A lot of folks want to know my opinion about the two new Tabby’s Star papers out this week:
Mohammed A. Sheikh, Richard L. Weaver, and Karin A. Dahmen
Phys. Rev. Lett. 117, 261101
With commentary by Steinn Sigurdsson here:
On this one, my opinion closely matches Steinn’s (because I asked him to explain it to me!). From what I understand of the paper, certain statistics of the dips follow a power law, and so-called “avalanche” models of certain phenomena associated with phase transitions follow a similar power law. The authors suggest that this means that the processes causing the dips are internal to the star, and represent some sort of transition it is undergoing, like a global magnetic field flip.
That’s interesting, but I don’t know what it really means. It may provide a way for physical models to try to reproduce the data, by asking if the dips they predict follow the same power law.
Neslušan & Budaj A&A accepted:
In this paper, the authors model four of the deeper dip complexes with a relatively simple but physically motivated model of massive objects (small planets) with very large, extended dust shrouds moving on highly eccentric orbits.
The physical motivation for this model is the same as for Boyajian et al.’s invocation of “comets”: eccentric orbits mean that the material will only spend a brief time near the star where it occults it, and so only be warm briefly. This concentration in time explains the lack of IR excess at other epochs, and the lack of repetition of the dips. Like Boyajian et al., Neslušan & Budaj posit that the bodies are the result of a single break-up event, and at least approximately share an orbit.
What Neslušan & Budaj add to this is a rough physical model for the clouds of material. They assume it appears around 4AU (around the time comets get their comae) and that the dust particles are orbiting the planet (not how I imagined they would behave—I would have guessed like a collisional gas). They have a variety of models for the initial conditions of these orbits, including some that generate spherical clouds and rings. They then modeled the gravitational interactions of the star and massless dust particles, and included PR drag (radiation pressure from the star and the dust particle re-emission). They used MERCURY to do the integrations.
They tried lots of variations of these parameters, and found a few that gave good fits. Here are some of their efforts:
Neslušan & Budaj
The green lines (very hard to see: don’t use bright green on white, and use heavier weight for your lines, people!) are good qualitative fits to the data. The right hand side shows the evolution of the dust cloud (lots of colors) and the orbit of the planet (red line) about the star (not shown, but at (0,0) and the focus of the red line).
In all cases what seems to be happening from the right hand side is that a dust cloud is released from the body, and then radiation pressure blows it away from the progenitor body near periapse. They get similar results and some better fits with other models, including the ring model.
The authors have not really addressed the long term dimming seen (they mention it but have only a hand-wavey explanation that it’s accumulated dust in the orbits), nor the lack if IR excess (qualitatively, these things are only IR-bright when they are close, but the long-term dimming demands significant dust all the time, so, one would think, IR emission all the time). As Steinn and I wrote, the “comets” explanation is “plausible for the dips, [but] very unlikely for the secular dimming.” I think that still stands.
Keep in mind that the reason Neslušan & Budaj get good results for the “comets” hypothesis while Bodman & Qullien had more equivocal results is that they have more free parameters to play with from a more sophisticated model. I don’t think they can do it with much less mass, but they can do it with fewer objects because their objects are bigger. They still need one object per dip complex.
I’m also surprised that they want to model dust like massless particles in orbit around the planet—I would have guessed that dust cloud would be better modeled with a gas dynamics code than an n-body code. I’d like to hear from someone who studies cometary tails or atmospheric escape about how physically plausible these initial conditions and equations of motion are.
But this is a great next step. This is how Tabby’s Star will be solved: a vague and qualitative hypothesis will get turned into a simple, quantitative model like this one, and that model’s success will inspire further work on more complex quantitative models. Eventually, these models will explain all of the data well and make some sort of prediction that will be confirmed by observations. Then we’ll say we have a good model for the system, and, if that model includes interesting features with wider applicability, we will use it to understand the Universe better.
Finally, Neslušan & Budaj conclude with “with such physical models at hand, at present, there is no need to invoke alien mega-structures into the explanation of these light-curves.” My thoughts on the propriety of the ETI hypothesis are well documented at this point, but let me say that I don’t think this paper takes the comets hypothesis across any critical threshold that we can say that we now have a good physical model for the system. They’ve shown that one can get the sorts of complex structures we see in four the dips from a very simple model that is still missing a lot of physics — but a spline is also a very simple model that will fit the data well, which shows that simple models that fit are not by themselves enough to close the book here. We still have a lot of work to do!
Some notes on the term “comets” here:
- Boyajian et al. invoked giant comets, meaning bodies on highly eccentric orbits with extended clouds of gas and dust, but much bigger than Solar System comets.
- Now, Neslušan and Budaj have taken exactly that idea and built a physical model for it. It seems to be able to generate the dips.
- Makarov & Goldin invoked interstellar “comets,” but that term usually refers to comets that have been ejected from the Solar System: such objects would not have comae, and so would not be big enough to cause Tabby’s Star to dim. Like Boyajian et al. and now Neslušan & Budaj, they mean things with big clouds of material around them, although it’s unclear why they would emit big clouds in interstellar space.
- In all cases, we’re not really talking about comets like in the Solar System. Both Boyajian et al. and Neslušan & Budaj are invoking a single break-up event that generates things that have orbits like comets and, like comets, generate big comae and tails when they get close to the star. But these are not the primordial “dirty snowballs” of our comets.