Using observations from two NASA missions, Kepler and Swift, astronomers have discovered a number of stars rapidly spinning and producing X-rays at levels that were more than 100 times the peak amounts ever recorded from the sun.
And because these stars are spinning so fast, they've been squashed, ending up looking like pumpkins. Astronomers believe the stars resulted from close binary systems where two stars, both sun-like, have merged.
On average, the recently discovered stars take just a few days to rotate, while it takes almost a month for the sun to do so.
Senior research scientist Steve Howell of the NASA Ames Research Center explained that this rapid rotation essentially puts the stars in overdrive, amplifying usual activity seen on the sun, like solar flares and sunspots.
Of the 18 "pumpkin stars" discovered so far, one stands out: a K-type orange giant called KSw 71. Aside from being more than 10 times bigger than the sun, the star also completes a rotation in a mere 5.5 days and releases X-ray emissions 4,000 times greater compared to what the sun is capable of at its solar maximum.
The Kepler And Swift Missions
"A side benefit of the Kepler mission is that its initial field of view is now one of the best-studied parts of the sky," said NASA Goddard Space Flight Center researcher Padi Boyd, who also designed the Swift survey.
Using the optical/ultraviolet and X-ray telescopes on the Swift spacecraft, astronomers carried out the Kepler-Swift Active Galaxies and Stars Survey (KSwAGS). They imaged about six square degrees of the Kepler field, or an area equivalent to the size of 12 times a full moon's apparent size.
The survey yielded 93 new sources of X-ray, many of which have never been observed before. For the brightest sources, the research team obtained spectra with a 200-inch telescope from California's Palomar University, which offered detailed chemical portraits for the stars.
Rare Star Formation
According to Howell and colleagues, the 18 stars discovered were likely formed via a model, put forth by astronomer Ronald Webbink, which stated that close binary systems merge while initially located in an "excretion" disk resulting from gas thrown out as the two stars merged. The disk eventually disappears over the course of 100 million years and leaves behind a rapidly spinning active star.
Following Webbink's model, the researchers estimate that there should be about 160 of the rapidly spinning stars in the Kepler field. They only discovered 18 given they were only able to observe a small portion of the field for the Swift mission.
To find more of the stars, the team has already moved to extend the Swift survey to accommodate additional fields being mapped by the K2 mission, an ongoing extension to the Kepler mission.
