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News Services
University of Arizona
Tucson, Arizona

Contact information: 
Elizabeth M. Green, 520-621-1144, bgreen{at}as.arizona.edu

Tuesday, 11 February 2003

University of Arizona Undergraduates Discover New Class of Star
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By Lori Stiles

University of Arizona astronomy undergraduates have serendipitously 
discovered a new class of star that thrills astronomers who specialize 
in a relatively new field called "asteroseismology."

Astronomers worldwide will collaborate in continuous observations of 
one of these newly found stars for several weeks in May 2003.

"Astronomers are always looking for new and better ways to study 
stars," said Elizabeth Green, assistant staff astronomer at Steward 
Observatory, who with her students discovered the new class of stars. 
They have found sub-dwarf B stars that pulsate like Jell-O, quivering 
in space through cycles that typically last an hour.

"We have incredibly sophisticated theoretical models that describe the 
interior evolution of stars from birth to death. But our observations 
are usually limited to only what we can see of the outer layers of a 
star's atmosphere.  It is very difficult to check the theoretical 
calculations with actual evidence of what is happening inside the
star," Green said.

Astronomers have begun to study fluctuating light from naturally 
pulsating stars to understand interior star structure, in much the 
same way that seismologists use earthquake-generated density waves to 
study the interior structure of the Earth.

This new community of "asteroseismologists" was delighted in 1997 with 
the discovery that a few sub-dwarf B stars were pulsating in several 
different modes during short periods, periods of 100 to 200 seconds.

Sub-dwarf B stars are far along in their stellar evolution.  These 
rare, very hot stars burn helium, rather than hydrogen, in their 
cores. They have somehow lost almost all of their obscuring red giant 
atmospheres, leaving their tiny helium-burning cores exposed for 
astronomical study. Pulsating sub-dwarf B stars promised to give
astronomers needed new evidence on interior star structure.

But during the past 5 years, astronomers have searched something like 
600 such stars and found only 30 "multimode" pulsators. More, the 
stars are typically faint, and extremely small changes in their 
brightness during 2-to-4 minute periods make useful observations 
difficult. 

The discovery of this new class of pulsating sub-dwarf B star is 
exciting because the stars' hour-long periods should make good 
observations much easier, and because these stars are more common than 
the short-period pulsators, Green said.

It was one of those discoveries you make but aren't looking for, she 
added.

When some of Green's undergraduate astronomy students three years ago 
asked her for hands-on experience in observational astronomy for 
independent study credit, she trained them to help on her National 
Science Foundation- funded survey of sub-dwarf B stars in binary 
systems, a project to better understand how stars evolve.

Students worked in pairs during weekends, changing off working on 
homework and observing, mostly at the 61-inch Kuiper Telescope on 
Mount Bigelow, and occasionally at the 90-inch Bok Telescope on Kitt 
Peak, which by now are two of Steward Observatory's more modestly 
sized telescopes.

They observed strange, irregular light curves like one that another UA 
undergraduate working with Green had seen in July 1999.

"The original discovery curve was done by Melissa Giovanni, an 
undergraduate working for me for the summer. She wanted to do some 
observing at a real telescope, and we had 5 nights of telescope time 
at the 90-inch in July," Green said. "But this was during the 
monsoons. It was raining cats and dogs every afternoon, and cloudy 
most of the nights. I decided to give up, but Melissa wanted to keep
going, hoping the skies might clear. In the last few hours of the last 
night, she got a light curve that was the funniest looking thing I'd 
ever seen," Green said.

Green said she knew the irregular light curve wasn't from a star 
eclipsing another, or reflection effects that she studies in her 
survey. "I honestly didn't know what it was. I carried this bizarre 
light curve around to meetings for the next year and half, and showed 
it to people who asked if it might be simply a result of observing 
through the Earth's own turbulent atmosphere."

Beginning spring semester 2000, Keith Callerame, Ivo R.  Seitenzahl 
(who have since graduated) Brooke White, Elaina Hyde and other UA 
undergraduates on Green's survey collected similar light curves on 
what are now known to be long-period multimode pulsating sub-dwarf B 
stars. The UA astronomy undergraduates did about two-thirds of the
observing work on the project. Astronomers from the University of 
Montreal and Missouri State University, from Germany, and from the La 
Palma Observatory in the Canary Islands collaborated with Green and 
her students in a research paper on the discovery, published Jan. 20
in the Astrophysical Journal Letters. 

Green, the UA undergraduates and their colleagues report seven 
confirmed such stars pulsating in 3 to 5 modes, and possibly in as 
many as 10 or more modes. And they have by now found 23 such stars in 
the group of 100 they have examined, including 18 found just last 
year.

Green and Gilles Fontaine of the University of Montreal are organizing 
a campaign from March to June 2003 to observe the brightest, coolest 
and most dramatically pulsating of these newly found stars, 
PG1627+107.

The star is easily visible from both the Northern and Southern 
Hemispheres. Astronomers from Germany, South Africa, Australia, South 
America and Spain will collaborate with Green and her team at Steward 
Observatory to get around-the-world and around-the-clock coverage of
the star for two weeks during the spring campaign.

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