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Celestial foundlings

Bethel Park native discovers unusual features of young star cluster

Monday, June 07, 1999

By Byron Spice, Science Editor, Post-Gazette

You're walking down Forbes Avenue in Oakland and suddenly come across a bunch of week-old babies, maybe 50 of them, lying together. Now, you wouldn't be surprised to see that many babies in a hospital nursery or dispersed among 50 households. But out on the street? Weird.

So imagine the surprise astronomers felt six months ago when they discovered a cluster of 8-million-year-old stars -- the stellar equivalent of week-old babies -- sitting out by themselves just over 300 light-years from Earth. Visible in the sky above the South Pole and bunched around a bright star called eta Chamaeleon, they are far from any cloud of gas that might have given birth to them.

"Science is weird," said Eric Feigelson, an astronomer at Penn State University, shrugging off the seeming absurdity. He and Warrick Lawson of the University of New South Wales in Australia had recognized these stars as youngsters and had been studying them off and on for several years, but had not suspected just how weird they might be until Eric Mamajek picked up on the work last fall.

Mamajek, a former student of Feigelson's who is spending a year in Australia with Lawson as a Fulbright scholar, showed that these stars weren't just young, but that they were close by and moving through the heavens as a group. It is the closest such cluster to Earth to be discovered this century, said Feigelson, who announced the find last week at an American Astronomical Society meeting in Chicago.

Its proximity makes the young stars relatively easy to study. Astronomers hope studying them will help fill some gaps in what is known about the evolution of stars.

X-ray astronomy has shown that young stars are seeded throughout the galaxy, but no one has been sure how they disperse from the clouds of gas that serve as stellar nurseries. The eta Chamaeleon cluster appears to have been caught in this middle phase, just before the cluster breaks apart and the stars scatter.

"This [cluster] won't be something that will be around in a million years," added Mamajek, a 1993 graduate of Bethel Park High School who is pursuing his doctorate in astronomy at the University of Arizona.

The young stars are what are known as pre-main-sequence stars. During most of their lifetimes, stars are in their main sequence phase, when they produce power primarily through nuclear fusion of hydrogen. Pre-main-sequence stars have yet to reach the temperatures necessary to burn hydrogen, but gravitational contraction is causing them to heat up, so they glow even more brightly than they will during their main sequences.

Pre-main-sequence stars traditionally have been found in or near molecular clouds, where stars form. But in 1984 Feigelson discovered that these young stars may be much more scattered through the universe.

He found the young stars emit distinctive pulses of X-rays. It seems that in their youthful convulsions, these stars generate magnetic pulses that are thousands of times more powerful than those of mature stars such as our Sun. When X-ray astronomers have looked for this distinctive signature, they have found the signals emanating from all over the sky, not just the obvious spots in molecular clouds.

It was this X-ray signature that drew the attention of Feigelson and Lawson to the eta Chamaeleon group. The three brightest stars in the group are well known, but when the orbiting European/U.S. X-ray telescope called ROSAT observed the group in 1997, it found at least 12 stars in the vicinity that were emitting those variable X-rays.

Follow-up observations last year with optical telescopes at Australia's Mount Stromlo and Siding Springs Observatories confirmed that they were pre-main-sequence stars.

But things didn't get interesting until Mamajek arrived in Canberra last year. He had intended to pursue a different project during his Fulbright year in Australia, but those plans fell through when he was unable to get the telescope observing time he needed. So he turned his attention to eta Chamaeleon, a project that had been a low priority for both Lawson and Feigelson.

Mamajek started analyzing the data about the stars' light spectra that had been gathered by the optical telescopes. It appeared there were at least 13 of the pre-main-sequence stars there, and perhaps another 20 or 30 candidates. But he also turned to the European satellite called Hipparcos, an orbiting observatory that specializes in measuring astronomical distances.

"Once he got the Hipparcos data, it was like ... BOING!" Feigelson said. The distance information helped Mamajek use the stars' luminosity as another indicator of their ages. But it also showed that they were nearby, tightly spaced -- the whole cluster is only one or two light-years across -- and moving together in the same direction.

By tracing their motion backward, Mamajek found that the stars might have originated in the Scorpius-Centaurus OB star association, a swarm of a couple thousand stars of similar age.

He suspects that a portion of the Sco-Cen cloud might have somehow been ejected and that the eta Chamaeleon stars formed from that cloud.

So what happened to the cloud? Using the babies-on-the-street analogy, Feigelson said it's as if a group of babies were lying in a hospital nursery when a big wind suddenly blew the hospital apart, but left the babies in place.

The wind that blew away the eta Chamaeleon cloud likely came from an exploding star, a supernova. Mamajek said a supernova within the Sco-Cen association appears to have been situated in the right place to have blown away that cloud.

But the group of babies won't hang together for long. The eta Chamaeleon is a so-called "open" cluster. The velocities of the stars and the size of the cluster indicate that the stars eventually will scatter. Small open clusters such as this are probably a very common phase in stellar evolution, Mamajek said, with only the largest clusters, containing thousands of stars, enjoying anything close to permanence.

Luck helped Mamajek and his colleagues find the eta Chamaeleon cluster during this brief interlude between star birth and the scattering of the cluster. Now that the discovery has been announced, the hard work of astronomy begins. Mamajek, who will be returning to the States in August, already has secured time on a South Pole telescope to survey the cluster in search of brown dwarfs -- star-like objects not quite massive enough to ignite hydrogen fusion.

Feigelson said they also will be looking for signs of planets and seeing if any of the stars still have accretion disks, the clouds of dust and gas from which planets form. In addition, they will be looking for binary stars and measuring speed at which the stars spin.



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