June 3, 2002
Binary Stars "Flare" With Predictable Cycles, Analysis of Radio Observations Reveals
Astronomers have completed a 5-year campaign to monitor continuously radio flares from two groups of binary star systems. This survey is of special interest because it provides evidence that certain binary star systems have predictable activity cycles like our Sun.
The survey, which ran from January 1995 to October 2000, was conducted with the National Science Foundation's (NSF) Green Bank Interferometer. The report was presented at the American Astronomical Society (AAS) meeting in Albuquerque, New Mexico, by Mercedes Richards of the University of Virginia, and her collaborators Elizabeth Waltman of the Naval Research Laboratory, and Frank Ghigo of the National Radio Astronomy Observatory (NRAO).
"This long-term survey was critical to our understanding of the short- and long-term magnetic cycles of these intriguing star systems," said Richards.
The survey focused on the binary star systems Beta Persei and V711 Tauri -- both are about 95 light-years from Earth. Beta Persei is the prototype of the "Algol" class of interacting binary stars. An Algol system contains a hot, blue, main sequence star, along with a cool, orange/red star that is more active than our Sun. V711 Tauri is an "RS Canum Venaticorum" binary, which contains two cool stars that behave like our Sun.
"Our survey was the longest-running continuous radio flare survey of Algol or RS Canum Venaticorum binary star systems," said Richards. A flare is an enormous explosion on the surface of a star, which is accompanied by a release of magnetic energy. Flares can be detected over the full range of wavelengths from gamma rays to the radio.
It is estimated that the energy release in a flare on the Sun is equivalent to a billion megatons of TNT. The strength of the magnetic field and the amount of activity it displays, like sunspots and flares, are directly related to the rotation or "spin" of the star. In Beta Persei and V711 Tauri, the cool star spins once every 3 days, compared to once every month in the case of the Sun. So the stars in these binary systems have magnetic fields that are ten times more powerful than our Sun, and they produce flares that are powerful enough to be detected with radio telescopes on Earth.
Richards and her collaborators used two different techniques to determine how often radio flares occur in these systems -- the "periodicity" of flaring activity. They found that flares occur every 50 to 60 days in both systems, but the strongest periodicity was 49 ± 2 days for Beta Persei and 121 ± 3 days for V711 Tauri. "This means that we can expect to see a strong flare on Beta Per every 1 to 2 months, while strong flares on V711 Tau occur about every 4 months," said Richards.
The researchers also identified some long-term flaring cycles that are 1 to 4 years long. These long-term cycles may be related to magnetic cycles like the 11-year sunspot cycle on the Sun. Richards said, "It would be exciting if these long-term cycles are linked to magnetic cycles, but our survey was not long enough to confirm this result without the shadow of a doubt."
The continuous monitoring program also demonstrated that Beta Persei and V711 Tauri have active and inactive cycles. "This fact would not have been established if the systems had only been monitored sporadically," said Richards. "We could never be absolutely sure that no flares occurred at certain times unless we were monitoring the system all the time."
Many flares occurred during the active cycles, and almost no flares, or very weak flares, were ejected during the inactive cycles. Flares usually began with a massive burst of energy and then decayed slowly as the gas cools. The radio flares on the Sun typically last up to 2 days, but those in the two binary systems lasted for 10 to 40 days.
"Continuous monitoring of radio flares requires the availability of a dedicated telescope like the Green Bank Interferometer," said Richards. The interferometer is composed of two 85-foot radio dishes separated by 2,400 meters. During the survey, this telescope was operated by the National Radio Astronomy Observatory, with funding from the United States Naval Observatory, Naval Research Laboratory, NASA High Energy Astrophysics Program, and NRAO. The monitoring program ended when the interferometer was closed in October 2000.
Richards received funding for this research from the Air Force Office of Scientific Research, the National Science Foundation, and NASA.