APOD: The Extraordinary Spiral in LL Pegasi (2021 Nov 29)

[APOD Robot]

The Extraordinary Spiral in LL Pegasi

Explanation: What created the strange spiral structure on the upper left? No one is sure, although it is likely related to a star in a binary star system entering the planetary nebula phase, when its outer atmosphere is ejected. The huge spiral spans about a third of a light year across and, winding four or five complete turns, has a regularity that is without precedent. Given the expansion rate of the spiral gas, a new layer must appear about every 800 years, a close match to the time it takes for the two stars to orbit each other. The star system that created it is most commonly known as LL Pegasi, but also AFGL 3068 and IRAS 23166+1655. The featured image was taken in near-infrared light by the Hubble Space Telescope. Why the [url=http://flyingpudding.com/projects/florets/applet/" >spiral glows</a> is <a href="https://www.intermountainpet.com/hs-fs/ ... -heads.jpg]itself a mystery[/url], with a leading hypothesis being illumination by light reflected from nearby stars.

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[Ann]

NGC 2857. Image: SDSS/Donald Pelletier.

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LL Pegasi. Image Credit: NASA, ESA, Hubble, HLA; Processing & Copyright: Jonathan Lodge

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Once in a while you find the perfect spiral out there.

How did it get that way? Who knows?

Ann

[VictorBorun]

NGC 2857. Image: SDSS/Donald Pelletier.

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LL Pegasi. Image Credit: NASA, ESA, Hubble, HLA; Processing & Copyright: Jonathan Lodge

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Once in a while you find the perfect spiral out there.

How did it get that way? Who knows?

Ann

wait a minute, what does perfect spiral mean here?
The distance between the two arms of NGC 2857 is growing with the distance from the centre.
The distance between the nth coil and (n+1)th coil of the posted image is constant.

[thearborist]

An amazing image. Is this object planar or spherical? How can you tell?

[Ann]

NGC 2857. Image: SDSS/Donald Pelletier.

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LL Pegasi. Image Credit: NASA, ESA, Hubble, HLA; Processing & Copyright: Jonathan Lodge

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Once in a while you find the perfect spiral out there.

How did it get that way? Who knows?

Ann

wait a minute, what does perfect spiral mean here?
The distance between the two arms of NGC 2857 is growing with the distance from the centre.
The distance between the nth coil and (n+1)th coil of the posted image is constant.

Good point, Victor!

Ann

[VictorBorun]

An amazing image. Is this object planar or spherical? How can you tell?

a cylinder roll with equal thickness of each layer would be hard to explain.
But the drawing tool does not have to be a pencil-thin jet (veering around and throwing a coil after coil).
If the dusty jet is thick and every portion it shoots in a short period of time is a spheric segment then what we the observers can mostly see
of each spheric segment is an arc in the plane perpendicular to our line of sight.

The spiral is not necessarily thin-planar; it may just seem so.

[VictorBorun]

somehow the northern segments of the coils are compressed in relation to the other segments

[JohnD]

My first thought on seeing that image was - Einstein Ring!

Could a black hole be between that star and us, causing the strange spiral to appear, when the star behind is relatively normal?

JOhn

[VictorBorun]

My first thought on seeing that image was - Einstein Ring!

Could a black hole be between that star and us, causing the strange spiral to appear, when the star behind is relatively normal?

JOhn

Then the lensing BH would have to be on the line between us and the centre of that stellar system.
Then this point would micro-lense some objects off that line.
Like the bright star thought to side-light the dust of this object.

Wait, that star is smeared and in the direction perpendicular to the direction to the center of lensing!

[JohnD]

"Relatively" to other stars, of course!

Thank you Victor! Surely the distortion of the adjacent star is the usual diffraction spikes, like the one to left. Is there any additional "smearing".

And which of those is the sidelighter? The right hand one looks as if it's a lot nearer than the spiral, probably because it's brighter. If nearer, wouldn't gravity lens its light much less?
John

[VictorBorun]

"Relatively" to other stars, of course!

Thank you Victor! Surely the distortion of the adjacent star is the usual diffraction spikes, like the one to left. Is there any additional "smearing".

And which of those is the sidelighter? The right hand one looks as if it's a lot nearer than the spiral, probably because it's brighter. If nearer, wouldn't gravity lens its light much less?
John

The obvious side-lighter has to be close to the dust it illuminates.
Therefore both would be at the same distance from us.
Therefore both would be lensed, according to their distance from the optical axis of the gravi-lens

[Chris Peterson]

Why the spiral glows is itself a mystery, with a leading hypothesis being illumination by light reflected from nearby stars.

That's odd. It should be trivial to distinguish between an emission nebula and a reflection nebula by a simple spectroscopic examination, or even a set of narrowband images. Maybe this object simply hasn't received that attention yet?

[neufer]

Millimeter ALMA image

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Click to play embedded YouTube video.

<<Although it looks like the pattern of a shell on the beach, this intriguing spiral is in fact astronomical in nature. The Atacama Large Millimeter/submillimeter Array (ALMA) captured this remarkable image of a binary star system, where two stars — LL Pegasi and its companion — are locked in a stellar waltz, orbiting around their common centre of gravity. The old star LL Pegasi is continuously losing gaseous material as it evolves into a planetary nebula, and the distinct spiral shape is the imprint made by the stars orbiting in this gas.

The spiral spans light-years and winds around with extraordinary regularity. Based on the expansion rate of the spiralling gas, astronomers estimate that a new “layer” appears every 800 years — approximately the same time it takes for the two stars to complete one orbit around each other.

LL Pegasi was first highlighted about 10 years ago when the NASA/ESA Hubble Space Telescope obtained a picture of the almost-perfect spiral structure. This was the first time a spiral pattern had been found in material surrounding an old star. Now, ALMA’s observations, of which this image only shows one “cross-section”, have added an extra dimension to reveal the exquisitely-ordered 3D geometry of the spiral pattern.>>
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This video shows the observed structure surrounding the binary star system LL Pegasi, as seen by the Atacama Large Millimeter/submillimeter Array (ALMA). ALMA’s observations were able to reveal the exquisitely-ordered 3D geometry of the spiral pattern (seen to the left). The observations match almost perfectly with the predicted theoretical model (to the right).

[orin stepanek]

Looks almost like a galsxy!

[JohnD]

Neufer's post above links to the ESO site where it says that, "Based on the expansion rate of the spiralling gas, astronomers estimate that a new “layer” appears every 800 years" A mis-description surely as the layers of the spiral are continuous, not from 800-year pulses? Do they mean, the layers 800 light years apart? Or that the turns of the spiral take 800 years to form.

Neufer also links to the video, which shows that the central 'blob' is the end of a cigar-shaped object, with its long axis pointing straight at us. Why a _cylinder_ of star-stuff? There are cylindrical star formations, Menzel3, the Ant, for instance, that is supposed to originate from a binary system, but very different.

Could we imagine a BH in front of the central blob, a binary BH with an orbital period that focusses a simpler nebula from LL Pegasi into apparent shells? A binary BH behind LL would require another large nebula behind that, whose light could be similarly focussed towards us, but that presumes more coincidences in the Heavens that even Hamlet could dream of?

[DRN]

Sorry for the laymen question but: Why are the progenitor binary stars (LL Pegasi) not visible in this image? Is the nebula so thick in near-IR that it masks them? That is reverse from what I would expect from IR sources so why are they not visible?
Also, is the other star (with the diffraction spikes) a foreground/background object or is it nearby to the spiral? That was assumed in one of the posts but I didn't see it mentioned in the text for the description.

Very beautiful object, regardless the explanation!

[Eclectic Man]

The spiral is not completely even, in that there are parts which have a smaller radius of curvature than the adjacent arcs to either side. These 'corners' appear to my eyes to be regular and repeated through the layers of the spiral so that each layer has the same number of corners, and in roughly the same place relative to the centre of the image.

Would this indicate that there are two (or more) bodies orbiting each other, acting in conjunction to create the 'spiral', and that when they are aligned there is a different amount of energy propelling gas / dust to create the spiral than when they are not?

I assume that we are, as it were, looking 'down' on one of the poles of the system to get this amazing view.

Just a thought.

[VictorBorun]

Then came neufer and told us how the things really are.

So the star I saw as a side lamp is much nearer and sheds no light upon this loosely rolled soot cigar pointing at us

[VictorBorun]

my fitting

[neufer]

Image of The Cat's Eye Nebula (NGC 6543) processed to reveal the concentric rings surrounding the inner core. Also visible are the linear structures, possibly caused by processing jets from a binary central star system.

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<<The Cat's Eye Nebula (NGC 6543) is structurally a very complex nebula, and the mechanism or mechanisms that have given rise to its complicated morphology are not well understood. The central bright part of the nebula consists of the inner elongated bubble (inner ellipse) filled with hot gas. It, in turn, is nested into a pair of larger spherical bubbles conjoined together along their waist. The waist is observed as the second larger ellipse lying perpendicular to the bubble with hot gas.

The structure of the bright portion of the nebula is primarily caused by the interaction of a fast stellar wind being emitted by the central PNN with the visible material ejected during the formation of the nebula. This interaction causes the emission of X-rays discussed above. The stellar wind, blowing with the velocity as high as 1900 km/s, has 'hollowed out' the inner bubble of the nebula, and appears to have burst the bubble at both ends.

It is also suspected that the central WR:+O7 spectral class PNN star, HD 1064963 / BD +66 1066 / PPM 20679 of the nebula may be generated by a binary star. The existence of an accretion disk caused by mass transfer between the two components of the system may give rise to polar jets, which would interact with previously ejected material. Over time, the direction of the polar jets would vary due to precession.

Outside the bright inner portion of the nebula, there are a series of concentric rings, thought to have been ejected before the formation of the planetary nebula, while the star was on the asymptotic giant branch of the Hertzsprung–Russell diagram. These rings are very evenly spaced, suggesting that the mechanism responsible for their formation ejected them at very regular intervals and at very similar speeds. The total mass of the rings is about 0.1 solar masses. The pulsations that formed the rings probably started 15,000 years ago and ceased about 1000 years ago, when the formation of the bright central part began (see above).

Further, a large faint halo extends to large distances from the star. The halo again predates the formation of the main nebula. The mass of the halo is estimated as 0.26–0.92 solar masses.>>

[VictorBorun]

Outside the bright inner portion of the nebula, there are a series of concentric rings, thought to have been ejected before the formation of the planetary nebula, while the star was on the asymptotic giant branch of the Hertzsprung–Russell diagram. These rings are very evenly spaced, suggesting that the mechanism responsible for their formation ejected them at very regular intervals and at very similar speeds. The total mass of the rings is about 0.1 solar masses. The pulsations that formed the rings probably started 15,000 years ago and ceased about 1000 years ago, when the formation of the bright central part began (see above).

Further, a large faint halo extends to large distances from the star. The halo again predates the formation of the main nebula. The mass of the halo is estimated as 0.26–0.92 solar masses.>>

It's hard to tell concentric rings from spiral coils when the series is eclipsed by later jets

[alter-ego]

Then came neufer and told us how the things really are.

So the star I saw as a side lamp is much nearer and sheds no light upon this loosely rolled soot cigar pointing at us

Prior to reading your post, I researched this a bit and agree with that conclusion, and for other reasons. Here are some details that I don't see presented:
1. That bright star, GSC01713_00903, is 893 pc ± 14 pc (GAIA, EDR3) from us, and 1 kpc is the estimated to distance to LL Persei,
→ separation = 349 ly ± ≈50 ly
→ Apparent magnitude, m_V = 12.2
→ Calculate absolute magnitude, M_V = 2.4, (Sun M_V = 4.8)
→ GSC01713_00903 is ~9x brighter than the Sun
→ Apparent magnitude, m_V as viewed from LL Persei ≈ +7.5
► Given the apparent low level of illumination, I don't see that's it's possible for GSC01713_00903 light up the nebula enough to see.
2. The illumination does appear to come from the right (coincidentally toward GSC01713_00903), but this can't be.
► Given the apparent line-of-sight displacement is < 1 ly, and the separation between LL Persei ~350 ly, then the illumination must be face on, not from the side.
3. A 2006 paper, A Binary-Induced Pinwheel Outflow from the Extreme Carbon Star, AFGL 3068 instead suggests the illumination is from the stars in the Galactic plane.
 

[alter-ego]

Sorry for the laymen question but: Why are the progenitor binary stars (LL Pegasi) not visible in this image? Is the nebula so thick in near-IR that it masks them? That is reverse from what I would expect from IR sources so why are they not visible?

The dust and expelled gas is very thick, and in fact, they are visible In NIR images with 1.8um to 3um filters, but not for images using filters ≤ 814nm filters which is what you may be referring to.
Here are Keck AO images resolving the closely separated (0.11") pair:
 

From a 2006 paper, A Binary-Induced Pinwheel Outflow from the Extreme Carbon Star, AFGL 3068

Also, is the other star (with the diffraction spikes) a foreground/background object or is it nearby to the spiral? That was assumed in one of the posts but I didn't see it mentioned in the text for the description.

It is a foreground star about 350 ly closer to us than LL Persei (at 1kpc distance.) The star is most likely separated too far and any illumination of the nebula is negligible.

[VictorBorun]

wiki says LL Pegasi is even farther away, 1,300 pc

I wonder how far it is from the disk of Milky Way and what bright stars are main illuminators

It's a surprise for me to know how dense the soot around a carbon red giant is, to cover it and its companion so thick that the Milky Way (dominating the starry sky outside the galaxy disk) is brighter than the stars within the shells.

Compare with an observer at Kuiper belt. There is our Sun, there are Sirius and Vega. The Milky Way is not dominating at all as a lamp for Pluto's hills.

[Astronymus]

somehow the northern segments of the coils are compressed in relation to the other segments
LLPegasi_HubbleLodge_1926..png

First guess would be that's the direction of movement by the system. The cloud falls behind.