University of AZ Undergrads Discover New Class Of Star

“It was one of those discoveries you make but aren’t looking for” says Elizabeth Green, assistant staff astronomer at Steward Observatory. Green’s research area is so-called sub-dwarf B stars that 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 which leaves their tiny helium-burning cores exposed for astronomical study. In 1997 a few sub-dwarf B stars were discovered pulsating in several different modes during short periods, periods of 100 to 200 seconds. Pulsating sub-dwarf B stars originally 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 that are typically faint, and extremely small changes in their brightness during 2-to-4 minute periods make useful observations difficult.

Enter the undergrads. “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. 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. In a Jan. 20 research paper on their discovery published in the Astrophysical Journal Letters, light curves for newly discovered long-period pulsating sub-dwarf B stars are announced. The time between peaks varies from about 35 minutes to nearly 120 minutes. 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. Organization is underway to coordinate a worldwide observing program from March to June 2003 to observe the brightest, coolest and most dramatically pulsating of these newly found stars, PG1627+107. Around-the-world and around-the-clock coverage of this star for two weeks during the spring campaign is expected to reveal…well, stay tuned.