vapor that we observed is the result of that collision," he says.

Such a high concentration of water in Orion's giant gas cloud, which swirls around millions of stars along our spiral arm of the Milky Way, 1,500 light years from the sun, could have implications for the origin of water in the solar system, says ISO team member David Neufeld, professor of physics and astronomy at Johns Hopkins University. "The interstellar gas cloud that we observed in Orion seems to be a huge chemical factory generating enough water molecules in a single day to fill the Earth's oceans 60 times over."

Eventually, he says, the water vapor will freeze, becoming small ice particles. Similar ice particles are thought to have been present within the gas cloud from which the solar system originally formed. "It seems quite plausible that much of the water in the solar system was originally produced in a giant water vapor factory like the one we have observed in Orion," Neufeld says.

Cornell's Harwit speculates that the shock waves observed in the Orion gas cloud could be a cause as well as the result of star birth. The shock waves might also trigger the formation of additional stars and planets as they compress the gas cloud -- if the heat can be radiated away, says Harwit. "Water vapor is a particularly efficient radiator at far-infrared wavelengths and plays a critical role in cooling the gas and facilitating star formation," he notes.

The concentration of water vapor measured by the team was about one part in 2,000 by volume. The new observations confirm predictions by astrophysicists over the past 25 years that whenever the temperature exceeds 200 degrees Fahrenheit, chemical reactions will convert most of the oxygen atoms in interstellar gas into water.

"An enhanced concentrator of water is precisely what we expected in this gas cloud," says team member Gary Melnick of the Harvard-Smithsonian Center for Astrophysics. He adds that the strength of the water radiation detected from Orion was in perfect agreement with theoretical predictions published in the doctoral thesis of team member Michael Kaufman, a former Johns Hopkins graduate student now at NASA's Ames Research Center.

Panels showing two examples of measurements carried out on board the ISO, together with an image of the Orion nebula taken with the Wide Field Planetary Camera 2 on NASA's Hubble Space Telescope, can be seen on the World Wide Web at http://www.pha.jhu.edu/~neufeld/orionwater.html.

Cornell University

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