FOR RELEASE: 9:20 AM PST, January 8, 2003

YOUNG STARS FOUND NEAR EARTH; IDEAL FOR STUDYING PLANET

FORMATION

Astronomers are announcing today the discovery of young stars that are only a few hundred light years away. This work, presented at the American Astronomical Society Meeting in Seattle, Washington by Dr. Eric Jensen and undergraduate student Rabi Whitaker of Swarthmore College (Swarthmore, PA), shows that these stars are about 30 million years old; scientists believe the Sun was about this age when planets were forming in our own solar system. Studying such stars may allow scientists to learn how often and how quickly planets form around other stars.

"What makes these stars interesting is that their ages are just right for them to be forming planets right now, and their proximity to Earth makes them easier to observe" said Jensen, Assistant Professor of Astronomy at Swarthmore College. Thus, these stars are perfect candidates for follow-up observations to help astronomers understand planet formation. He cautioned that it is not yet known whether there are planets orbiting any of these stars right now; rather, the stars will provide a test of how well we understand planet formation. By observing these stars as part of a larger sample of stars of similar ages, astronomers hope to get an idea of how frequently stars form planetary systems, and exactly when in a star's life cycle planets are formed.

Like many astronomical discoveries, this work did not involve discovering new stars, but rather learning new things about stars whose existence has been known for a long time. "What's new here is our realization of how young these stars are," said Rabi Whitaker, a Swarthmore College senior who helped with this project as part of her senior thesis. "If you think of our Sun as middle-aged, these stars are like babies that are only a few weeks old." That recent birth may herald the birth of planets around the stars, too.

The evidence for the stars' youth comes from observations made with the National Science Foundation's Blanco 4-meter (159-inch) Telescope at Cerro Tololo Inter-American Observatory in Chile. Spectra of the stars show the presence of a large amount of the element lithium (see illustration). As a star ages, nuclear reactions gradually destroy the lithium atoms that were part of its initial chemical makeup. Thus, the more lithium present in a star, the less time the star has had to destroy it, and the younger the star.

While it was once believed that star formation occurred only in large clouds of gas and dust, observations over the past ten years have indicated that at least some stars are formed in relatively small groups, and that some of these groups are nearby, only 100 to 200 light years away. In the early 1990s, a Brazilian team led by Carlos Torres and Ramiro de la Reza uncovered a nearby group of stars in the constellation of Hydra. Since then, a number of small, nearby groups of stars have been found by the Brazilian team and by other researchers. The discovery announced today adds additional stars to the list of young, nearby stars. "We've been trying to go about this systematically, searching across the entire sky for stars like these," said Jensen, "and we're finding a number of previously-undiscovered groups of young stars." In addition to the stars announced today, Jensen said that more groups are likely to be found in the near future; the team has more observations of promising candidates scheduled for this coming April to check additional stars for signs of youth.

Even when considering only relatively nearby stars, there are millions of objects, so the researchers needed some way to narrow down the list of possibilities before going to the telescope to make new observations. Such needle-in-a-haystack work was made possible by recent space missions that have mapped the sky at various wavelengths. Young stars emit copious amounts of X-rays, so the team concentrated on bright X-ray sources found by the ROSAT satellite and found the stars announced today by observing stars with particularly strong X-ray emission. To select additional stars for follow-up observations, the scientists used the fact that groups of stars that form together tend to share a common motion through the Milky Way galaxy. Thus, the scientists found additional young stars around their initial discoveries by looking for stars that are moving across the sky in the same direction.

Astronomers hope that discoveries like those announced today will lead to a better understanding of planet formation through additional observations over the coming years. If planet formation in other solar systems proceeds at the same rate as it did in our own, then these stars are at just the right age to be forming planets. But that assumption---that our own solar system is typical---remains to be tested. These nearby young stars can provide such a test. Because all these stars have size and temperature characteristics similar to those of the Sun, they provide an excellent basis for comparison. If these stars are forming planets right now, they should still be surrounded by planetesimal or "debris" disks, collections of small chunks of rock that were built up from coagulation of dust grains during the process of planet formation. Such rocky disks are harder to detect than the dusty disks seen around much younger stars like those in the Orion Nebula; as the dust clumps together into larger and larger bodies it has less total surface area and thus emits and absorbs less light, just as a single piece of chalk is much harder to see from a distance than it would be if it were ground up and dispersed as a cloud of chalk dust. But soon, detecting such planet-forming disks around nearby stars may no longer be beyond our capabilities. The launch of NASA's Space Infrared Telescope Facility (SIRTF) this spring will allow detection of the very faint infrared light emitted by these disks. "We hope that within the next few years, SIRTF observations of these stars will give us a much better idea of how quickly and how often planet formation occurs," Jensen said.

This work was supported by generous funding from the National Science Foundation and Swarthmore College.

For more information:

Dr. Eric Jensen (610-328-8249, ejensen1@swarthmore.edu)

Rabi Whitaker (610-690-5237, rwhitak1@swarthmore.edu)

Illustrations:

Click on each image to access a larger version of that individual image.

Figure 1: Left: The region centered on one of the newly-discovered young stars, TYC 5853-1318-1, located in the constellation of Cetus. Because these stars probably formed from relatively small clouds of gas and dust, they are no longer surrounded by the gas clouds (such as those in the Orion Nebula, image at right) that scientists have long used as an indicator of active star formation. This lack of accompanying gas makes such stars much harder to find. Both images are one degree across, an area roughly four times the size of the full moon seen in the sky. (Credit: Digitized Sky Survey; left image, Anglo-Australian Observatory; right image, California Institute of Technology.)

Figure 2: The scientists used the presence of the element lithium in the stars' spectra to determine that they were young. These spectrum plots show how bright a star is at different colors, or wavelengths of visible light. The dips in the spectrum show wavelengths at which the star appears darker due to absorption of its light by atoms of a particular element. At left is the spectrum of one of the newly-discovered young stars, showing a strong dip at a wavelength (6707.8 Angstroms) where lithium atoms absorb light. The spectrum at right, of a much older star that has already destroyed all of its lithium, is identical except for the lack of a lithium feature. These spectra were obtained using the National Science Foundation's Blanco 4-meter (159-inch) Telescope at Cerro Tololo Inter-American Observatory in Chile. (Credit: Eric Jensen and Rabi Whitaker, Swarthmore College)

Alternate version of Figure 2, showing the two spectra superimposed; the young star is shown in orange:

(Credit: Eric Jensen and Rabi Whitaker, Swarthmore College)