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MIT astronomer advances study of planet formation

'Rag-tag group' of young stars a rich source of data
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Sarah H. Wright
Phone: 617-258-5400
MIT News Office

CAMBRIDGE, Mass. -- An MIT scientist 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, Science magazine reported on July 4.

The scientist, Joel H. Kastner, PhD, a research scientist at the MIT Center for Space Research, is the first author of the article, titled "X-ray and Molecular Emission from the Nearest Region of Recent Star Formation."

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 testbed for study of X-ray emission from young stars and the formation of planetary systems around Sun-like stars," the article in Science stated.

"We aren't the first to observe this rag-tag group of young stars, or even to note that they're unusual," said 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, over 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 in Science said.

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," Kastner and collaborators wrote in Science.

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.

Kastner and colleagues also used a radio telescope atop Mauna Kea in Hawaii to detect faint, characteristic emission 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," noted Kastner. "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," he said.

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, 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 MIT's Center for Space Research.

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

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.

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