Tabby’s star is probably just dusty, and still not an alien megastructure

Alien megastructures are out. The unusual fading of an oddball star is more likely caused by either clouds of dust or an abnormal cycle of brightening and dimming, two new papers suggest.

Huan Meng of the University of Arizona in Tucson and his colleagues suggest that KIC 8462852, known as Tabby’s star, is dimming thanks to an orbiting cloud of fine dust particles. The team observed the star with the infrared Spitzer and ultraviolet Swift space telescopes from October 2015 to December 2016 — the first observations in multiple wavelengths of light. They found that the star is dimming faster in short blue wavelengths than longer infrared ones, suggesting smaller particles.
“That almost absolutely ruled out the alien megastructure scenario, unless it’s an alien microstructure,” Meng says.

Tabby’s star is most famous for suddenly dropping in brightness by up to 22 percent over the course of a few days (SN Online: 2/2/16). Later observations suggested the star is also fading by about 4 percent per year (SN: 9/17/16, p. 12), which Meng’s team confirmed in a paper posted online August 24 at arXiv.org.

Joshua Simon of the Observatories of the Carnegie Institution for Science in Pasadena, Calif., found a similar dimming in data on Tabby’s star from the All Sky Automated Survey going back to 2006. Simon and colleagues also found for the first time that the star grew brighter in 2014, and possibly in 2006, they reported in a paper August 25 at arXiv.org.

“That’s fascinating,” says astrophysicist Tabetha Boyajian of Louisiana State University in Baton Rouge. She first reported the star’s flickers in 2015 (the star is nicknamed for her) and is a coauthor on Meng’s paper. “We always speculated that it would brighten sometime. It can’t just get fainter all the time — otherwise it would disappear. This shows that it does brighten.”

The brightening could be due to a magnetic cycle like the sun’s, Simon suggests. But no known cycle makes a star brighten and dim by quite so much, so the star would still be odd.
Brian Metzger of Columbia University previously suggested that a ripped-up planet falling in pieces into the star could explain both the long-term and short-term dimming. He thinks that model still works, although it needs some tweaks.

“This adds some intrigue to what’s going on, but I don’t think it really changes the landscape,” says Metzger, who was not involved in the new studies. And newer observations could complicate things further: The star went through another bout of dimming between May and July. “I’m waiting to see the papers analyzing this recent event,” Metzger says.