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

September 12, 2017 by Jessica Sieff
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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1.6 / 5 (7) Sep 12, 2017
Four other examples of violations of the HR diagram the article opts to omit:

(1) FG Sagittae:

"Around 1900 FG Sge was an inconspicuous hot star (T = 50,000 K) of magnitude 13. During the next 60 years it cooled to about 8000 K and brightened in the visual region to magnitude 9, as its radiation shifted from the far-UV to the visual region. Around 1970 a whole new bunch of spectral lines appeared due to elements such as Sr, Y, Zr, Ba and rare earths. .... The star cooled further in the 1970s and 80s and then all of a sudden in 1992 its magnitude dropped to 14. Further drops occurred from 1992 to 1996 with a very deep minimum near magnitude 16 in June of 1996."

(2)(3) V605 Aquilae and 4334 Sagittarii (Sakurai's object):

"Virginia Trimble, professor of physics at the University of California, Irvine, and visiting professor of astronomy at the University of Maryland, has said recently ..."

1.6 / 5 (7) Sep 12, 2017

"We don't often see stars change their spectral types in a human lifetime. Thus, FG Sagittae, which brightened, cooled from about BO to K, and added lines of carbon, barium, and other elements to its spectrum in the century after 1890 was long seemingly unique. The standard interpretation has been that it experienced its very last flash of helium shell burning (the products are carbon and oxygen) and was about to become an R Coronea Borealis variable. These are carbon-rich stars that fade suddenly and unpredictably (which FG Sge started doing a couple of years ago) and that have hydrogen-depleted atmospheres (which FG Sge has just developed). In addition, the "galloping giant" is no longer alone ..."

1.6 / 5 (7) Sep 12, 2017

"... Examination of old images and spectrograms reveal that V 605 Aquilae, studied by Knut Lundmark in the 1920's was a similar sort of beast, though it is now very faint And the latest recruit is V 4334 Sagittarii, better known as Sakurai's object, for its 1994 discoverer. It, too, changed both spectral type and surface composition very rapidly, and is now hydrogen-poor and carbon-rich, and well on its way to becoming the century's third new R CrB star."

(4) V838 Monocerotis


"V838 Mon was discovered to be in outburst in January of this year. Initially thought to be a familiar type of classical nova, astronomers quickly realized that instead, V838 Mon may be a totally new addition to the astronomical zoo ..."

1.6 / 5 (7) Sep 12, 2017

"... Observations indicate that the erupting star transformed itself over a period of months from a small under-luminous star a little hotter than the Sun, to a highly-luminous, cool supergiant star undergoing rapid and complex brightness changes. The transformation defies the conventional understanding of stellar life cycles. A most notable feature of V838 Mon is the 'expanding' nebula which now appears to surround it."

Don Scott appears to be the only person tracking the issue. These quotes come from his website at http://electric-c...agr.htm.
4.1 / 5 (13) Sep 12, 2017
violations of the HR diagram

The HR diagram is a plot of derived quantities, it's not violated by variables. Just because something is plotted at one moment does not imply it is fixed for all eternity, the HR diagram has tracks for this reason. That's like saying a map is violated by continental drift. It's a plot, it's not violated by anything. Not even Scott is making such nonsensical statement.

Don Scott appears to be the only person tracking the issue.

There are hundreds of papers written about those stars, please keep your delusions to yourself.
1.7 / 5 (6) Sep 12, 2017
(semantic nitpicking ignored)

Re: "There are hundreds of papers written about those stars, please keep your delusions to yourself."

But, notice the problem: Since the papers include SPECULATIONS about what COULD HAVE caused these sudden changes, the changes are not tracked as anomalous, and the science journalists fail to recognize that there exists a growing list of stars which change over human-historical timeframes.
not rated yet Sep 12, 2017
Must wonder at the spin-up mechanism...
1 / 5 (5) Sep 12, 2017
Somehow things like white dwarfs have become fact and not hypothesis. Of course nobody will ever get to go look at such objects so the concept is not falsifiable. Even Proxima Centauri can not be proven to be a star, so why it is accepted that all those points of light in the sky are stars is puzzling to say the least.
Da Schneib
5 / 5 (2) Sep 12, 2017
Very interesting. We'll find out more about gravity when they work this one out.
5 / 5 (3) Sep 13, 2017
Must wonder at the spin-up mechanism...

Direct result of a conservation law (conservation of angular momentum). As size decreases the spin must speed up for angular momentum to stay the same.

It's the effect you see in ice skaters when they start a pirouette and pull in their arms.

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