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

Last time we finished up our roundup of hypotheses for Boyajian’s Star. Let’s summarize them:

Plausible Hypotheses

#3 Small-Scale ISM structure: This will find support if future dimmings are accompanied by increased reddening and sodium absorption.

#4 An Intervening Bok globule: In addition to the above, this would be supported if we discover a cloud of neutral gas near Boyajian’s Star.

#7 Comets and other circumstellar material: Plausible for the dips. Would be supported if future dips are accompanied by significant infrared flux and absorption features consistent with cometary or other circumstellar material.

Less Plausible Hypotheses

#5 An Intervening Black Hole Disk: Would find support if we can detect the central object or if the dips repeat in reverse order, as with J1407, or if more rigorous analysis shows this to be more likely than we have estimated.

Hypotheses with Unclear Plausibility

#2 A Solar System Cloud: Would have plausibility if such a cloud is theoretically realistic, if the cloud were found by other means, or if the dimmings are accompanied by absorption features characteristic of Solar System ices.

#9 Alien Megastructures: Would find support if all natural hypotheses are ruled out, we detect signals, or if star suffers significant achromatic extinction.

#13 Post-Merger Return to Normal: Would have plausibility if Boyajian’s Star is found by Gaia to be overluminous, and if simulations show this could explain the dips and dimming timescale.

Not Likely Hypotheses

Very Unlikely Hypotheses

And here’s a handy guide for what the Gaia parallax, which will determine the distance to Boyajian’s Star and is due out on September 14, will tell us:

Within 400pc: Means that ISM extinction cannot explain current level of dimming, so favors non-ISM and non-dust explanations. These are #9 ,11, and 12. Could also imply that the cause is very opaque dust blocking part of the stellar disk, so #5, 6, 7, and 8 might still be OK

Around 450pc: Means that all of the secular dimming can explained by the dust we observe along the line of sight. Favors #2, 3, 4, 5, 6, 7, and 8.

Beyond 500pc: Means that the star is more luminous than we expected from the reddening. Favors #13: return to normal brightness.

No answer: Star does not have a good astrometric solution. Unclear how this could happen given the lack of RV variation, but could that the 2″ companion is much bluer than we think, which would be annoying and not tell us very much except that it’s not an M dwarf, maybe boosting #5 (if it’s really the central black hole). Might imply that there is another star nearby that we haven’t accounted for, suggesting new hypotheses.

You can vote on what you think Gaia will find here.

The paper is Wright & Sigurdsson, on the arXiv here. Thanks for reading, and hopefully we’ll know a lot bit more in 11 days!

Update: Jason Curtis points out that the first Gaia release will not actually have the precision to distinguish among these cases! So unless the parallax comes out really for from 450, we’ll have to wait longer than Sep. 14.