Major flares are predictable on far-away stars

University Park, Pa. -- For the first time, astronomers are able to predict when major flares -- enormous explosions that shoot hot gases into space -- will erupt on stars outside our solar system, according to research to be published in an upcoming issue of the Astrophysical Journal.

The research is based on data from the longest-running continuous radio survey of flares produced by two types of binary systems, each containing a pair of stars under the influence of each other's gravity. Stars in both binary systems, located about 95 light years from our solar system, are like a younger version of our Sun. "Studying the flares on these stars can help us understand more about how life evolved on Earth because they indicate the kind of environment that was bombarding our planet during an earlier age," says Mercedes Richards, professor of astronomy and astrophysics at Penn State and leader of the survey team.

During their 5-year-long observations, the researchers used the Green Bank Interferometer in West Virginia to continuously monitor radio waves produced by flares on pairs of stars as they circle each other like partners in a dance, regularly eclipsing each other when viewed from Earth. They studied two systems of such stars, one known as "The Demon Star," or "Beta Persei," which is the brightest and closest eclipsing binary pair in the sky. It contains a hot, blue star along with a cool, orange-colored star that is like our Sun but a bit more active. The other system, known as "V711 Tauri" to indicate its location in the constellation Taurus, also contains relatively cool stars like our Sun, one orange-colored and the other slightly hotter and yellow-colored.

Cool, Sun-like stars have an outer convective zone that produces a magnetic field. The pattern of a star's flares reveal how its magnetic field is changing. "We were trying to discover the magnetic cycle within these stars by detecting a pattern in their strongest flares," Richards explains. The strength of flares in a binary pair is related to the age and speed of rotation of the cooler star. "Because we discovered that these flares occur at regular intervals, we now can predict accurately when future flares will occur," she says.

Since the strength of the Sun's magnetic activity is relatively weak, astronomers have needed to accumulate close to 100 years of observations in order to get enough data to determine the Sun's cycle of flare strength. The binary stars the team studied are younger than our Sun and are spinning about 10 times faster, so their flares are about 10 times more powerful and the astronomers were able to discover their interval pattern much more quickly.

The team's observations of these two objects lasted from January 1995 until October 2000, when the Green Bank Interferometer was shut down. "Our continuous monitoring demonstrated that Beta Per and V711 Tau have active cycles and inactive cycles," Richards says. "This fact would not have been established if the systems had only been monitored sporadically. We could never be absolutely sure that no flares occurred at certain times unless we were monitoring the system all the time."

Richards and her collaborators used two independent statistical techniques to find out how often radio flares occur in these systems. They found that flares occur every 50 to 120 days in both systems. The survey also suggested a longer cycle of flares that lasted more than 500 days, or 1.4 years, with a pattern of active flaring and then very little flaring activity, but this long-term cycle could not be confirmed by the statistical analysis because the survey was not long enough to yield results that reach the usual criterion for statistical significance.

When Richards divided the long-term flare cycle by the rotation period of the cool star, she realized that the flaring cycles in the two binary systems may be related to magnetic cycles like the 11-year sunspot cycle on the Sun. "Now that we have begun to understand more about the flaring cycles on other stars, we may be able to better understand flaring in general, including the 11-year cycle of flares from our Sun, which regularly disrupts communications satellites on Earth," Richards says.

In addition to Richards, the research team includes Elizabeth Waltman of the Naval Research Laboratory, Frank Ghigo of the National Radio Astronomy Observatory, and Donald Richards of Penn State.

Continuous monitoring of radio flares requires the availability of a dedicated telescope like the Green Bank Interferometer -- a facility of the National Science Foundation that was operated during the collection of these data by the National Radio Astronomy Observatory with funding from the United States Naval Observatory, the Naval Research Laboratory, the National Radio Astronomy Observatory, and NASA's High Energy Astrophysics Program. The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. Richards received funding for this research from the Air Force Office of Scientific Research, the National Science Foundation, and NASA.