November 03, 2016

Possible reasons for the unusual dimming of Tabby's star include super-advanced aliens

About 1480 light-years away from Earth, a star is doing something astronomers have never seen before. Every now and then, at random intervals, its light dips by as much as 22 percent (observed by the Kepler space telescope). That's way too much to be a planet. And the star (officially named KIC 8462852, but informally called "Tabby's Star" or "Boyajian's Star") seems to have gotten dramatically darker over the past century.

This odd behavior defies all known explanations, and astronomer Jason Wright has pointed out that these light patterns are similar to what we might expect if aliens built a complex of machines around the star to harvest its energy. But even Wright admits it's much more likely there's a natural explanation.

Bradley Schaefer looked at old DASCH photometry and found that Boyajian’s Star has been fading over the past 100 years, a claim at least as extraordinary as the star’s Kepler light curve.

Ben Montet and Josh Simon very cleverly recently used the Kepler full-frame imagery—some calibration data that doesn’t get much attention because you can’t use it to find planets—to get accurate long-term photometry of Boyajian’s Star over the course of the mission. Amazingly (to everyone but Bradley, I suspect), they found that the star got 4% dimmer over 4 years, in a monotonic but irregular way. What’s more it is the only star out of > 200 that show this effect.

This independent confirmation of the unprecedented effect Schaefer claimed—even if not covering the same time period—shows that Shaefer’s analysis is correct and the star really has dimmed a lot. Adding the two effects, the star is now apparently at least 17% dimmer than it was in 1890.

We now have two inexplicable things going on: long-term, secular dimming of 17% in 115 years, and these days-long, deep “dips” of up to 22%. Both are very hard to explain.

Some call it “Boyajian’s Star“. Dr Tabetha Boyajian herself calls it the “WTF” star, ostensibly after the subtitle of her paper (“Where’s the Flux?”) Others call it Tabby's star.





Observational Constraints on possible reasons

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.


Red points showing the spectral energy distribution of Boyajian’s Star, with arrows indicating upper limits. The right-most measurements are from Thompson et al. They rule out big clouds of 65K dust blocking more than 0.2% of the star’s light in all directions, because the dust would reradiate at levels easy to detect. The two curves are models consistent with the short-wavelength data and differing only in the fraction of starlight being intercepted and re-radiated as heat.

Boyajian and others have noted that the deepest dips seem to have some patterns to them.

A 2 year period?

For instance, two of the deepest dips occur 2.000 years apart, which is an awfully precise number to be mere coincidence.

But, coincidence it is: remember that Kepler does not orbit the Earth, it orbits the Sun, and it does so in an Earth-trailing orbit, meaning that it has a different orbital period than the Earth! So the relevant year is not an Earth year, but a Kepler year. This makes those two dips’ interval less suspiciously precise: they are 1.96 Kepler years apart. Also, if the Kepler orbit were responsible for those two dips, then one would also expect to have seen dips 0.98 Kepler years before and after the first of those—and we don’t.

A 48.4 day period?

Another suspicious bit of timing is that many of the deepest dips seem to occur with an interval of 24.2 days. Boyajian noted that this looks like a 48.4 day orbital period, in which something luminous blocks the star every 48.4 days, and is itself blocked half a cycle later as it goes behind Boyajian’s Star.

With only 6 dips, it’s quite common to see phantom periodicities if you check enough periods. True, these are a bit better lined up than random, but that’s probably why they were noticed and I did this analysis in the first place.

Sodium absorption and reddening

Boyajian’s Star’s sodium lines are very broad because the star is rotating rapidly. Between us and the star, there are clouds of neutral gas, and this gas contains sodium and dust (and lots of other stuff). As a result, the dust makes the star a bit too faint in the bluer wavelengths, and the sodium absorbs light at its characteristic wavelengths near 590 nm. This is not unusual: at the nominal distance of Boyajian’s Star (450 pc) we expect there to be plenty of this interstellar material.

The reddening from dust makes the blue wavelengths about 10% fainter than it should be compared to green-ish wavelengths (we write E(B-V) = 0.1). This also implies that about 35% of all of the visible light from the star in our direction is attenuated by dust before it reaches Earth.

Reasons reviewed

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 and Simon discovery of similar dimming during the Kepler mission (and careful use of control stars) provides independent confirmation of the phenomenon.

Jason Wright thinks we can put this one to bed. Jason 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?

Hypothesis 3) Absorption from the Interstellar Medium

Between the stars there is lots of gas and dust. The densest, coldest parts of this “interstellar medium” (ISM) form neutral gas and dust, which cause reddening, dimming, and line absorption in stars. For instance, all evidence points to Boyajian’s Star having its light pass through enough dust and neutral gas on the way to Earth to make it about 35% dimmer at visible wavelengths than it would be without that stuff in the way. This is expected for any star in its part of the Galaxy.

But, one asks, maybe there are some especially dense pockets along that our line of sight occasionally sweeps past? I had always rejected this line of thought because if that were a thing, all sorts of stars would do that. After all, Kepler looked at over 100,000 other stars and none of them ever showed this behavior.

But, it turns out rare dense patches are a thing! I was at Berkeley recently and chatted with Carl Heiles, and he pointed me to SINS — small ionized and neutral structures in the diffuse interstellar medium. He was actually one of the folks who first brought these things to the attention of the broader community and postulated that they are short-lived, overdense, corrugated sheets and/or filaments in the ISM, and that when our line of sight aligns with a tangent point of such a structure we get a temporary jump in absorption.

So it turns out that this is a whole field—Carl pointed me to this conference on the topic 10 years ago. So, could this be the answer? Well, one problem is that the columns and sizes implied by Boyajian’s Star are much different from the “tiny scale atomic structure” that Carl describes. Those structures are typically around 30 AU across and block maybe 0.1% of the visible light that passes through them. We need about 100 times as much dust, and we need structures maybe 100–1000 times smaller than that.

This hypothesis would find support if during a future dip (or if the star’s long-term brightness continues to change a lot, in either direction) we see a corresponding change in the reddening, and in the sodium absorption features.

Jason Wright Subjective verdict: plausible!

Hypothesis 4) Absorption from an Interstellar Molecular Cloud

This hypothesis would find support if we could find the cloud, perhaps with a gas map done by the VLA or something. Even a single-dish radio telescope might at least spot some molecular gas in that general direction compared to neighboring direction, which would lend this hypothesis support.

This one isn’t quite as nice as SINS for Jason, but he still think it’s got a lot going for it. Jason Wright Subjective verdict: plausible!


Hypothesis 5) An Interstellar Black Hole Disk

A possible black hole (in the way) is one where the close-in disk is gone, and only a very wide, very cold disk of dust and debris remains. Also, the black hole is several solar masses, not several million.

They 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

Jason really likes 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.

Jason's subjective verdict of: less plausible.

This hypothesis would find support if the dips repeat in reverse order while the star starts brightening again


Hypothesis 6) An Orbiting Black Hole Disk

Jason 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.

Hypothesis 7) Comets or other circumstellar debris

Jason's subjective verdict on this is: plausible for the dips, very unlikely for the long-term dimming

There doesn’t seem to be any way the comets can explain the long-term dimming, especially without an infrared excess.

Hypothesis 8) A cool annulus of material

This is basically the hypothesis that there is a disk of material, but there is a very large (10 AU, at least) gap between the star and the inner edge of the disk (annulus). The disk is almost, but not quite, edge-on: then, we could invoke corrugation or other irregularities that occasionally block part of the star for the dips. This needs to be a real protoplanetary disk, not just a debris disk, because it needs to be optically thick.

There are lots of problems with this

It has no theoretical justification, and it requires a very unlikely geometry and high frequency of such disks in the field.

Subjective verdict: not likely.

Hypothesis 9) Alien Megastructures

This one has very little to hang any physics on. Ancient alien civilizations could be arbitrarily advanced, and so it’s not clear what physics we’re allowed to assume. We don’t know why they would create megastructures (though energy collection seems like a good guess to me) so we have no good reason to expect any particular shape or size for them.

That said, we can build up a straw man model, and see how well it holds up.

Imagine that it is advantageous to collect solar energy to be used for some purpose, and that there is enough material to do so, but not an infinite supply. Imagine that the panels have a range of sizes and orbit the star in a range of orbital periods. In this case, the cost (in time, energy, and mass) to construct a panel is balanced by the benefit (the energy intercepted, which favors close-in panels, times the efficiency of the panels, which favors far-out panels). If too many such panels are created, the close-in ones will shadow the farther-out ones, reducing their efficiency.

In this toy model, one expects (in a rough order-of-magnitude sense) to end up with a swarm of panels that have an optical depth near 1. That is, they should not capture, say 99.999% of the photons, because the marginal efficiency of the next panel is reduced by a factor of 100,000 compared to the first one. And they shouldn’t intercept only 0.001% of the light, because 99.999% of the light still free to take with virtually zero efficiency hit. You’d expect somewhere between, say, 10% and 90% of the light to get absorbed.

The smaller panels will appear as a translucent fluid around the star, constantly blocking some fraction between 10-90% of the light, roughly speaking. This fraction will vary as denser parts of the swarm come into and out of view, and as chance alignments of parts of the swarms at different orbital distances align. We might see variations in brightness on scales from hours to centuries. Particularly large panels—even bigger, perhaps, than the star itself—will cause large, discrete dips as they transit, with profiles according to their shape. The timescale for crossing may not be a good indicator of their distance from the star, because they might be so thin that radiation pressure is important.

This is actually right in line with the observations of Boyajian’s Star: we see a constant dimming of the star, one that erratically has increased by about 15% in the past 115 years, and occasional dips.

The long-wavelength constraints apply almost as well to megastructures as they do to dust. We argued in that paper that it is reasonable to expect nearly all of the energy collected to be reradiated as waste heat. As we saw from the long-wavelength limits, it just can’t be that 15% of the starlight is being reprocessed at 150K, or at any other temperature.

Now, there are a couple of ways out. First, the aliens might be doing some non-dissipative work with this starlight. Perhaps they are launching interstellar probes. Perhaps they are sending out powerful laser or radio transmissions. Anything that puts all of that stellar luminosity into energy that leaves the system in a low-entropy way would reduce their waste heat luminosity.

If the work is all done at 65K, then the maximum (Carnot) efficiency allowed by thermodynamics is about 99%. This means that they could reduce their long-wavelength luminosity down from 15% of the star’s, to only 0.15%. The upper limit at this temperature, as you can see in the figure above, is 0.2% (the purple line).

So… not completely ruled out, yet, but Jason says we’re close enough that he put this one down as very unlikely, even after normalizing for the unknown chance that there are megastructures there in the first place.

Another way out is that it’s not a spherical swarm. Make it an edge-on ring, and you can further reduce the waste heat. Make them radiate towards their ecliptic poles, and you can reduce it further. Turn the energy into low entropy radiation at high efficiency and do both of the above, and they’d be nigh undetectable.

SEveral other star related reasons

Hypothesis 10) Starspots and Magnetic Cycles
very unlikely.

Hypothesis 11) Polar Spots
not likely.

Hypothesis 12) Stellar Pulsations
Subjective verdict: not likely

Hypothesis 13) Post-Merger Return to Normal

What if the star isn’t dimmer than it should be: what if it’s brighter, and we’re seeing it return to normal brightness? In that case Gaia will reveal that it is much farther away than we think based on its brightness and reddening. This would imply that the long-term “dimming” we see is the star returning to a normal state after somehow getting way too bright.

But what could that mean? The best idea we have is some sort of merger: perhaps the star recently coalesced with another star, or a brown dwarf or planet. This would deposit a lot of orbital and gravitational potential energy into Boyajian’s Star, which would eventually get dissipated as heat and escape as starlight, causing a temporary brightening.

One issue here is that the dimming is too fast. When confronted with big changes in energy content or flux, stars evolve on the Kelvin-Helmholz timescale, roughly the time it takes for all of the energy in the star at a given moment to finally escape the surface (while being constantly replenished by the fusion in the star’s core). For Boyajian’s Star this timescale is about 1 million years. This means that if the entire star is processing a big change in internal energy or luminosity, it takes around 1 million years to complete the adjustment. Changing by 15% in 100 years is therefore about 10,000 times too fast.

But, the star’s radiative envelope is not very massive, so perhaps the energy never made it deep into the star? In that case the Kelvin-Helmholz timescale is a bit shorter, so maybe we’re off by only 1,000 times. It’s an order of magnitude argument, so maybe we’re being too pessimistic by a factor of 10, so we’re only off by 100 times. It’s possible that a detailed simulation of such a merger will reveal shorter timescale events, perhaps even things that might produce the dips

Subjective verdict: unclear.

SOURCE - Jason Wright blog

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