A team of U.S. astronomers, including a member of the
Harvard-Smithsonian Center for Astrophysics, has discovered a large
concentration of water vapor within a cloud of interstellar gas close
to the Orion nebula.
The concentration is 20 times larger than that measured previously in other interstellar gas clouds and may provide an important clue to the origin of water in the solar system.
The discovery was reported in an article published Monday in the Astrophysical Journal Letters.
Using the European Space Agency's Infrared Space Observatory satellite, ISO, which was launched in November 1995, the astronomers observed water vapor within the Orion Molecular Cloud, a giant interstellar gas cloud composed primarily of hydrogen molecules. The new observations were carried out in October 1997 with the Long Wavelength Spectrometer, one of four instruments on board ISO.
"The Orion Molecular Cloud is a site of particularly active star formation within our galaxy," said Gary Melnick of the Harvard-Smithsonian Center for Astrophysics, a member of the team that reported the finding. "Several thousand stars have been born in this region in the last million years -- which is very recent from the cosmic perspective.
"For reasons that aren't entirely understood, when stars are born, their birth is accompanied by a strong outward wind of gas and dust. When this outflowing material eventually impacts the surrounding gas, the shock waves that are created compress and heat the gas. The water we observe is rapidly produced in this
warm dense gas."
The concentration of water vapor measured by the U.S. team was some 20 times larger than that measured previously in other interstellar gas clouds.
"An enhanced concentration of water is precisely what we expected in this gas cloud," said Melnick "We are looking at a region of interstellar space where shock waves have made the gas abnormally warm. For the past 25 years, astrophysicists have been predicting that whenever the temperature exceeds about 200 degrees Fahrenheit, chemical reactions will convert most of the oxygen atoms in the interstellar gas into water. And that's exactly what we've observed in Orion."
Melnick also noted that the strength of the water radiation detected from Orion was in perfect agreement with theoretical predictions.
The high concentration of water measured in Orion may have
implications for the origin of water in the solar system. "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 sixty times over," said Professor David Neufeld of Johns Hopkins University. "Eventually that water vapor will cool and freeze, turning into small solid particles of ice. Similar ice particles were presumably 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."
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