Amidst the huge task of collating all of the data coming in from the May 20, 2017 dip, two papers have hit the arXiv. I don’t have any updates on the data from the dip (we haven’t had time to do any detailed analyses yet), but the live chat I did on Friday is still mostly valid:
except to say that the dip has maybe ended:
— Jason Wright (@Astro_Wright) May 22, 2017
Today there are two new papers on the arXiv on the subject. I haven’t had time to do deep dives on them (and neither is refereed yet) but here are my hot takes:
The first is by Ballesteros et al. (MNRAS, submitted) and they try to model the dips with a gigantic planet with a huge ring system and huge swarms of trojan asteroids. In other words, their model puts a lot of stuff in a 6 au orbit around the star, which is far enough away that it would be pretty cold. They point out that the deep, asymmetric dip at Kepler day 793 occurs about half way in the middle of a pretty quiescent period for Tabby’s Star. They associate the other dips with swarms of trojan asteroids—asteroids in the same orbit as the planet but leading or trailing the planet by 60 degrees.
Some strengths of the model:
- They claim that they can model the deep D793 event as a giant (0.3 solar radii!) planet with a tilted ring system and that they will do this in a later paper
- They get the overall pattern of the dips explained: Kepler just caught the back of the pack of leading trojan asteroids when it started observing, then the planet at day 793, then the trailing swarm at the end of its mission
- In what must have been a hastily written addition, they attribute the May 20 event to a secondary eclipse of the planet behind the star. This comes with a prediction: the event will be no longer than the D793 event (which was actually very long), but they say no more than 2-4 days. They say that the secondary eclipse depth could be as deep as 3% (about what we see). I note it should also be pretty achromatic, unless the reflectivity of the planet is a strong function of wavelength.
- They emphasize that their model appeals only to likely, conventional astrophysics (though when it comes to 0.3 solar radius planets and a Jupiter-mass of asteroids in a swarm, your mileage may vary on that one).
- They have a really nice diagram!
- They need a lot of asteroids: they don’t actually say how much, but the number they do give is huge: over a Jupiter mass of them! It’s not clear to me how stable such a swarm could be co-orbital to an actual planet. Part of the reason Jupiter’s trojan asteroids work as they do is that they don’t really perturb Jupiter. Also, how do you keep a Jupiter mass of material from collapsing or falling into the planet? Also, where would you get a Jupiter mass of rock?!
- They cannot explain the secular dimming seen by Montet & Simon and Schaefer, which they say must have a different cause.
- They do not confront the infrared and mm upper limits, especially those of Thompson et al. (whom they do not even cite) that put no more than a millionth of an Earth mass of dust hotter than 160K. I would think that an asteroid swarm dense enough to have an optical depth near 1 along some lines of sight (22% dips!) would also generate some serious dust, as would those rings.
- They will need a pretty strange sort of planet to have a detectable secondary eclipse out at 6AU. They claim that a Bond albedo of 0.34 will do it, but my back-of-the-envelope calculation says no way this could work (a perfectly reflective 15 solar radius circle (for the ginormous rings of this planet) at 6 au intercepts about 1 ten thousandth of the stellar flux, not 3% of it). If it’s really emitted light then it should be pretty red, so the May 20, 2017 dip should be hard to see in the blue.
- I think the slopes of the dips are too steep; material at 6 au moves pretty slowly. They could easily calculate this.
But kudos to them for putting an idea out there with concrete predictions!
The second paper is by J. Katz. I’m glad to see this one in principle—Steinn and I suggested an object in the outer solar system could be responsible and hoped someone would work that out, and here’s a paper working it out! Weirdly, Katz cites us but don’t mention our suggestion. Anyway glad to see it.
This is a strange paper, though. There is no comment that it has been submitted to any journal to be refereed—it’s possible this is all we get. It’s called “Tabetha’s Rings”—I don’t think I’ve ever seen just a modern astronomer’s first name in a paper title before. Katz refers to the star as “Tabetha’s Star” which is also strange (because the star needed another name, right?).
Katz suggests that a ringed object in the outer solar system could be responsible for the dips…and not much else. Some of the implications are worked out, but some of the math seems wrong to me (he predicts that the dips will be visible every 365.25 days from earth, which ignores the orbital motion of that object). I kept expecting Katz to bring up the rings of asteroids but it never came up.
Anyway, I hope Katz develops this model further and describes things like the spectral and photometric properties of the dips his model implies, and discusses, for instance, the mass of the object hosting the rings (at least!). I’d really like to see a fleshed out version of this paper in the refereed literature.
OK, the kids are off to school so time to get back to the disaster area that is my inbox…