University of Arizona astronomer Huan Meng and co-authors have found the long-term dimming of KIC 8462852 — a main-sequence F-type star located in the constellation Cygnus, about 1,480 light-years from Earth — appears to be weaker at longer infrared (IR) wavelengths of light and stronger at shorter ultraviolet (UV) wavelengths. Such reddening is characteristic of dust particles and inconsistent with more fanciful ‘alien megastructure’ concepts, which would evenly dim all wavelengths of light. The study is published in the Astrophysical Journal (arXiv.org preprint).
This illustration depicts a hypothetical uneven ring of dust orbiting KIC 8462852. Image credit: NASA / JPL-Caltech.
KIC 8462852, also known as Boyajian’s star, Tabby’s star, TYC 3162-665-1 or 2MASS J20061546+4427248, captured astronomers’ imagination in September 2015 with its strangely fluctuating brightness.
Ever since then, the scientific community has been observing this enigmatic character and sifting methodically through the data in search of an answer.
Speculation to account for KIC 8462852’s dips in brightness has ranged from it having swallowed a nearby exoplanet to a swarm of exocomets orbiting the star to a Dyson sphere, a hypothetical energy-gathering structure built by an advanced civilization.
To test multiple hypotheses for the behavior of the star, Dr. Meng and colleagues obtained measurements of the star over a wide wavelength range from the UV to the mid-IR from October 2015 through December 2016, using NASA’s Spitzer and Swift missions, as well as the Belgian AstroLAB IRIS observatory.
They found less dimming in the IR light from the star than in its UV light. Any object larger than dust particles would dim all wavelengths of light equally when passing in front of KIC 8462852.
“This pretty much rules out the alien megastructure theory, as that could not explain the wavelength-dependent dimming,” Dr. Meng said.
“We suspect, instead, there is a cloud of dust orbiting the star with a roughly 700-day orbital period.”
The objects causing the long-period dimming of KIC 8462852 can be no more than a few micrometers in diameter (about one ten-thousandth of an inch), according to the team.
“Based on the strong UV dip, we determined the blocking particles must be bigger than interstellar dust, small grains that could be located anywhere between Earth and the star,” the astronomers said.
“Such small particles could not remain in orbit around the star because pressure from its starlight would drive them farther into space.”
“Dust that orbits a star, called circumstellar dust, is not so small it would fly away, but also not big enough to uniformly block light in all wavelengths.”
“This is currently considered the best explanation, although others are possible.”
While the scientists have a good idea why KIC 8462852 dims on a long-term basis, they did not address the shorter-term dimming events that happened in three-day spurts in 2017 and two brightening episodes in 2007 and 2014.
“KIC 8462852 could have something like a solar activity cycle,” said co-author Dr. Siegfried Vanaverbeke, an AstroLAB volunteer and a physicist at the University of Leuven in Belgium.
“This is something that needs further investigation and will continue to interest scientists for many years to come.”
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Huan Y. A. Meng et al. 2017. Extinction and the Dimming of KIC 8462852. ApJ 847, 131; doi: 10.3847/1538-4357/aa899c
