A one-of-a-kind star found to change over decades

AR Scorpii consists of a rapidly spinning, magnetized
white dwarf star that mysteriously interacts with its companion
star. Credit: M. Garlick/University of Warwick, ESA/Hubble

Astronomers studying the unique binary star system AR Scorpii
have discovered the brightness of the system has changed over
the past decade. The new evidence lends support to an
existing theory of how the unusual star emits energy. AR
Scorpii consists of a rapidly spinning, magnetized white
dwarf star that mysteriously interacts with its companion
star. The system was recently found to more than double in
brightness on timescales of minutes and hours, but research
recently published in The Astrophysical Journal
Letters
found variability on a timescale of decades.

Researchers at the University of Notre Dame analyzed data on
the unique system from the Kepler Space Telescope’s K2 mission
taken in 2014 before the star was known to be unusual. The data
was then compared with archival sky survey images going back to
2004 to look for long-term changes in the light curve of AR
Scorpii. The binary’s light curve is unique, in that it
exhibits a spike in emission every two minutes as well as a
major brightness variation over the approximately 3.5-hour
orbital period of the two .

“One model of this system predicts long-term variations in the
way the two stars interact. It was not known what the time
scale of these changes might be—whether 20 to 200 years. By
looking at the K2 and archival data, we were able to show that
in addition to hourly changes in the system, there are
variations occurring over decades,” said Peter Garnavich,
professor and department chair of astrophysics and cosmology
physics at Notre Dame.

A white dwarf is a very dense remnant of a star like the sun.
When a solar-like star runs out of energy, gravity compresses
its core to about the size of the Earth but with a mass 300,000
times higher. A teaspoon-sized piece of a white dwarf would
weigh about 15 tons. The compression of the star can also
amplify its and its spin rate.

The unique system became famous in 2016 when researchers in
England discovered that AR Scorpii, believed to be a mundane
solitary star, was actually a rapidly varying binary. The
system is remarkable as the white dwarf spins on its axis at an
incredibly fast rate, causing flashes in luminosity every two
minutes. The amplitude of the flashes varies over the 3.5-hour
orbital period, something no other white dwarf binary system is
known to do.

“We found that back 12 years ago, AR Scorpii’s peak brightness
came a bit later in its orbit than it does now,” said Colin
Littlefield, research associate working with Garnavich. “This
does not solve the mystery, but it is another piece to the
puzzle that is AR Scorpii.”

The team at Notre Dame has been observing the system with the
Sarah L. Krizmanich Telescope at the University’s Jordan Hall
of Science, and they plan to publish those results in an
upcoming paper.

Explore further:

Astrophysicists discover dimming of binary star

More information: Colin Littlefield et al. Long-term
Photometric Variations in the Candidate White-dwarf Pulsar AR
Scorpii from K2, CRTS, and ASAS-SN Observations, The
Astrophysical Journal
(2017). DOI: 10.3847/2041-8213/aa8300

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