November 14, 2001

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A team of astronomers using the National Science Foundation's Very Large Array (VLA) radio telescope has caught an old star during the very brief period of its transformation into a planetary nebula, a shining bubble of glowing gas with a hot remnant star at its center.

"This is the first time that anyone has seen a star that is so clearly going through this transformation stage," said Yolanda Gomez, an astronomer at the Institute for Astronomy at the National Autonomous University in Mexico City, Mexico. "We believe this star began to enter its planetary-nebula phase only after 1984," she added. The researchers reported their findings in the November 15 edition of the scientific journal Nature.

At the end of their lives, stars like our Sun eject gas into space before starting to contract under their own gravity into white dwarf stars. The gravitational contraction heats up the star, making it pour out energetic ultraviolet light. The ultraviolet light tears apart molecules in the gas ejected earlier by the star and rips electrons from the atoms in the gas. This makes the gas glow, producing often-beautiful shining shells and other shapes.

Once the remnant star has heated up sufficiently to produce large amounts of ultraviolet light, molecules in the gas ejected earlier are destroyed rapidly. "We are seeing radio waves emitted by water molecules in this planetary nebula," said Gomez, who added, "The water molecules, we believe, are all destroyed within only 100 years of the beginning of this stage, so we are seeing this star during an extremely brief transition period of its life." This is the first time that water has been detected in a planetary nebula.

The astronomers used the VLA to observe a planetary nebula called K3-35, 16,000 light-years from Earth in the constellation Vulpecula (the small fox). This object has a doughnut-shaped ring of gas around its center and lobes of outflowing material, similar to structures seen in other planetary nebulae.

The researchers were surprised to find regions near the star in which water molecules are amplifying, or strengthening radio-wave emission at a frequency of 22 GigaHertz, in the same manner that a laser amplifies light waves. They found these regions, called masers, in the doughnut-shaped structure surrounding the central star, as well as at the end of much larger lobes of gas extending from the star. The doughnut-shaped ring has a radius of more than twice the distance from the Sun to Pluto. The masers at the ends of the lobes are more than 100 times more distant from the star.

By analyzing their VLA observations as well as earlier observations of the object by other astronomers, the research team concludes that K3-35 has only just begun its transformation into a planetary nebula.

"This is extremely exciting, because we now have a 'laboratory' for watching this process take place over the next few years," Luis F. Miranda of the Astrophysical Institute of Andalucia (IAA) in Granada, Spain said. "We don't fully understand everything we see in this object, but know that we are going to learn much valuable information about this process by watching it develop," he added.

"We are very lucky to have caught this star during such a very brief but important period of its life," agreed Guillem Anglada of the IAA and the Harvard- Smithsonian Center for Astrophysics in Cambridge, MA and Jose Torrelles of the Institute for Space Studies of Catalunya in Barcelona, Spain, who are, with Miranda and Gomez, members of the research team.

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The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.