Undergrads Discover New Class Of Star; "They Pulsate Like Jell-O"

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 Jello, 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 "astroseismologists" 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.