KIC 8462852: A Century Long Fade?

by Paul Gilster on January 14, 2016

I hadn’t expected a new paper on KIC 8462852 quite this fast, but hard on the heels of yesterday’s article on the star comes “KIC 8462852 Faded at an Average Rate of 0.165±0.013 Magnitudes Per Century From 1890 To 1989,” from Bradley Schaefer (Louisiana State University). Schaefer takes a hard look at this F3 main sequence star in the original Kepler field not only via the Kepler data but by using a collection of roughly 500,000 sky photographs in the archives of Harvard College Observatory, covering the period from 1890 to 1989.

The Harvard collection is vast, but Schaefer could take advantage of a program called Digital Access to a Sky Century@Harvard (DASCH), which has currently digitized about 15 percent of the archives. Fortunately for us, this 15 percent covers all the plates containing the Cygnus/Lyra starfield, which is what the Kepler instrument focused on. Some 1581 of these plates cover the region of sky where KIC 8462852 is found. What Schaefer discovers is a secular dimming at an average rate of 0.165±0.013 magnitudes per century. For the period in question, ending in the late 1980s, KIC 8462852 has faded by 0.193±0.030 mag. From the paper:

The KIC 8462852 light curve from 1890 to 1989 shows a highly significant secular trend in fading over 100 years, with this being completely unprecedented for any F-type main sequence star. Such stars should be very stable in brightness, with evolution making for changes only on time scales of many millions of years. So the Harvard data alone prove that KIC 8462852 has unique and large-amplitude photometric variations.

That’s useful information, especially given the possible objection to the Kepler findings that they might be traceable to a problem with the Kepler spacecraft itself. Evidently not:

Previously, the only evidence that KIC 8462852 was unusual in any way was a few dips in magnitude as observed by one satellite, so inevitably we have to wonder whether the whole story is just some problem with Kepler. Boyajian et al. (2015) had already made a convincing case that the dips were not caused by any data or analysis artifacts, and their case is strong. Nevertheless, it is comforting to know from two independent sources that KIC 8462852 is displaying unique and inexplicable photometric variations.

As Schaefer notes, KIC 8462852 can now be seen to show two unique episodes involving dimming — the dips described here yesterday for the Kepler spacecraft, and the fading in the Harvard data. The assumption that both come from the same cause is reasonable, as it would be hard to see how the same star could experience two distinct mechanisms that make its starlight dim by amounts like these. The timescales of the dimming obviously vary, and the assumption would be that if the day-long dips are caused by circumstellar dust, then the much longer fading that Schaefer has detected would be caused by the same mechanism.

KIC8462852EfraSAC

Image: KIC 8462852 as photographed from Aguadilla, Puerto Rico by Efraín Morales, of the Astronomical Society of the Caribbean (SAC).

Thus we come to the comet hypothesis as a way of explaining the KIC 8462852 light curves. Incorporating the fading Schaefer has discovered, a cometary solution would require some mind-boggling numbers, as derived in the paper. From the summary:

With 36 giant-comets required to make the one 20% Kepler dip, and all of these along one orbit, we would need 648,000 giant-comets to create the century-long fading. For these 200 km diameter giant-comets having a density of 1 gm cm−3, each will have a mass of 4 × 1021 gm, and the total will have a mass of 0.4 M. This can be compared to the largest known comet in our own Solar System (Comet Hale-Bopp) with a diameter of 60 km. This can also be compared to the entire mass of the Kuiper Belt at around 0.1 M (Gladman et al. 2001). I do not see how it is possible for something like 648,000 giant-comets to exist around one star, nor to have their orbits orchestrated so as to all pass in front of the star within the last century. So I take this century-long dimming as a strong argument against the comet-family hypothesis to explain the Kepler dips.

If Schaefer’s work holds up, the cometary hypothesis to explain KIC 8462852 is deeply compromised. We seem to be looking at the author calls “an ongoing process with continuous effects” around the star. Moreover, it is a process that requires 104 to 107 times as much dust as would be required for the deepest of the Kepler light dips. And you can see in the quotation above Schaefer’s estimate for the number of giant comets this would require, all of them having to pass in front of the star in the last century.

The paper is Schaefer, “KIC 8462852 Faded at an Average Rate of 0.165+-0.013 Magnitudes Per Century From 1890 To 1989,” submitted to Astrophysical Journal Letters (abstract).

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{ 273 comments… read them below or add one }

Harry R Ray April 28, 2016 at 10:54

A very interesting new paper is up on http://www.voxcharta.org: “Type III Societies (Aparently) Do Not Exist” by Brian C Lacki, dealing with what he calls “blackboxes”, which are artificial megastructure “screens” made of artificial “dipole antennae” dust from the scale of ENTIRE GALAXIES down to Kuyper belts, with a temperature JUST ABOVE THE CMB. If ET’s at KIC8462852 WERE making such a screen(for what purpose I have NO IDEA), a century-long timescale would seem a lot more doable WITHOUT invoking Von Neuman machines than anything else I can think of. ALSO: It SHOULD be DETECTABLE by telescopes LESS POWERFUL than ALMA, which CANNOT OBSERVE KIC8462852. Please check this paper out!

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Dirk Bontes April 28, 2016 at 16:15

In this brief article I argue that its dimming is reminiscent of the Cepheid mechanism, in which helium atoms are completely ionized. https://goo.gl/ng7ey9

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EricSECT April 28, 2016 at 20:48

Dirk: Can you elaborate on the dimming mechanism without us having to purchase the book, because otherwise it’ sounds pretty suspect.

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Dirk Bontes April 29, 2016 at 8:26

The book temporarily is for free. It is about two thousand words.
The cover says it all, actually.
In Cepheids the light is temporarily blocked inside the star itself by a layer of completely ionized helium, preventing some light from escaping from the star.
My solution is that in KIC 8462852 something similar is at work, though the star itself is not a Cepheid.
Cepheids and various other types of star are in the instability strip of the Herzsprung-Russell diagram and KIC 8462852 is in a position on the Main Sequence of this diagram that is very close to this instability strip.

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Michael Hutson April 29, 2016 at 10:17

See Eddington valve, or k-mechanism

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Harry R Ray April 29, 2016 at 12:35

I have ALWAYS FAVORED some natural INTERNAL PROCESS to be the cause of the flux variations of KIC8462852. Dirk’s proposal works, but ONLY if there is NO gradual century-long fade. No cephied that I know of has gradually faded for that long of a time period. Still waiting for the Sonneberg plates analysis to be completed

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Dirk Bontes April 29, 2016 at 15:57

I do not see the century-long fade as an obstacle. We are not dealing here with an increase in the helium concentration in the atmosphere of the star, but with an increase in its ionization. That might increase very quickly, on a century time scale.
Moreover, we are not dealing with a Cepheid star here. Cepheid stars are not in a hydrostatic equilibrium; this star still is, though. So it is no use comparing Cepheids re the century-long fade with this star.
Anyway, there are other types of stars in the instability strip that this star more resembles than a Cepheid star.
For now, it appears – and is reasonable to suspect – that this star is about to move from the Main Sequence into the instability strip. I am sure that we will learn a lot from observing its evolution.

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EricSECT April 30, 2016 at 9:07

Dirk: Thanks for the free link to your book.

Tabby’s Star has two deep dips in luminosity that are 750 days apart. This does not mean that it has a regular 750 day period, we would need to witness at least one other dimming event.

But there is something going on with Tabby’s flux at about a 20 day period, if it is He double ionization caused by core magnetic field reversal, why does this period not dominate over any longer term flux oscillations?

Your hypothesis does appear to be able to explain the cause of the dimming events, but what unique predictions can it make that would differentiate it from others?

I suppose it is possible that of the (millions?) of similar F3V class stars we’ve observed, Tabby’s is unique, the only one so far to exhibit this behavior, and that we caught it just the right time as it starts to turn off the main sequence, but that seems suspect given it’s multi-billion year life span. Supporting evidence would be to find similar class stars doing similar flux changes.

In your hypothesis, does the entire star dim or do patches form, coalesce and then disperse? How would (or could?) a dimming event signature discriminate between the two causes?

I am not sure I follow the angular momentum contribution of Tabby’s possible red dwarf companion, could you please elaborate? Does this companion somehow excite or de-stabilize Tabby’s flux? Do we see this in other star pairs? If it is a companion, even at 1000 AU maximum separation, it’s orbit and it’s influence is much more than 750 days.

Is there any evidence that the red dwarf is indeed a gravity-bound companion and not just a foreground star, besides their apparent proximity? It’s radial velocity or apparent motion?

Is there any other supporting evidence that Tabby’s is at an unstable region and about to exit the main sequence, rather than being at about mid-life and still on the main sequence?

Why is this entire hypothesis not submitted in a scientific paper?

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Dirk Bontes April 30, 2016 at 12:13

The e-book will be for free until there have been one hundred downloads. After that I will probably ask a small fee (99 cents).

I agree that we all are waiting for another major dimming event.

As for the twenty days period, one hypothesis of mine is that there are two sources of the stars magnetic field and that the twenty day period comes from the magnetic polarity reversal of the core of the star. Another possible explanation is that there maybe is some innate ringing of the star going on with that frequency. But as a matter of fact I have no idea, which is why I focussed on the major dips.
The twenty day period clearly has only a minor effect on the light output of the star. I do not know much about this period, its variations and possible associations and correlations with other phenomena, so neither can I draw any conclusions about it.

A unique prediction? I do not know. Fact is that I have used similar principles to explain other phenomena in other stars, especially re sunspots.

I do not consider the uniqueness of Tabby’s star suspect. We have got to consider that it is relatively very close to us and that it is one of if I recall correctly, only one to two hundred thousand stars that are watched by a team of variable star observers, who alerted the scientists. There are about a hundred billion stars in our Milky Way who are not watched by that group. Who knows how many similar stars there are among those?

Indications are that the entire star dims, like Cepheids do. On the other hand there also is a small group of stars that form extensive sunspots, as I have discussed in another article, and there is Algol, also discussed by me in another article. In KIC 8462852 we do not observe the same phenomenon / -na is in these latter stars.

As for the hypothetical companion. (It actually is so close to KIC 8462852 that I consider it more likely to be a companion than for it to not be a companion.) A secondary orbiting a primary will cause tides in the primary, that decrease its angular momentum. Since there is a law of conservation of angular momentum, the angular momentum from the primary is transferred to the secondary, causing it to increase the radius of its orbit. (The same happens in the Earth-Moon system.)
So if this is a genuine companion, we would expect the primary to scarcely have any angular momentum left. However, on the contrary it does have a rapid rotation. Some internal phenomenon must be spinning up the star. My money is on a contracting core and a strengthening magnetic field.
The companion does not have any effect on the dimming of KIC 8462852; it simply is too far away.

No, there is no evidence whatsoever, apart from its proximity, that the red dwarf either is or is not a companion star.

Other than the absence of helium, and the presence of some heavy elements in its atmosphere, the various dimmings, and its rapid spin, and its proximity to the instability strip, there is no reason to suspect that KIC 8462852 may leave the Main Sequence, no.

Why is this hypothesis not submitted to a scientific paper? 1. They would not pay me. 2. I would require extensive guidance by a professional astronomer who knows his subject to be able to write such a submission, I suspect. 3 & 4. Well, there are a 3 & 4.

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EricSECT May 1, 2016 at 8:15

Thanks Dirk, your hypothesis makes sense and I can’t shoot any holes in it. It’s only weakness is that although it explains what we see it does not make unique predictions to distinguish it from other theories. If we find a handful of other stars (of the same class?) that act similarly, that seems like that would be your supporting evidence. I like yours much more than the (to me) contrived massive comet swarm(s), the only other still standing “natural” alternative.

I will hazard my own prediction: Someone will hijack your idea and present it as a published scientific paper. You should think about jumping on that!

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Coacervate April 30, 2016 at 17:19

I think you expand a bit more on step 2 where you say “…and then a miracle occurred.”

-Sidney Harris

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Jon K May 2, 2016 at 14:07

Interesting theory, but the ionization of all the atmosphere need to be perfectly homogeneous, or we expect to see the 0.88 day rotation effect on the long-day dips.
Cepheids can do that ?

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Dirk Bontes May 2, 2016 at 15:45

A reversal of the magnetic poles will affect the entire star at once.

Besides, it is not a Cepheid.

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Harry R Ray May 9, 2016 at 15:57

LATEST BREAKING NEWS: The REVISED Hippke paper has FINALLY been accepted, and the Schaefer paper has been revised AGAIN! I don’t know what this means but it doesn’t look very good for Schaefer!

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Harry R Ray May 9, 2016 at 19:24

OMG! Schaefer’s revised paper has ALSO been accepted by a DIFFERENT journal! I have ABSOLUTELY NO IDEA WHAT TO MAKE OF THIS NOW! Anybody else know what’ going on? I guess it is now TOTALLY UP TO the Sonneberg plates now since the DAASCH plate results have seemed to be TOTALLY CANCELLED OUT(i.e., no way to tell whether there WAS a fade or NOT)! Oh well. Patience. PATIENCE!

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Paul Gilster May 9, 2016 at 20:34

Michael Hippke will have an update on all this here on Centauri Dreams in a couple of days.

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Fred Parker May 30, 2016 at 12:33

Folks, if you are interested in pledging to pay for telescope time necessary to solve the Tabby Star mystery, they are trying to raise funds on KickStarter.com. The link for this is:

https://www.kickstarter.com/projects/608159144/the-most-mysterious-star-in-the-galaxy

They only have 17 days left to go to reach their goal of $100,000. Let’s help them out, please.

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Paul Gilster May 30, 2016 at 16:19

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