Astrolunch: Nicholas Stone

Tue, 10/12/2019 - 12:30 to 13:30

How Exomoons can Solve the Ongoing Puzzle of Tabby's Star, Without The Fuss
Tabby's Star, also known as Boyajian's Star, KIC 8462852, or sometimes the "Where's The Flux" object, is one of the most enigmatic stars in the Milky Way. It is doubly unusual, in that it exhibits both (i) irregular transits of depths reaching 20%, and (ii) a long-term "secular dimming" that has reduced its average luminosity by a few percent over the lifetime of the Kepler mission, and perhaps ~10% over the 20th century. While many hypotheses have been proposed to explain these two observations individually, Occam's Razor suggests that the conjunction of two very rare phenomena requires a common cause. I will discuss my recent theoretical work to explain both of the unusual properties of KIC 8462852 through a chain of events initiated by a single cataclysmic trigger: the collision of the star with its former gas giant, and the tidal detachment of volatile-rich exomoons that once orbited the late planet. As these exomoons are transferred from planetocentric orbits (safely beyond the ice line) to heliocentric ones, with pericenters less than 0.1 AU, massive outgassing will ensue, producing gas- and dust-rich debris clouds that explain the observed transits. During the planetary impact event, substantial heat will be deposited into the outer layers of KIC 8462852, temporarily moving it off the main sequence. As the star contracts back on to the main sequence over a time period of decades to millenia, it will appear to dim, and this may be a major contributor to the observed secular dimming. However, I will show that recent observations argue for a more complicated modulation of the star's luminosity over decade-long timescales, in a way that is likely related to the long-term evolution of the exomoon debris clouds.