Astronomers using observations from NASA’s Kepler and Swift space observatories have spotted 18 fast-spinning stars that produce X-rays at more than 100 times the peak levels ever seen from the Sun.
This artist’s concept illustrates how KSw 71 compares with the Sun. Both stars are shown to scale. Image credit: NASA’s Goddard Space Flight Center / Francis Reddy.
“These stars rotate in just a few days on average, while the Sun takes nearly a month. 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,” explained team leader Dr. Steve Howell, from NASA’s Ames Research Center.
The most extreme member of the group is called KSw 71. It is a K-type orange giant star, more than 10 times larger than our Sun.
It rotates in just 5.5 days, about four times faster than the Sun. Rapid spin causes the star to flatten into a pumpkin shape, which results in brighter poles and a darker equator.
Rapid rotation also drives increased levels of stellar activity such as starspots, flares and prominences, producing X-ray emission over 4,000 times more intense than the peak emission from the Sun.
KSw 71 is thought to have recently formed following the merger of two Sun-like stars in a close binary system.
The designations and locations of 18 KSwAGS stars are shown on part of the first-light image of the original Kepler field. KSw 71 is located in the second field from the left. Image credit: Francis Reddy / J. Jenkins / NASA’s Goddard Space Flight Center / NASA’s Ames Research Center.
This group of stars was found as part of the Kepler-Swift Active Galaxies and Stars Survey (KSwAGS), an X-ray survey of the so-called Kepler field — a patch of the sky comprising parts of the constellations Cygnus and Lyra.
“The NASA Kepler mission was launched in 2009 and completed four years of photometric observation of over 150,000, mainly late-type stars in a single field of view,” Dr. Howell and his colleagues said.
“Our team conducted the KSwAGS survey using the Swift X-ray Telescope.”
“This survey of the Kepler field covered about 6 square degrees imaging a strip of sky roughly perpendicular to the galactic plane in order to sample a range of galactic latitude.”
“We were looking for variable X-ray sources with optical counterparts seen by Kepler, especially active galaxies, where a central black hole drives the emissions,” said team member Dr. Padi Boyd, from NASA’s Goddard Space Flight Center.
“With KSwAGS we found 93 new X-ray sources, about evenly split between active galaxies and various types of X-ray stars,” added team member Krista Lynne Smith, from the University of Maryland, College Park.
“Many of these sources have never been observed before in X-rays or ultraviolet light.”
The team also used Kepler measurements to determine the rotation periods and sizes for 10 of the stars, which range from 2.9 to 10.5 times larger than the Sun.
Their surface temperatures range from somewhat hotter to slightly cooler than the Sun, mostly spanning spectral types F through K.
Astronomers classify the stars as subgiants and giants, which are more advanced evolutionary phases than the Sun’s caused by greater depletion of their primary fuel source, hydrogen.
All of them eventually will become much larger red giant stars.
In the 1970s, Dr. Ronald Webbink at the University of Illinois, Urbana-Champaign noted that close binary systems cannot survive once the fuel supply of one star dwindles and it starts to enlarge.
The stars coalesce to form a single fast-spinning star initially residing in a so-called ‘excretion’ disk formed by gas thrown out during the merger.
The disk dissipates over the next 100 million years, leaving behind a very active, rapidly rotating star.
The astronomers suggest that their 18 KSwAGS stars formed by this scenario and have only recently dissipated their disks.
The findings were published online this week in the Astrophysical Journal (arXiv.org preprint).
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Steve B. Howell et al. 2016. Rapidly Rotating, X-ray Bright Stars in the Kepler Field. ApJ 831, 27; doi: 10.3847/0004-637X/831/1/27
This article is based on a press-release issued by NASA.
