Unusual Cluster of Young Stars Discovered over the South Pole

The discovery of an unusual cluster of very young stars near Earth in the sky over the South Pole is being announced today by Graduate Student Eric E. Mamajek and Senior Lecturer Warrick A. Lawson, of the University of New South Wales, and Professor Eric D. Feigelson, of Penn State. Feigelson presented the discovery at the 100th-anniversary meeting of the American Astronomical Society in Chicago, Illinois, and discussed the find during a press conference there on June 1, 1999. Because of its location close to Earth and its highly unusual characteristics, the cluster is expected to become an important laboratory for understanding a number of mysteries concerning star formation and the early stages of stellar evolution.

"We have used the enhanced X-ray emission of young stars to discover an unusual compact cluster, about 8 million years old. Among over a thousand known open clusters of stars, it is the first to be discovered with X-rays and the nearest to Earth discovered this century. It also is odd because it appears isolated in space, far from any stellar nursery in which they could have been born," Feigelson says. Young stars usually are found in or near the dark clouds of gas and dust from which they are born.

"The brightest members of this new cluster may be the only infant stars that can be seen from Earth with just binoculars or a small telescope," Mamajek says. "Through binoculars, you can see the brightest three members in a tight group." These stars are visible in the sky over Australia, South America, and southern Africa about 11 degrees from the south celestial pole in a rather featureless region of sky. Its coordinates are 08h 42m -79.0d (epoch 2000).

The star cluster, centered around the bright star, eta Chamaeleon, is located in the Chameleon constellation about 97 parsecs, or 316 light years, from Earth. "Few very young stars are as close to Earth as these," Lawson says. "Most very young stars are harder to study because they are significantly farther away.

"It is surprising that the unusual nature of this cluster of stars was not discovered many years ago because its location is relatively close to Earth," Mamajek says. "The stars are not obscured by dust, and the cluster's three brightest stars have been on star charts for more than a century."

The discovery announced today reveals that these previously known stars are members of a previously unrecognized cluster of at least a dozen very young "pre-main-sequence" stars. Most stars quickly pass through their pre-main-sequence phase and spend most of their lives, typically billions of years, burning hydrogen during their "main sequence" phase. Pre-main-sequence stars have not yet started to burn hydrogen but they nevertheless glow brightly, primarily as a result of the intense heat generated by the gravitational compression during their formation.

"It is very hard to find pre-main-sequence stars far from their clouds without an X-ray telescope," says Feigelson, who discovered in 1981 that pre-main-sequence stars have a dramatically distinctive X-ray signature that can be used to locate them. "The intensity of X-ray signals emitted by pre-main-sequence stars is elevated far above that of normal main-sequence stars because they have more powerful magnetic flares," Feigelson explains. "These flares are similar to those on the Sun today, but are thousands of times more powerful."

The astronomers made their X-ray observations with the German/U.S./U.K. satellite called ROSAT, which was launched in 1990 and turned off earlier this year. They made spectroscopic measurements to confirm the stars' young age with the 2.3-meter optical telescope at Mount Stromlo and Siding Springs Observatory in New South Wales, Australia. They also used data from the European Space Agency satellite Hipparcos for gauging the distance to the stars in the cluster in order to determine their luminosity as another indicator of their age.

The astronomers speculate that the eta Chamaeleon stars were born in the Scorpius-Centaurus OB star association (Sco-Cen), a huge swarm of stars about the same age but dozens of light years away from the eta Chamaeleon cluster. "We suspect that the eta Chameleon cluster was once located within the Sco-Cen Association but its surrounding gas cloud was stripped away by powerful explosions of massive Sco-Cen stars," Mamajek says. The team suggests the eta Chamaeleon group might evaporate fairly rapidly because it is an "open cluster" whose stars are not strongly bound by the forces of gravity. "Open clusters like this may form more often than we think, but their transient nature makes them difficult to find before they dissipate," Lawson says.

Lawson says he also hopes to learn something from the eta Chameleon cluster about brown dwarfs-the smallest, lowest-mass, and faintest stars in the universe. "Only a few dozen brown dwarfs have been discovered since the first one was identified in 1997 and we often don't know their age and their distance from Earth--but we do know the age and the distance of this cluster," he says. "If it turns out to have some brown dwarfs in it, we could use them as a particularly effective and powerful laboratory for understanding the birth of these mysterious objects." Brown dwarfs are of interest to astronomers because their mass is between stars, which have enough mass to burn hydrogen in their cores, and giant planets like Jupiter, which do not.

This research was supported by the J. William Fulbright program, the Australian-American Education Foundation, and the U.S. National Aeronautics and Space Administration.

CONTACTS:

Eric D. Feigelson at the AAS meeting from May 30 to June 3 via the press room: PHONE (+1) 312-294-6607, -6608, and -6609; FAX (+1) 312-294-6610 Eric Feigelson after June 5 at Penn State: PHONE (+1) 814-865-0162, E-MAIL efeigelson@astro.psu.edu
Eric E. Mamajek, PHONE (+61) 2-6268-9215, E-MAIL eem@phadfa.ph.adfa.oz.au
Warrick A. Lawson, PHONE (+61) 2-6268-8810, E-MAIL wal@ph.adfa.edu.au
Barbara K. Kennedy (PIO): PHONE (+1) 814-863-4682, E-MAIL science@psu.ed

ILLUSTRATION:

EDITORS: Images of the cluster in optical and X-ray wavelengths are available on the World Wide Web at <http://www.ph.adfa.edu.au/e-mamajek/etacha> beginning on May 31.

PHOTO CREDIT:

Eric E. Mamajek, Warrick A. Lawson, and Eric D. Feigelson

CAPTION FOR X-RAY IMAGE:

FOR RELEASE: 2:00 p.m. CDT, May 31, 1999 A soft X-ray image of the eta Chamaeleon region obtained with the ROSAT space observatory, showing 12 X-ray sources (white) embedded in noise (red). The angular extent of the image is about the size of the full moon.

CAPTION FOR OPTICAL IMAGE:

FOR RELEASE: 2:00 p.m. CDT, May 31, 1999 The same region of the sky seen in visible light, with the stars associated with the 12 X-ray sources indicated. This image can be obtained from NASA's SkyView Web site at <http://skyview.gsfc.nasa.gov>.

PENNSYLVANIA EDITORS:

Mamajek, a graduate student at the University of Arizona and a Fulbright Fellow now studying with Lawson in Australia, did undergraduate research with Feigelson when he was a student at Penn State. Mamajek is from Bethel Park, Pennsylvania. He attended the Penn State Beaver and University Park campuses as a University Scholar.