Shock zone found where planets are born
Posted: January 9, 2002

In research that may help reveal how planets form, astronomers have gathered evidence that a shock is created when material falls in toward a dust disk around a growing star.

"This is a very important step in our understanding of how stars and planets develop," said Dr. Thangasamy Velusamy, principal research scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "It gives us some insight into the process by which planetary systems could begin around stars." Velusamy and his colleagues presented their findings Tuesday at the American Astronomical Society winter meeting in Washington, D.C.

The interstellar dark cloud L1157 is located in the Cepheus constellation. The multi-colored area shows a dust disk surrounding a newborn star. The red-orange area at the center represents the brightest region, which contains the young star. It is surrounded by the cooler, dusty disk, which appears as yellow, green and blue. The diameter of the disk is about 20 times larger than our entire solar system. The white lines trace the radio wave emission of methanol. Note that the methanol emission comes only from the outer parts of the disk. That is the zone where a warm shock occurs when the cloud material moves in toward the star and meets up with the outer surface of the disk. Photo: NASA/JPL
Dusty disks around stars, called protostellar disks, form from gas and dust in interstellar clouds. The disks grow as they accumulate material falling in from the parent cloud. Where the cloud material meets the disk, astronomers had predicted that a warm shock would occur. This new discovery confirms that prediction.

Scientists believe dust particles in these disks clump together, eventually leading to small rocks, which can join together to form planets and comets. Scientists believe that a similar process formed Earth and the other planets of our solar system from a dusty disk around our parent star, the Sun.

At the shock zone, the newly introduced material slows down and redistributes itself throughout the disk. Some will make its way toward the center and become part of the developing star, while some will eventually become part of the planets.

Protostellar disks, like the one observed by Velusamy and his colleagues in cloud Lynds 1157 (located about 1,300 light years from Earth in the constellation Cepheus), contain mostly molecular hydrogen. Other ingredients include tiny dust particles and trace amounts of such chemicals as carbon monoxide and methanol, or rubbing alcohol. Because the falling material and the disk are so cold, these chemicals freeze on the dust particles and can not be seen. However, when a shock is produced at the disk's surface, the resulting heat warms the dust particles and releases their icy mantles. This transforms the ice into a gas that emits radiation in radio wavelengths, which can be detected.

Using the Owens Valley Radio Observatory Millimeter Array near Bishop, Calif., the team of scientists, including Dr. William Langer, senior research scientist at JPL, and Dr. Paul Goldsmith, a professor of astronomy at Cornell University, Ithaca, N.Y., confirmed the presence of a dusty disk in L1157. They also found the first-ever evidence for the shock region. It was detected when telltale disk emissions indicated the presence of methanol only at the predicted shock zone.

"The methanol emission looks different than the distribution of material already in the disk," noted Goldsmith.

"Methanol is intriguing, since it is abundant in both interstellar space and in comets, and is a chemical starting point for more complex molecules, including those that eventually form life," said Langer.

Protostellar disks are planetary construction zones. Over the past several years, astronomers have discovered planets orbiting dozens of other stars. The key mystery waiting to be solved is whether any extrasolar planet may have conditions suitable for life, and if so, whether life exists there.

The latest findings will appear in the January 20, 2002 issue of the Astrophysical Journal Letters. The scientists plan further research on the chemical makeup of planet-forming disks. Future NASA missions will also study these dust disks. The Space Infrared Telescope Facility may study ice crystals in the disks as part of its investigations. Other planned missions, including the Terrestrial Planet Finder, will be able to image planetary systems forming in disks around other stars. Both missions are managed by JPL.

The Owens Valley Radio Observatory Millimeter Array, operated by the California Institute of Technology in Pasadena, is supported in part by the National Science Foundation. Caltech manages JPL for NASA.