If you’re looking for a guide to this series, click here.
Last time, we discussed the periodicities (or lack thereof) in the dips of Boyajian’s Star, and the implications of the sodium absorption and reddening we see. The last piece of observational evidence I’d like to introduce is whether any neighboring stars are variable. After all, if there is a large cloud between us and Boyajian’s Star, it should block light from other stars, too.
Photometry of Nearby Stars
Well, there is a star, KIC 8462860, which sits 25′′ NNW of Boyajian’s Star. It doesn’t have the same sort of time-series as Boyajian’s Star, so we can’t look for dips, but Ben Montet did extract its long-term light curve from the Kepler full-frame imagery, and he graciously gave them to me for publication:
Nothing to see here, apparently—this looks like an ordinary constant star, like most of the >200 others Montet & Simon looked at. Nonetheless, Steinn and I strongly encourage photometry monitoring of all stars within 1′ of Boyajian’s Star, just in case.
OK, so that’s going to help with explanations invoking a cloud of material. Now on to the hypotheses!
Hypothesis 1) It’s Instrumental
That is, there is no dimming to explain, it’s an artifact of the instruments. Dr. Boyajian ruled this out in her paper regarding the dips, and since then many groups have gone back to the Kepler data to see if they can find a problem, including Kepler team members. The dips are real.
The long-term dimming Schaefer sees could be instrumental — indeed there was a big food-fight over this issue, but, as I wrote in Part I, the Montet & Simon discovery of similar dimming during the Kepler mission (and careful use of control stars) provides independent confirmation of the phenomenon.
I think we can put this one to bed. Subjective verdict: very unlikely
Hypothesis 2) A Solar System Cloud
Could there be a cloud of — something — in the Solar System blocking light from Boyajian’s Star? Let’s set aside where it came from for now—does the hypothesis have explanatory power?
The Earth, Kepler, the Sun, and Boyajian’s Star are all moving. The motion of the star is called its space motion and the apparent change in position of the star due to its motion and the Sun’s is called the star’s proper motion. If there is something between us and the star, then proper motion should change our line of sight through it. Kepler also moves around the Sun, and that results in an apparent, annual elliptical motion across the sky we call parallax. Nearby stuff seems to move due to parallax much more than background stars, so if there is a Solar System cloud, then we would expect our line of sight to trace an annual ellipse through it, in addition to a slow drift due to proper motion.
For the moment, let’s put the hypothetical cloud out at 10,000 AU. Parallax would make it appear to move by about 20 arcseconds, and its orbital motion would move it by about the same amount over 100 years. So if the cloud is 20″ across, it could be responsible for the long-term dimming. This would also help explain the 1.96 Kepler year gap between the two dips (although not the lack of dips at 0.98 years): that’s the time it takes our line of sight from Kepler to return to about the same place, with ~1% taken off due to the cloud’s own orbital motion.
So, in this scenario the long-term dimming is just a density gradient in the cloud, and the dips are from dense knots in the cloud that we briefly look through when Kepler and Boyajian’s Star line up just so.
So, what are the problems? Well for one thing, the density gradient doesn’t seem modulated on an annual timescale. For another, the sodium lines in the spectrum don’t seem to be at zero velocity with respect to the Solar System barycenter. Finally, why would there be a cloud of dust out there? Not only is Boyajian’s Star way above the ecliptic (but does that even matter at 10,000 AU?), but a 20″ cloud at 10,000 AU would be 1 AU across. What could cause it?
Steinn likes the idea of a very low-velocity cometary collision that kicked up ice and dust in a big cloud. It wouldn’t take much mass to hold the dust in a Hill sphere 1AU across (an asteroid would do it). I prefer a big KBO-type object with a very gentle geyser that burps out dense plumes occasionally.
If we see another dip, and the spectrum of Boyajian’s Star shows evidence of Solar System ices in absorption then, I think this idea might gain traction.
Nonetheless, the combination of a highly speculative cloud, a missing annual timescale, no obvious dimming of neighbor stars, and no clear sodium absorption feature cause me to give a subjective rating of unclear to this one. That is, I’ll need some outer Solar System people to weigh in on likelihoods before I give it a grade.
Credit Where Credit Is Due
I should take this opportunity to note that many of these hypotheses are not original to Steinn and me. Many have been suggested independently to us privately, both in person and over email, at Q&A sessions after talks, in social media, and among our professional colleagues. If in this or future installments you recognize an idea as one you pitched to me, let me say:
- Thanks for your suggestion; it helped me think about the problem
- You weren’t the only one to suggest it, and you probably weren’t the first
- I’m sorry if you are not in our acknowledgements section; we could not acknowledge everybody or track down every suggestion, but even so our original acknowledgements were so extensive that the editor asked us to scale them back (Letters are supposed to be short).