If you’re looking for a guide to this series, click here.

Last time we covered SINS and Bok globules as potential sources of the dimmings in Boyajian’s Star. This time we’ll finish up the interstellar solutions and start to look at circumstellar solutions.

Our topic? BLACK HOLES!

Hypothesis 5) An Interstellar Black Hole Disk

Now, a very popular suggested explanation from laypeople has been “a black hole”. For a long time, I treated this as a total non-starter: black holes are tiny, tiny, tiny, and don’t have the geometric cross-section to block any noticeable amount of light. Further, they tend to gravitationally lens light, so if there really were a black hole between us and Boyajian’s Star we’d expect a brightening, not dimming. Finally, if it’s close enough to suck up any material, we’d see Boyajian’s Star get brighter from the accretion, we’d see light from radio to X rays from the interactions, and we’d see RV variations to boot. So, no black hole, right?

So it’s ironic that we found another way to do it (though not what those who suggested it were thinking). This solution is more analogous to J1407, the “planetary system under construction” discovered and characterized by Eric Mamajek and Matt Kenworthy.

Here’s the idea: the dips really suggest small-scale structure in the intervening material, but the long-term dimming suggests a smooth distribution. One solution is a gigantic disk with annular features, like a protoplanetary disk with gaps and waves from planets and protoplanets. The timescales involved (115 years!) mean that it must be HUGE — like 600 AU across. What could host such a disk, but have totally escaped notice? After all, Boyajian et al. used adaptive optics to hunt for anything nearby that could do the trick, and only found an M dwarf 2 arcseconds away.

Well, it must be very massive, and it must be dark. Stellar remnants do the trick: a cold white dwarf, a quiescent neutron star, or a black hole. We focus on the black hole solution in our paper.

The idea is that after a supernova explosion, there will be material that falls back towards the remnant black hole, where conservation of angular momentum requires it to collapse to a disk, and some amount of the material to move outward while a lot of it spirals in and accretes onto the black hole. Eventually, the black hole finishes its meal, and goes quiescent, leaving behind a big inner gap within a large ring of debris outside. This ring can now get very cold (the central object is dark! nothing to heat it), thin, and wide. Perna et al. (2014) describe the process.

So, does it work? Could there really be a black hole aligned with Boyajian’s Star? Well, the alignment doesn’t have to be very good: the disk needs to span a big chunk of the sky for the dimming to persist over 115 years (a couple of arcseconds, at least). The black hole itself is small and would be unlikely to actually go between us and Boyajian’s Star (if it did it would lens it, but its Einstein radius is only ~4 mas).

We calculated the volume of space probed by Kepler for objects with that size (angle squared times typical distance cubed), multiplied by the number of stars Kepler observed, and decided you needed about 10 billion disk-bearing black holes in the Milky Way for one to have had a good chance to wonder in front of ~1 such star in the field. That’s not too far off from the estimated number of black holes in the galaxy!

So, the numbers aren’t too bad!

I really like this one, but there still isn’t any observational evidence that such disks exist, or that they are common enough for Kepler have found one. We haven’t done a rigorous calculation of the probabilities, so it could still fall apart upon closer inspection.

Given the uncertainties, I give it a subjective verdict of: less plausible.

This hypothesis would find support if the dips repeat in reverse order while the star starts brightening again, like J1407 did. Alternatively, that so-called “M dwarf” 2″ away could turn out to be the central object, in an almost-but-not-totally quiescent state. Someone get a spectrum of that thing!

Hypothesis 6) An Orbiting Black Hole Disk

I had hoped that we could make an alignment more likely by putting the black hole in orbit around Boyajian’s Star, but it turns out that makes things much harder. In addition to the low probability of such a binary companion in the first place, the chances that it would be in a part of its orbit such that we would see it are very low, like 1 in a million low. Since Kepler only looked at 100,000 stars, and since every star does not have such a companion, this one doesn’t work.

Subjective verdict: not likely.

OK, enough with the black holes. Next time: Circumstellar material.

Update: Commenter Herp McDerp (obviously their real name) points to this Nature article by Alastair G. W. Cameron (Bethe Prize winner and originator of the Giant Impact Hypothesis for the formation of the Moon). In it, Cameron tries to explain the eclipses of the ε Aur system with our Hypothesis 6! I’d write “great minds think alike” but I’m totally out of my league on this one, so I’ll just write that we’re in very good company with this hypothesis!