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MIT-led group studies recently formed stars

Dr. Joel H. Kastner, a research scientist at the Center for Space Research, has led a group of astronomers in research that offers a new, clearer understanding of how planets like those in our own solar system are born.

The team studied a group of five recently formed stars known as the TW Hya Association. The Association, comprised of the isolated, young, Sun-like star TW Hya and four other young stars in its vicinity, "serves as a test bed for study of X-ray emission from young stars and the formation of planetary systems around Sun-like stars," a July 4 article in Science states.

Dr. Kastner is the first author of the article, titled "X-ray and Molecular Emission from the Nearest Region of Recent Star Formation."

"We aren't the first to observe this rag-tag group of young stars, or even to note that they're unusual," said Dr. Kastner. "But hopefully we've made clear that they deserve a lot more attention from astronomers in the future."

The astronomers established that TW Hya itself is a "classical T Tauri star." It emits X-rays and is surrounded by a cloud of molecules that probably orbit the star in a disk similar to the disk thought to have spawned our own solar system more than four billion years ago.

Astronomers have long suspected that planet formation is a natural byproduct of the process of star formation, and that T Tauri stars were the most likely candidates to harbor "protoplanets."

"T Tauri stars are so young--a hundred thousand to a few million years--that the process of core nuclear fusion, which powers the sun and all other stars for most of their lifetimes, is only just beginning. There is evidence from infrared and radio astronomy that many T Tauri stars������������������ are potential sites of planet formation," the article says.

But it has been difficult to scrutinize such potential "protoplanetary systems," since the nearest well-studied T Tauri stars lie at least 400 light years from Earth. At this distance, even the Hubble Space Telescope is unable to easily resolve solar system-size structures.

Using X-ray data obtained with the Roentgen Satellite (ROSAT), the MIT-led team determined TW Hya and the four other stars in TW Hya's group to be about 20 million years old. At this age, "TW Hya may be undergoing, or perhaps has recently ended, a period of giant planet building," Dr. Kastner and collaborators wrote.

By combining this age estimate with the previously established optical brightnesses of the stars, the scientists established that the TW Hya Association lies between about 130 and 200 light years from Earth--about three times closer to Earth than any other well-studied region of recently formed stars.

The researchers also used a radiotelescope atop Mauna Kea in Hawaii to detect faint, characteristic emissions from a handful of familiar molecules, including carbon monoxide and hydrogen cyanide, in orbit around TW Hya. Such a molecular cloud is probably similar to the one out of which formed Jupiter, Saturn and the other gas giant planets in our solar system.

"Certainly TW Hya is the nearest known 'classical T Tauri star,' and it's probably one of the oldest known, too," Dr. Kastner said. "Given that we now know that it's surrounded by a molecular disk, we stand a good chance of moving the study of planet formation ahead by closer study of TW Hya."

Dr. Kastner has just completed observations of TW Hya with the Japanese X-ray satellite ASCA which, coincidentally, features MIT-built X-ray detectors. In the future, he and his collaborators plan to observe the TW Hya Association with the Hubble telescope, as well as with the Advanced X-ray Astrophysics Facility (AXAF), which is scheduled for launch by NASA next year. Some of the instrumentation aboard AXAF was also developed at the Center for Space Research.

Dr. Kastner's research at the AXAF Science Center at MIT is supported in part by the NASA Marshall Space Flight Center as well as by the NASA Origins of Solar Systems program.

Other authors of the Science article were Ben Zuckerman of UCLA, David Weintraub of Vanderbilt University, and Thierry Forveille of Grenoble Observatory in France.

A version of this article appeared in MIT Tech Talk on July 16, 1997.

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