It’s time for another edition of “Why is Tabby Dimming Now?” Tabby is the mysteriously dimming star known to astronomers and sticklers for detail as KIC 8462852 and nicknamed Tabby for its discoverer, Tabetha Boyajian. Previous episodes have covered popular theories that the star is fading because it’s being covered by an alien-built Dyson sphere to harvest its energy, because it has an fast spin and irregular shape or because it’s being blocked by swarms of comets. None of these theories explain the occasional bright flickers that suddenly interrupt the long-term dimming.
That brings us to the latest episode: “Secular Dimming of KIC 8462852 Following its Consumption of a Planet.” This is also the title of a paper submitted to the astrophysical journal Monthly Notices of the Royal Astronomical Society by Brian Metzger, Ken Shen and Nicholas Stone of Columbia's Departments of Physics and Astronomy. Their plot centers on the idea that the dimming began when a large planet with multiple moons collided with Tabby.
The planet may have spent millions of years slowly decreasing its pericenter [closest approach to Tabby's star on its orbit] before it finally impacted the star, causing a brightening and then the slow dimming we have observed.
In an interview in Gizmodo, Nicholas Stone explained the dimming. What about the occasional but temporary brightness?
Towards the end of this slow process... any moons around the planet would have been detached by the tidal field of Tabby's star.
So the moons moved to the star’s orbit after their planet was consumed. Unfortunately, they’re destined to the same fate as their planet. Each time Tabby eats a large moon, another belch of brightness occurs, followed by another period of digestive dimming. That explains the dims and the bright flare-ups. What about the irregular periods of greater dimness? Stone blames these on those lingering moons as well.
Moreover, these moons now have pericenters around the star that are closer than the orbit of Mercury around our own Sun, so if they have icy outer layers, they should be evaporating and outgassing rapidly, blowing clouds of vapour and dust off their surfaces. The dust in these expanding evaporation clouds may be able to explain the irregular transits of Tabby's Star seen by Kepler.
The vapors and clouds of dust block our view of Tabby, causing minor dips and flickers in its light. Put all of these pieces together and it sounds like the Great Tabby Mystery Series has reached the end of its run, right?
Not so fast, Tabby fans. Brian Metzger says the theory needs to be tested the next time Tabby dims. If they’re correct, they should next see clouds of gas and dust followed by a temporary increase in luminosity followed by a return to dimness.
And if it doesn’t happen, new episodes of “Tabby and the Dyson Sphere” are ready to go.