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

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.