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KIC 8462852 Observations
JRehling
post Jan 10 2018, 10:28 PM
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KIC 8462852 has 4.7x solar luminosity and has a surface temperature of 6750K. That's far above the boiling point of iron and carbon. Any substance that is credibly abundant in its atmosphere would quickly be heated to gaseous, and one gas that's 6750K will radiate the same blackbody radiation as another. Any dust or chemicals that block the light and are high enough up from the star to avoid radiating would radiate, instead, in the IR, producing an IR excess that has not been observed.

This also applies to gargantuan sunspots. Sunspots aren't black, but cooler areas that radiate in redder wavelengths. If you image sunspots on the Sun in IR, you see them as dark but not black regions, and if you sub IR for red in an RGB image, they look red.

So I don't think any explanations in terms of atmospheric dynamics proximate to the star will buy us a 22% dip in luminosity with no IR excess.
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HSchirmer
post Jan 10 2018, 11:30 PM
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QUOTE (stevesliva @ Jan 8 2018, 09:58 PM) *
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."


Well, there is one really interesting part about stellar "weather", Tabby's star is right near the balance for convection versus conduction.

When you look at our sun, the surface you actually see is composed of convective cells. (Heat causes the matter to criculate).
However, at the core, conduction (radiation) occurs, not convection i.e. the matter stsys still, the photos circulate)
As stars get bigger, the outer convective layer gets smaller and smaller, until it becomes unstable and vanishes.

Well, Tabby's star is an F3, which should just be at the cusp between total conduction and the faint hint of convection cells.

So, it's at least POSSIBLE that we're seeing transient "convection" ripple across the face of the star.
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Gerald
post Jan 11 2018, 12:22 AM
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Might be, that I'm not quite up to date here, but could you provide a link to a paper, that explains, why the inner of an F3 star can be assumed to be dominated by conduction? Within my maybe not quite complete picture of the interior of main sequence stars, I'd expected, that heat transfer is mainly dominated by radiative transfer, which I'm understanding as a diffusion process taking millions of years to transport energy from the stellar core to the convective outer layers. A reference to a stellar model which shows the core of an F3 being essentially a white dwarf star would be sufficient, since white dwarfs have been assumed to be dense enough to allow for thermal conduction as the main heat transfer process, e.g. this paper, p.858:
QUOTE
In the interior of a white dwarf heat conduction by the highly degenerate electrons is the dominant mechanism, leading to a very small temperature gradient and an almost isothermal interior, a fact exploited widely by early calculations of white dwarf evolution.
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HSchirmer
post Jan 11 2018, 02:33 AM
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QUOTE (Gerald @ Jan 11 2018, 01:22 AM) *
Might be, that I'm not quite up to date here, but could you provide a link to a paper, that explains, why the inner of an F3 star can be assumed to be dominated by conduction? Within my maybe not quite complete picture of the interior of main sequence stars, I'd expected, that heat transfer is mainly dominated by radiative transfer, which I'm understanding as a diffusion process taking millions of years to transport energy from the stellar core to the convective outer layers. A reference to a stellar model which shows the core of an F3 being essentially a white dwarf star would be sufficient, since white dwarfs have been assumed to be dense enough to allow for thermal conduction as the main heat transfer process, e.g. this paper, p.858:


-edited to clarify-
Point taken, it's actually a mix of radiative transfer and conduction
Radiative transfer by fusion derived gamma rays random-walking around, scattering until they exit the star as photos in the visible light range,
Conduction by plasma as protons and electrons bouncing around due to that gamma-ray stirring.

Anway, the emphasis was about energy moving through matter, versus the bulk movement of matter.
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Gerald
post Jan 11 2018, 01:21 PM
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Ok, thanks, this fits better with my understanding of stellar physics.
So, are you suggesting something like giant star spots? I'd think, in that case, we should see changes in the black body component of its emission, or in the intensity of spectral emission lines of "metals", provided the metallicity isn't too low to perform that kind of spectral analysis.
The variability of temperature should result in a variable star in terms of stellar class, i.e. some kind of intrisically variable star. But, as far as I understand, KIC 8462852, doesn't show the intrinsic variability needed to explain the dips in brightness, see this paper, subsection 3.2. However, the paper might not be quite water-proof in this respect.
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HSchirmer
post Jan 11 2018, 06:22 PM
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QUOTE (Gerald @ Jan 11 2018, 01:21 PM) *
Ok, thanks, this fits better with my understanding of stellar physics.

So, are you suggesting something like giant star spots?

But, as far as I understand, KIC 8462852, doesn't show the intrinsic variability needed to explain the dips in brightness, see this paper, subsection 3.2. However, the paper might not be quite water-proof in this respect.


Not specifically, and not accepting any particular theory, e.g.



just find it interesting that
THIS star is doing somethig weird,
and
Tabby's star should be near the transition point between- 1 layer (radiative) and 2 layers, (radiative & convective)
before the light/energy reaches the surface and escapes into space.

My point is only based on gedanken experiment- it would not be "sunspots" e.g. magnetic effects, but
currently (AFAIK) it's an F3 star, so the radiative zone is directly exposed to the sky.

Imagine a transient convective layer forming in the coolest ourter plasma-
That would trigger a phase change from a 1 layer star to a 2 layer star.

So, in terms of stellar "weather" this is "instability driven convection- a transient troposphere
thnk
"stellar thunderstorm" - an instability driven convection that dies out.
or
"stella derecho" - an instability driven convection that translates across the surface.
or
"stellar typhoon" - an instability driven convection that incororates angular momentum.
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Gerald
post Jan 12 2018, 02:01 AM
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QUOTE (HSchirmer @ Jan 11 2018, 07:22 PM) *
... it's an F3 star, so the radiative zone is directly exposed to the sky.

According to Wikipedia, it's an F3V star, i.e. a main sequence star, with an estimated 1.43 solar masses.
So, we are above the 1.2 solar masses, where the upper bound for the existence of a significant outer convective zone is assumed. I can follow to that point.
But if there would be an instability, as you propose, wouldn't we get significant changes in the spectra instead just a dimming? In the simplest case, the mean surface temperature would change, in order to explain the dimming, which would then result in a shift of the Planck portion of the emission spectrum. At the same time, such a shift in the temperature would mean a shift in the spectral type, hence kind of an intrinsic variable star. Other features of the spectrum would indicate a change of surface temperature, too.
So, even if your thought experiment would be physically possible, can this be made consistent with actual observations, especially with the observations of the spectral type of the star, a variability of which doesn't appear to be reported?
-- We are actually in the realms of MHD, which is a little beyond my current skills.
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stevesliva
post Jan 14 2018, 03:22 AM
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QUOTE (JRehling @ Jan 10 2018, 05:28 PM) *
So I don't think any explanations in terms of atmospheric dynamics proximate to the star will buy us a 22% dip in luminosity with no IR excess.


I have to quote the summary, but I just want to thank you for the rest of the comments and assure you that I appreciate them! I think I just keep returning to this feeling that, even though this is a kepler star, that the star's observed behavior is more likely to be attributable to the star itself than its, uh, panoply. I'm just seizing on the transparency/dustiness/ephermeralness of the occulting body to decide there isn't such a separate thing at all. You have good reasons why that's not likely.
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Gerald
post Jan 14 2018, 07:35 PM
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If it's on the star itself, it needs to be very symmetrical, since otherwise it would oscillate approximately with the reported rotational period of 0.8797 days.
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stevesliva
post Jan 16 2018, 07:59 PM
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QUOTE (Gerald @ Jan 14 2018, 02:35 PM) *
If it's on the star itself, it needs to be very symmetrical, since otherwise it would oscillate approximately with the reported rotational period of 0.8797 days.


Right, big polar circular... thing. biggrin.gif A "hood," say.

I suppose the dust could be at the pole and NOT on the star and be similar. Perhaps it's concentrated there at certain intervals relating to interactions with satellites.
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Gerald
post Jan 17 2018, 02:12 AM
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How does the presumed dust avoid falling into the star by gravity, and being heated up to vaporization and ionization? Wouldn't these large amounts of dust show up in fluctuating absorption and emission lines once they get close enough to the star to evaporate?
Do you presume a "star-grazing" satellite with a perihelion over one of the poles of the star? But then, how is the dust slowed down from 100s of km/s to almost zero to stay over the pole?
Wouldn't it be more reasonable to assume the dust being considerably farther away from the star?
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stevesliva
post Jan 17 2018, 02:40 AM
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Magic, magic, and magic, of course. I try to make it clear I invoke no expertise and no particular ardent tie to this, but... the irregular period of the dimming doesn't seem to suggest an orbit, but instead a more irregular interval. Or do I recall incorrectly? It's been longer, so perhaps there are predictable periods now. (?)

But, good reasons as to why the non-opaque stuff which is causing dimming is in an orbit. I posit ill-conceived hypotheses and get interesting counterpoints. I occasionally try to do that here without taking away from the more informed discussion.
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hendric
post Jan 18 2018, 04:48 PM
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But we don't know for sure if it's in orbit, since we don't have repeated events with similar profiles.

I suppose it's possible a polar hood that has N/S waves similar to the Earth's jet streams could be the cause, with the excursions towards the stellar equators being destroyed before the next rotation. But that would make the material have to travel very fast, and dissipate fast as well. Also, a star rotating as rapidly as that probably behaves very differently than our own Sun at 30ish days rotation. Maybe instead of magnetic flips every 11 years, it happens every 1-2 years, with the flips causing large areas of the star to suddenly go relatively dark. But I can't buy those kinds of plasma speeds across a whole star without it causing enormous magnetic activity - flares etc - that should be pretty obvious.


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