An international team of astronomers has discovered a new class of rapidly rotating stars. Their red-orange color and squished shaped has earned them the name "pumpkin stars", and although in line with the season, these stars are actually the result of stellar mergers.
These stellar mergers were first proposed 40 years ago by Ronald Webbink, but researchers have now combined the power of the Kepler telescope and Swift observatory to find stars emitting a lot of X-rays, a predicted sign for these objects.
In a paper published in the Astrophysical Journal, the researchers discuss the properties of 18 newly discovered stars that emit at least 100 times more X-rays than the Sun during its solar maximum.
Artist's impression of KSw 71 compared to the Sun. NASA's Goddard Space Flight Center/Francis Reddy
"These 18 stars rotate in just a few days on average, while the sun takes nearly a month," said lead author Steve Howell, from NASA's Ames Research Center in Moffett Field, California, in a statement. "The rapid rotation amplifies the same kind of activity we see on the sun, such as sunspots and solar flares, and essentially sends it into overdrive."
The most extreme case in the sample is KSw 71, a star 10 times larger and producing 4,000 times more X-rays than the Sun. The object spins on its axis in just 5.5 days.
The stars were discovered using Swift, the gamma-ray and X-ray observatory of NASA, on the Kepler field – one of the best-studied areas of the Sky. The Kepler telescope has measured the luminosity of over 150,000 stars, and the region (which is 12 times the area of the full moon) has also been studied in infrared and UV.
"Webbink's model suggests we should find about 160 of these stars in the entire Kepler field," said co-author Elena Mason, a researcher at the Italian National Institute for Astrophysics Astronomical Observatory of Trieste. "What we have found is in line with theoretical expectations when we account for the small portion of the field we observed with Swift."
Webbink proposed that binary systems where stars orbit very close to each other could not survive one of the stars becoming a red giant. He proposed that one of the stars would envelope the other, coalescing into a rapidly spinning star.