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KIC 8462852 Observations
HSchirmer
post May 23 2017, 05:39 AM
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QUOTE (HSchirmer @ May 23 2017, 03:36 AM) *
Well, on the other hand, the recent 3% dip MIGHT indicate that the pattern with the 22% dip is repeating.
If so, then this is VERY interesting for 2 reasons.

First, we actually get to analyze the dips as they happen, with spectra.
Second, this would place whatever-is-causing-the-dips roughly within the goldilocks zone of the star


Oh, wow... a third point...
https://twitter.com/david_kipping/status/866127740776456192
These dips last for days.
If this is something with a 750 day circular orbit, then each day of occultation represents an arc 700,000km long.
Then the recent 3% dip over 2.5 days requires something opaque that is
roughly 5 sun (sol) diameters in length, and perhaps 2 Jupiters wide,
moving between Tabby's Star and us. If it's not completely opaque, then it's got to be even bigger.

Wow, that's big.
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alan
post May 24 2017, 07:12 PM
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KIC 8462852: Will the Trojans return in 2021?

QUOTE
We aim at offering a relatively natural solution, invoking only phenomena that have been previously observed, although perhaps in larger or more massive versions. We model the system using a large, ringed body whose transit produces the first dimming and a swarm of Trojan objects sharing its orbit that causes the second period of multiple dimmings. The resulting orbital period is T≈12 years, with a semi-major axis a≈6 au. In this context the recent observation of a minor dimming can be explained as a secondary eclipse produced by the passage of the planet behind the star. Our model allows us to make two straightforward predictions: we expect the passage of a new swarm of Trojans in front of the star starting during the early months of 2021, and a new transit of the main object during the first half of 2023.


https://arxiv.org/abs/1705.08427
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HSchirmer
post May 24 2017, 11:56 PM
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QUOTE (alan @ May 24 2017, 07:12 PM) *
KIC 8462852: Will the Trojans return in 2021?

https://arxiv.org/abs/1705.08427


Good paper, great graphics.

They're proposing a mega-saturn, with rings that reflect 3% of the starlight back at us as it transits in 3 days.
At 6AU, [edit] not sure that a 3 day transit fits with the orbital velocity or ring size.

[revised the calculations..]
The area of Tabby's Star that we can see is basically a circle.
The star is listed as being about 1.5 solar radii across, (radius of sun is ~700,000 km)
Consider the area of a circle is ¶r^2 so 1.5 radius =~7 area in srs (sol radii squared)

To reflect 3% more light, you need at minimum, 3% more surface area for the rings,
And 3% of 7 gives 0.21 srs in area. Divide by ¶ gives .067, take square root gives .25 as radius, or 180,000 km.
That's workable, Saturn's rings go to eh, 80,000 km, so it's a bigger version of something we've seen.
[edit] and it's within observed J1407 "super saturn" with rings 90,000,000 km in radius.

Main question for that, is whether the planet and rings that size would transit the star in 3 days.
Orbiting at 6AU, distance around the orbit would be ¶ x diameter or 12¶AU traveled over 12 years.
[edited to add the correct number of zeros]
Each year it travels ¶AU, or 3.14 x 150,000,000 km or 471,000,000 km.
Over 365 days, that's 1,290,000 km per day, over the 3 days of the transit,
the planet moves 3,870,000 km.

So, at 6AU, you need rings almost 3,870,000 km across to cause a 3 day event.
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hendric
post May 25 2017, 04:56 PM
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1 AU is 150,000,000 km, not 150,000.


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HSchirmer
post May 25 2017, 05:33 PM
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QUOTE (hendric @ May 25 2017, 04:56 PM) *
1 AU is 150,000,000 km, not 150,000.


Sorry about that, went back and added the correct number of 000s.


Actually, this proposal is rather similar to J1407, the "super saturn".
That's a planet with rings 90,000,000 km in radius, orbit of 4-14 years, and a 56 day transit.
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dudley
post May 26 2017, 04:15 PM
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This new hypothesis calls for a planet about 30 percent the size of KIC 8462852, exclusive of the rings, so much larger than Jupiter. Isn't it thought that planets don't grow much larger than Jupiter, but merely become denser, compressed by their own gravity?
Then, too, an object on this scale would fuse hydrogen at its core, wouldn't it? That would make for a conspicuous second star in the Boyajian's Star system. Such a star has not been reported.
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HSchirmer
post May 26 2017, 10:34 PM
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QUOTE (dudley @ May 26 2017, 05:15 PM) *
This new hypothesis calls for a planet about 30 percent the size of KIC 8462852, exclusive of the rings, so much larger than Jupiter. Isn't it thought that planets don't grow much larger than Jupiter, but merely become denser, compressed by their own gravity?
Then, too, an object on this scale would fuse hydrogen at its core, wouldn't it? That would make for a conspicuous second star in the Boyajian's Star system. Such a star has not been reported.


Here's an interesting theory - good attempts to combine explanation of long term dimming AND mega transits.

What if Tabby's star is slowly dimming because it ate a Jovian sized planet about a thousand years ago?
What if Tabby's star is obscured by the Jovian icy moons which are now short period mega comets?

System appears to have a distant companion star, which could pull gas giants into elliptical orbits.
Models suggest that a Jovian size planet forced into an elliptical orbit will end up stripped of it's large icy moons.
The moons go into short period comet style orbits, i.e. imagine Europa, Ganymede, Callisto as sun grazing comets.
Those icy moons would produce massive outgassing and massive dips in brightness.
The Jovian planet ends up disrupted and digested, the star's brightness peaks due to a dump of gravitational energy.
You get a slow dimming as the star returns to normal brightness over hundreds or thousands of years.

QUOTE (https://www.youtube.com/watch?v=risNfZxz6DQ)
Cool Worlds video by Brian Metzger and Nick Stone on their hypothesis for Tabby's Star behaviour
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ngunn
post May 27 2017, 09:26 AM
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For me that's the first suggestion that sounds really plausible. In their video they mention that a possible argument against is the issue of frequency, in other words that the presumed rarity and short-lived nature of such an event makes it very unlikely that one would have been observed. I don't see that as much of a problem given the uncertainties involved and the fact that this is (so far) a unique example.

They consider also in the video the likely effects of a star swallowing anything ranging from Moon-sized to Jupiter-sized, but why stop there? How about Trappist-1-sized? That recent discovery, though also unique so far, must be upping our estimates of the prevalence of very compact systems available for disruption by stellar companions.
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HSchirmer
post May 27 2017, 10:27 AM
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QUOTE (ngunn @ May 27 2017, 10:26 AM) *
They consider also in the video the likely effects of a star swallowing anything ranging from Moon-sized to Jupiter-sized, but why stop there? How about Trappist-1-sized?


Yep, that could happen.
I think the emphasis is more about a gas-giant that forms at the snowline, and ends up eccentric,
which isn't uncommon, e.g. https://arxiv.org/ftp/arxiv/papers/1205/1205.2429.pdf

because you'd also expect exo-Galilean moons (e.g. icy bodies which would otherwise qualify as dwarf planets)
which remain after the planet is gone, but end up on short period comet style orbits,
so the 3% and 20% dips result from truly giant comets. (Giant as in a nucleus the size of Mars)

Think of an orbit something like HD 80606b
e.g. http://www.space.com/6364-exoplanet-sees-e...heat-waves.html
but an ice-ball might have a freeze-thaw water atmosphere, and perhaps a freeze-thaw ocean...
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Mongo
post Jun 14 2017, 02:49 PM
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Another dip starting?

From Kepler Star KIC 8462852 Amateur Photometry Monitoring Project:


Attached thumbnail(s)
Attached Image
 
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Mongo
post Jun 19 2017, 01:56 AM
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A short update on the current dip, so far it seems to be tracking the previous dip on days 1566-1569.

https://www.youtube.com/watch?v=U30v_jlk3GY
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HSchirmer
post Dec 11 2017, 02:24 PM
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QUOTE (Mongo @ Jun 19 2017, 01:56 AM) *
A short update on the current dip, so far it seems to be tracking the previous dip on days 1566-1569.

https://www.youtube.com/watch?v=U30v_jlk3GY


And, another dip now in December 2017.

Most recent paper from discover's team is considering a theory about breakup of comets on sungrazing orbits-

Modelling the KIC8462852 light curves: compatibility of the dips and secular dimming with an exocomet interpretation
M. C. Wyatt, R. van Lieshout, G. M. Kennedy, T. S. Boyajian
http://www.ast.cam.ac.uk/~wyatt/wvkb17.pdf
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HSchirmer
post Dec 30 2017, 02:34 PM
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New paper coming out by Boyajian et al, with hundreds of co-authors, embargoed until Jan 3rd.
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alan
post Jan 5 2018, 08:57 PM
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New Data Debunks Alien Megastructure Theory on the ‘Most Mysterious Star in the Universe’

“Dust is most likely the reason why the star’s light appears to dim and brighten. The new data shows that different colors of light are being blocked at different intensities. Therefore, whatever is passing between us and the star is not opaque, as would be expected from a planet or alien megastructure,” Boyajian said.

https://www.lsu.edu/mediacenter/news/2018/0...oyajian_apj.php


The First Post-Kepler Brightness Dips of KIC 8462852

We distinguish four main 1-2.5% dips, named "Elsie," "Celeste," "Skara Brae," and "Angkor", which persist on timescales from several days to weeks. Our main results so far are: (i) there are no apparent changes of the stellar spectrum or polarization during the dips; (ii) the multiband photometry of the dips shows differential reddening favoring non-grey extinction. Therefore, our data are inconsistent with dip models that invoke optically thick material, but rather they are in-line with predictions for an occulter consisting primarily of ordinary dust, where much of the material must be optically thin with a size scale <<1um, and may also be consistent with models invoking variations intrinsic to the stellar photosphere. Notably, our data do not place constraints on the color of the longer-term "secular" dimming, which may be caused by independent processes, or probe different regimes of a single process.

https://arxiv.org/abs/1801.00732


Non-grey dimming events of KIC 8462852 from GTC spectrophotometry

We report ground-based spectrophotometry of KIC 8462852, during its first dimming events since the end of the Kepler mission. The dimmings show a clear colour-signature, and are deeper in visual blue wavelengths than in red ones. The flux loss' wavelength dependency can be described with an \AA ngstr\"om absorption coefficient of 2.19±0.45, which is compatible with absorption by optically thin dust with particle sizes on the order of 0.0015 to 0.15 μm. These particles would be smaller than is required to be resistant against blow-out by radiation pressure when close to the star. During occultation events, these particles must be replenished on time-scales of days. If dust is indeed the source of KIC 8462852's dimming events, deeper dimming events should show more neutral colours, as is expected from optically thick absorbers.

https://arxiv.org/abs/1801.00720


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stevesliva
post Jan 8 2018, 09:58 PM
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QUOTE (stevesliva @ Jan 27 2016, 03:47 PM) *
I have been thinking a lot about the geometry and the rotation rate's signature in the light curve. Perhaps we're seeing a pole in the earth-facing hemisphere, and we're seeing a polar hood form and dissipate, with just part of the hood rotating out of view, to give the big dip some brighter shoulders. I think along these lines because crazy stuff like enormous starspots or metal clouds would show that 0.88 day rotation.

Polar phenomena like aurorae-- who knows what would cause a very transient one on a star? Not me. But it's interesting to think about, and do you call that endogenous if the trigger is exogenous, like on earth?


I am now even more inclined to think this is effectively weather, in the star's own atmosphere. But like aurorae, I suppose it could be an atmospheric phenomenon with an external trigger. If the star has a small companion that it's consuming... that sort of "weather."
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