If you’re looking for a guide to this series, click here.
Last time, I laid out the background for Boyajian’s Star. Let’s look at some of the observational constraints on families of possible solutions.
First, there’s the lack of infrared excess. This actually is the main reason Boyajian’s Star is weird. Other stars, like this “dipper”, behave similarly to Boyajian’s Star but have whopping infrared excesses. That is, there is a lot of extra infrared light in excess of what you would expect from a “naked” star. This is because there is a lot of circumstellar material around those stars in the form of a disk, and parts of that disk sometimes occult the star, causing big drops in brightness. In those cases there is some puzzle in the exact geometry of the situation, but there is no mystery regarding what’s causing the dips.
With Boyajian’s Star, the lack of infrared excess is very puzzling in the context of its dips. If something is blocking the starlight, why is there no infrared light coming from it? Massimo Marengo and Casey Lisse published papers recently showing that the lack of IR excess in archival data is also a lack of IR excess today, using recent Spitzer and IRTF data. This rules out many categories of solutions where all the circumstellar dust formed recently due to a planetary collision (for instance).
More recently, Thompson et al. showed that there is no detectable millimeter flux from Boyajian’s Star. This upper limit is weaker, but rules out cooler dust. Specifically, they claim there can be no more than 7.7 Earth masses of dust within 200 AU, and no more than about 10-3 Earth masses of dust at the orbital distances implied by the durations of the dips. This rules out some origins for the long-term secular dimming seen by Schaefer: there can’t be a cloud of stuff around the star constantly blocking 15% of the light, or it would certainly intercept and reradiate more light than we see. Here’s what I mean:
In this figure, you can see the optical spectrum of the star on the left, with the red points being the measured brightnesses. It follows a normal curve for a star. Disks of the sort that cause the “dippers” are firmly ruled out: they have hot, close-in dust that have lots of emission at 5-20 microns, which we don’t see in the red points.
The black line is actually a model of the star that matches the detailed spectrum we really see 20 microns and shorter. Longward of 10 microns, there are two curves: the black curve is what you would see if the star were surrounded by a cloud of dust absorbing 15% of its light. I chose 15% because that is the total amount of dimming seen by Shaefer and Montet & Simon. The red arrows are upper limits: the star is dimmer than that at those wavelengths. Clearly, the long-term dimming can’t be created by such a cloud, or the dust would be warm enough to be seen by Thompson at 1 mm ( =1000 microns). You can’t save the hypothesis by making the dust colder, either, because that just moves the curve to the right.
In fact, if there is warm-ish dust aroud the star, it cannot be blocking more than 0.2% of the starlight (the purple curve), and even then it only squeezes between the existing upper limits if it’s at 65K, which is pretty cold. This constrains possible solutions to the long-term dimming.
Now, if the dust is in an edge-on ring, then it could absorb only 0.1% (or less) of the starlight in total, but still block 15% along our line of sight. In that case, it would not be seen by the current observations, as long as it was cold enough. So: disks and clouds are out, but rings (and comets) are still allowed by the IR and mm observations.
Ok, that’s enough words for one post. Next time: If the long-term dimming is going on right now, shouldn’t we be able to determine its composition from absorption features in the star’s spectrum? Also, aren’t the dips periodic, and can we predict the next one?