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KIC 8462852 Observations
Hungry4info
post Jan 28 2016, 10:11 PM
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Response from Schaefer. Rather direct...
http://www.centauri-dreams.org/?p=34933


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JRehling
post Jan 29 2016, 12:24 AM
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It seems like Schaefer has a definitively better grasp of the issue than his detractor.

I'm not sure why he said, "This dimming from the DASCH data is just a long-time scale version of the dimming also seen with the Kepler spacecraft…" but that may be nitpicking. Given one star and two anomalies, there's good reason to suspect that the anomalies might be related, but one of these, over a long timescale, is monotonic and the other, over a short timescale, was weirdly varying, and it's not certain that one is a version of the other.

However, I think we can go back to seeing this as a three-part mystery: The century-long dimming, the planet-like dimming event at one point, and the weird, up-and-down dimming about 740 days later.
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Gerald
post Jan 29 2016, 09:11 AM
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Since intrinsic causes are considered unikely, as well as large numbers of large comets, but a single unknown cause is supposed in Schaefer's initial paper, I'm wondering whether kind of a Bok globule moving in front of the star, or interacting with the star, could catch the observations. Interaction might cause some detectable wobble, and is less likely due to lack of an infrared signature.
To explain lack of absorption and emission lines, the presumed globule should have been swept free of gas for some reason.

So, kind of an almost gas-free Bok globule moving into the line of sight could be a construct explaining the observation.
Observation of visually close-by background stars might help to discern this kind of scenarios.
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JRehling
post Jan 29 2016, 07:37 PM
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A Reddit chat about this phenomenon raised, and summarily rejected, the idea of a Bok globule:

https://www.reddit.com/r/IAmA/comments/3set...sts_and_planet/

The reasoning given is: "we can constrain the position of the globs to be as far as Jupiter is to our Sun, but likely not much farther."

Given the brevity of the response, I'm not sure if all parties understood one another. There is discussion of circumstellar clumps in the Boyajian, et al, paper, but I'm not sure how interstellar clumps are excluded.
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JRehling
post Jan 29 2016, 09:39 PM
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I've found a statement of the problem with an "interstellar" explanation – that the occulting material/objects are located between Tabby's star and Earth. It's posted here:

http://sites.psu.edu/astrowright/2015/10/1...heres-the-flux/

"It’s very hard to get an interstellar occulter model to work. In order for the occulter to be physically small, it would have to be much closer to the Solar System than the star. In that case, the parallax from Kepler’s motion around the Sun would enter into things, and you’re back to large mutual velocities, which means short timescales."
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dvandorn
post Jan 30 2016, 01:38 AM
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A perfectly aligned stream of cometary bodies (or at least bodies of similar size), that was gravitationally liberated from some star system by a passing star or other massive body, could appear dark from Earth if there is no nearby star to light it up, and could be narrow enough that it would not occult other stars in the nearby starfield. It's the latter constraint, the lack of any abnormal dimming in the nearby starfield, that constrains the option of interstellar occulters more than anything else, but a one-in-a-billion configuration might result in such a stream of small bodies that would only occult the one star. Variations in its density along the stream could result in the odd ups and downs we are seeing -- the stream might be passing between us for some time, but only the densest stretches of it are detectable.

It's just not a very likely explanation, I fear.


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Gerald
post Jan 30 2016, 03:17 AM
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It cannot be anything likely, since otherwise we would see many similar copies elsewhere.

The next idea I could offer would be two or more filters of rotating polarisation due to aligned interstellar dust in magnetic fields to explain the short dimming peaks.
As strange as it sounds, it seems something along these lines has been considered already, although maybe for different reasons.
(Pete Mancini, October 21, 2015 at 3:07 pm, in AstroWright)

Other ideas would be bandwidth-limited pulses similar to a sinc pulse by adding up oscillations of neightbouring frequencies, since we see both, small high-frequency oscillations and sudden peaks of dimming, e.g. by intrinsic infrasound waves,
and Feigenbaum-ish bifurcation of a dynamical system hinted to by double- or triple dimming peaks, the physical realization unknown as of yet.

So we have the choice between various unlikely scenarios.
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JRehling
post Jan 30 2016, 03:55 AM
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A problem with comets that are dark and far from any star is that they would therefore be cold and not generating tails, and therefore not producing so large of a cross section.

The same explanation could work if one imagines more material, but one of the appeals of the comet explanation in the first place was that they can generate a lot of cross section when warm.
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Gerald
post Jan 30 2016, 11:02 AM
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The "it cannot be a young star due to the distance from star formation regions" argument, when thinking at intrinsic reasons, could be overcome by assuming a recent merger of a binary, or swallowing of a brown dwarf companion.
Such a merger may also explain the short rotation period.

When persuing the occultation scenario, the dimming events could be used to reconstruct properties of the structure of the cloud.
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dudley
post Jan 30 2016, 05:21 PM
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The luminosity class of KIC 8462852 seems to be consistently given as V-IV. Does this indicate a star of considerable age, still on the main sequence, but soon, in the astronomical scale of things, to leave it? If so, the solar system debris associated with young stars, and something similar to our late heavy bombardment seem unlikely to play a part in the solution of this mystery.
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HSchirmer
post Jan 30 2016, 07:19 PM
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QUOTE (dudley @ Jan 30 2016, 05:21 PM) *
The luminosity class of KIC 8462852 seems to be consistently given as V-IV. Does this indicate a star of considerable age, still on the main sequence, but soon, in the astronomical scale of things, to leave it? If so, the solar system debris associated with young stars, and something similar to our late heavy bombardment seem unlikely to play a part in the solution of this mystery.


Ok, I'm curious now, is there any definite framework or theory to correlate planetary arrangments and how long solar system debris stay around?
For example, IIRC, systems with hot Jupiters are assumed to have shovel everything else into the parent star in relatively short time frame, but is there any theory that predicts stability ranges? E.g. "one gas giant is stable for 2 G yr, but because of resonance issues, two gas giants are only stable for 2 M yr?" Or, " a system with a gas giant should eject planetismals out to stable Oort orbits in 100 M yr, but a system with ice giants will take 900 M yr to 'blenderize' the planetismals into dust."
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Gerald
post Jan 30 2016, 09:08 PM
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I'm still trying to find out some hint to the age of KIC 8462852. There seems to be some correlation between age and rotational period, saying that fast-rotating sun-like stars are more likely to be young than slowly-rotating, more detail e.g. here.
A rotation period of only 0.88 days would be a vague hint, that KIC 8462852 might be relatively young.
A second indicator of age is metallicity. It's estimated to be low for KIC 8462852 (0 +/- 0.1, the '-' doesn't make much sense). Another vague hint to not being too old (less than about half the main sequence lifetime). But data may be inaccurate.
A third indication is the mass. Since KIC 8462852 is assumed to be heavier (by a factor of 1.43, according to Wikipedia), its total lifetime should be shorter than that of the Sun.
More precise: The estimated main sequence lifetime for a 1.43 solar masses star is 10e10 x 1.43^(-2.5) years = 4.1e9 years, hence below the age of our solar system.

The radius of 1.58 solar radii, however, is larger than that of a typical main sequence star of 1.43 solar masses , although this might partially be explained by some oblateness due to rapid rotation. But generally, stars leaving the main sequence towards giants are near the end of their lifetime.
Luminosity may be used as another indicator of age. Stars tend to brighten over time. But oblateness needs to be considered, when inferring the luminosity.


... regarding lifetimes of planetary systems, this has been investigated extensively. Roughly speaking it's more stable for very different orbital radii.
So I'd say an instability can be constructed after any given timespan.
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dudley
post Jan 31 2016, 12:53 AM
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I believe the metallicity figure is compared to that of the Sun, which is set at zero. If so, KIC 8462852 is apparently thought to have the same metallicity as the Sun, plus or minus 10 percent.
I read that the typical rotation velocity of an F0 star is 95 km/sec. and an F5 star 25 km/sec. At 84 km/sec. KIC 8462852 (an F3 star) falls between these two.
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Gerald
post Jan 31 2016, 02:40 AM
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Metallicity:
QUOTE
In astronomy and physical cosmology, the metallicity or Z is the fraction of mass of a star or other kind of astronomical object that is not in hydrogen (X) or helium (Y)

Metallicity of the Sun:
QUOTE
...The new solar abundances are lower than previously recommended values and the present solar metallicity, Z, and Z/X, decrease to Z = 0.0122 and Z/X = 0.0165 respectively ...

The 84 km/s is a little faster than average F3 as far as I understand this paper about rotation velocities of stars, e.g. fig 1.6:
QUOTE
Note, that the open cluster F dwarfs rotate more rapidly than their older, field counterparts.

The more likely interpretation would then be, that KIC 8462852 is considerably younger than 4.1 Gy.
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JRehling
post Feb 1 2016, 05:35 PM
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Over the weekend, I downloaded the light curve data and started examining the various dips / dimming events. As noted in v2 (but not v1) of the "Where's the Flux?" paper, there are 10 events that are fairly significant in size (≥0.2%). As the biggest one is about 100x bigger than the smallest, the discussion has focused on the bigger dips, but all 10 are interesting.

What's odd is that the dips show different shapes, but are easily grouped into about four categories – three types that repeat three times each and do not look planetlike, and one that looks more planetlike, but is asymmetrical, and occurs once. I will probably organize my observations into a blog post, but some of the main points are:

1) The ones that appear similar are probably not, for the most part, repeat transits of single objects. They are similar in shape but very different in magnitude, and the triple occurrences are not spaced out evenly in time.

2) The types are:
T1) The single planetlike dimming event, which is very large.
T2) V-shaped dips, a shape which is in other systems indicative of star-star eclipses.
T3) A small peak followed by a brief brightening, then a larger peak. Interestingly, there are none in the opposite direction (large, then small). Two of these are very large.
T4) A symmetrical sequence of three subpeaks, with the largest one in the middle. This is not what you would see if a Saturn-like planet transited the Sun, but might see if a truly massive ring system transited a star. One of these is very large.

By "very large," I mean the maximum decrease in illumination is larger than could be explained by a planet more than twice the diameter of Jupiter.

The (T4) cases make me wonder if we're seeing something like the Jovian or Saturnian systems in a state of formation, with the respective satellite systems still like huge rings/toruses around the protoplanet. To show no IR excess might (my supposition) be explained if these systems are very far out. Perhaps some of the other types could be explained by various geometries. Then, the larger explanation would be that Tabby's star has a lot of giant protoplanet systems that are still evolving, with their Ganymedes, Titans, and Callistos, etc., still in the form of massive dust rings. What would need to be explained, still, is:

E1) Could this be consistent with the lack of IR excess if those systems are very far out? They still would be retransmitting the blocked light as longer-wavelength radiation, but perhaps at longer wavelengths than have yet been observed.

E2) Could one F dwarf star plausibly have so many giant planet systems? If we are witnessing ten of them transit in a span of four years, then the star would seemingly have a huge number of other ones that we didn't happen to observe. Is this plausible?

Fomalhaut has a dust ring at ~130 AU, but it emits a lot of IR, which indicates that Tabby's star is something different than a Fomalhaut system that simply happens to be seen edge-on.
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