#avalanche statistics

One of the most head-scratching stars in the night sky (at the moment) may be starting to finally give up some of its apparent mystery. The unceremonious-sounding KIC 8462852 is better known these days as “Tabby's Star” or “Boyajian's Star” after Tabetha S. Boyajian, professor of astronomy at Louisiana State University, who lead the team that first described the star in a 2015 paper. The paper is provocatively subtitled “Where’s The Flux?”, leading to the star’s other occasional nickname: the “WTF Star”. 

KIC 8462852 is an otherwise unremarkable 12th-magnitude star in the constellation Cygnus. What makes it remarkable is the recent history of its light output, which has baffled nearly every stellar astronomer who has examined its light curve. A light curve is a graphical representation of the light output of some astronomical thing over time. Most stars’ light curves are flat and boring: they emit essentially the same amount of light at all times. Other stars are known as “variables”. Their light curves show all manner of variation, from the highly regular to the highly irregular. In all those cases, however, we understand at least something about the underlying physics that causes these stars to vary.

Tabby’s Star, however, is in a league of its own. The first indications that something is up with this star came from the Kepler spacecraft mission in 2011. Kepler’s M.O. is pretty simple: in its primary mission, lasting from 2009-2013, it stared at a region of sky containing Tabby’s Star continuously, recording the brightness of every object in the field. Unlike previous instruments used in this manner, it achieved an unprecedented level of measurement precision, and was used to discover thousands of potential exoplanets. It also provided new information on the inner workings of stars themselves.

And it showed us that something about one particular star in its field of view is very, very weird.

Kepler light curves of Tabby’s Star (above) give some idea about its odd behavior. Long periods of stability (i.e., when the light curve is very flat) give way to inconsistent drops in the light received from the star. The star was seen to dim for many days at a time, losing up to 20% of its brightness, before stabilizing again at apparently full power. It didn’t at all resemble the light curve of a star hosting one or more exoplanets. Nor did it look like any other stellar light curve seen before.

Boyajian’s team wrestled with the cause of the star’s behavior. It considered explanations like a very young star prone to erratic light output as it settled onto the Main Sequence (nope: too old and mainstream for this kind of display). Or maybe it had a dusty disk around it that periodically blocked some of the light (nope: such a dust disk would re-radiate starlight in the infrared, leading to an obvious signal). After discarding some ideas like this for lack of evidence, the team proposed a radical idea: a chunky dust cloud made up of many dissolved comets (artist impression, below). 

The comets would probably tend to cluster, it was argued, if they originated as some larger object around the star that broke apart under the influence of the star’s gravity.Lots of other explanations have been proposed, and mostly dismissed, for want of supporting evidence, including:     

• Burps from a star that had actually eaten one of its planets;     

• A big planet with a set of rings tilted with respect to its orbit, periodically blocking some starlight from view;

• Chaotic stellar physics manifesting through something called “avalanche statistics”; and

• Some previously unobserved physical effect at or near the star’s surface.

The most exotic theory of all, and the one that got the most press, was that Tabby’s Star was being dimmed by a “Dyson swarm” (below): a ring or other structure around the star, composed of artificially designed and built objects, indicative of intelligent life.

Needless to say, it wasn’t the simplest explanation for the observations, and probably isn’t the right one.

New results seem to discount most of those theories and return to one of the original thoughts: the thing dimming Tabby’s Star is nothing more than garden-variety dust. New observations from NASA's Spitzer and Swift spacecraft, as well as the Belgian AstroLAB IRIS observatory, show that whatever is doing the light absorbing is better at it in the ultraviolet part of the spectrum rather than the longer-wavelength infrared.

That’s an indication that the absorbing particles are fairly tiny compared to the wavelengths of light illuminating them. But how do we know that? It turns out to be the same reason that the sky is blue: light interacts more strongly with particles when its wavelength is about the same size as the particles themselves. It’s for this same reason that the daytime sky on Earth is blue. So whatever is causing the dimming of Tabby’s Star can’t be made of particles much larger than the wavelength of light. Such particle sizes are consistent with the known properties of cosmic dust.

Lead study author Huan Meng (University of Arizona) says: “This pretty much rules out the alien megastructure theory, as that could not explain the wavelength-dependent dimming.” Instead, Meng says, the most likely reason for the observations is boring, old dust. “We suspect, instead, there is a cloud of dust orbiting the star with a roughly 700-day orbital period.”

However interesting Meng’s findings, they don’t completely explain what’s up with the “WTF Star”. There is no known mechanism to produce a dust ring or disk around a star that matches the observed properties of the KIC 8462852 system over the length of time we have seen its light varying. Other processes could be combining with the dust dimming to yield the observed variations. For instance, says AstroLAB volunteer Siegfried Vanaverbeke, “Tabby's Star could have something like a solar activity cycle.”