Binary Star System Fading, Unclear Why King Of The Intermediate Polars Dimming

A team of University of Notre Dame astrophysicists say they cannot yet explain why an interacting binary star known as the king of the intermediate polars about 500 light-years from Earth appears to be fading.

The binary star FO Aquarii is in the Aquarius constellation and is made up of a white dwarf star and a companion low-density star — known as an intermediate polar. The companion feeds gas to the white dwarf.

The team led by physics Professor Peter Garnavich first observed the system when NASA’s Kepler Telescope was pointed toward it for three months. The star was rotating every 20 minutes and Garnavich wanted to know whether that rotation varied. He tasked a student with precisely measuring the spin rate.

“We can do that by looking at the interval between flashes from the star just like we use the ticks in a clock to tell time. The star turned out to have other plans for the summer,” Garnavich said in a statement Monday.

Once Kepler’s attention was turned elsewhere, the team switched observations to Notre Dame’s Sarah L. Kizmanich Telescope, which was installed in 2013.

“Just after the star came around the sun last year, we started looking at it through the Krizmanich Telescope, and we were shocked to see it was seven times fainter than it had ever been before,” said Colin Littlefield, a member of the Garnavich lab. “The dimming is a sign that the donating star stopped sending matter to the compact dwarf, and it’s unclear why. Although the star is becoming brighter again, the recovery to normal brightness has been slow, taking over six months to get back to where it was when Kepler observed.”

The recovery has been remarkably slow, Garnavich said. The team theorizes the companion rotated to a position that disrupted the flow of hydrogen to the white dwarf, causing the dimming, but that doesn’t explain the long recovery period. The team has noticed the star’s pulsing now varies from 11 to 21 minutes.

The findings were published in the Astrophysical Journal.

Intermediate polars are considered an important subset of what are called cataclysmic variables. Essentially, the companion star is gravitationally stripped by the white dwarf in the form of an accretion disk. NASA confirmed 47 of 49 ironclad or confirmed intermediate polars as of October 2014.