APOD: Ou4: A Giant Squid Nebula (2014 Jul 18)

[APOD Robot]

Ou4: A Giant Squid Nebula

Explanation: A mysterious, squid-like apparition, this nebula is very faint, but also very large in planet Earth's sky. In the mosaic image, composed with narrowband data from the 2.5 meter Isaac Newton Telescope, it spans some 2.5 full moons toward the constellation Cepheus. Recently discovered by French astro-imager Nicolas Outters, the remarkable nebula's bipolar shape and emission are consistent with it being a planetary nebula, the gaseous shroud of a dying sun-like star, but its actual distance and origin are unknown. A new investigation suggests Ou4 really lies within the emission region SH2-129 some 2,300 light-years away. Consistent with that scenario, the cosmic squid would represent a spectacular outflow of material driven by a triple system of hot, massive stars, cataloged as HR8119, seen near the center of the nebula. If so, this truly giant squid nebula would physically be nearly 50 light-years across.

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[geckzilla]

Interesting nebula highlighting the difficulty in measuring the distance to such objects. It's either a very close planetary nebula or a rather distant outflow without any particular name. I can't say it makes much sense to me either way.

[Ann]

The remarkable but exceedingly faint Ou4 nebula is extremely well brought out in today's APOD. The details of the shape of the nebula are excellent. But for me, with my strong color hangups, I find it hard to accept the glaringly green color of the stars, even though I can understand that this may be a side effect of the attempt to bring out as much OIII light as possible from the nebula.

Click to view full size image

Photo: Scott Rosen

I prefer this image by Scott Rosen, which was recently posted in the Observation Deck forum at Starship Asterisk*. Do read Scott Rosen's own description of how he made his image!

Ann

[Ann]

Interesting nebula highlighting the difficulty in measuring the distance to such objects. It's either a very close planetary nebula or a rather distant outflow without any particular name. I can't say it makes much sense to me either way.

My complete amateur impression is that Ou4 is not a planetary nebula. The reason is that the nebula is centered exactly on a hot, massive star, HR 8119. According to my software Guide, which quotes Bright Star Catalog, HR 8119 is the exciting star of the red Ha emission nebula, SH2-129. I find it too much of a coincidence that the blue-green OIII nebula, which is exactly centered on HR 8119, could be a nebula centered on a white dwarf exactly along the line of sight of HR 8119.

HR 8119 is undoubtedly a powerful star. According to Guide, which cites the most recent version of the Hipparcos Catalog, this bright sixth magnitude star (V = 5.728 ± 0.010) has a parallax of 0.76 ± 0.42 milliarcseconds, which suggests that its distance maybe about 4,000 light-years and its (reddened) V luminosity may be equal to about 8,000 Suns. Its proper motion is small too, reinforcing the impression that HR 8119 is a far-away, bright star. Guide classifies HR 8119 (or HD 202214) as a star of spectral class B0V. If this is correct, the star is still on the main sequence and bright for its spectral class. I have spent quite some time checking the color and the Hipparcos distance to hot blue stars, and my impression is that stars of spectral class B0V typically have a V luminosity between 1,000 and 2,000 Suns. For comparison, well-known Alnitak, the star "on the Betelgeuse side" of Orion's Belt, is a supergiant star of spectral class O9.5Ib, but it is about equally bright in V light as HD 202214, or about 8,500 times the Sun. Stars of the same spectral class may vary considerably in brightness, and HD 202214 is most likely a B0V whopper.

HR 8119 or HD 202214 is not a variable star, according to Guide. Or rather, its only variability has to do with the fact that it is a binary star, probably an eclipsing binary. But that strange green nebula certainly speaks of some kind of outburst in the past.

Ann

[Joules]

Do read Scott Rosen's own description of how he made his image!

So the Squid IS an emission nebula:

the Squid Nebula shines by the blue/green light of OIII - the forbidden line of doubly ionized Oxygen.

[RaygunRn]

Little Green Man Nebula?

[The Tremor]

Looks more like a graboid than a squid!

[geckzilla]

Interesting nebula highlighting the difficulty in measuring the distance to such objects. It's either a very close planetary nebula or a rather distant outflow without any particular name. I can't say it makes much sense to me either way.

My complete amateur impression is that Ou4 is not a planetary nebula. The reason is that the nebula is centered exactly on a hot, massive star, HR 8119. According to my software Guide, which quotes Bright Star Catalog, HR 8119 is the exciting star of the red Ha emission nebula, SH2-129. I find it too much of a coincidence that the blue-green OIII nebula, which is exactly centered on HR 8119, could be a nebula centered on a white dwarf exactly along the line of sight of HR 8119.

It doesn't have to be exactly centered on HR8119. It just has to be sort of near it. A dim star mixed up in a multiple star system could be difficult to find. This is a perfect example of human tendency to draw associations based on the appearance of a pattern. The paper mentioned in the description provides some evidence that it may be a giant, distant structure but it's worth noting that the authors could not rule out the other possibilities.

[Chris Peterson]

So the Squid IS an emission nebula:

I don't think it was suggested otherwise.

[ta152h0]

you never know what is in a box of chocolates, do you ?

[Ann]

Geck wrote:
The paper mentioned in the description provides some evidence that it may be a giant, distant structure but it's worth noting that the authors could not rule out the other possibilities.

Romano L.M. Corradi, Nicolas Grosso, Agnès Acker, Robert Greimel and Patrick Guillout wrote:
The observed radial velocities of Ou4 and its reddening are consistent with those of Sh 2-129 and HR 8119.
...
The apparent position of Ou4 and the properties studied in this work are consistent with the hypothesis that Ou4 is located inside the Sh 2-129 HII region, suggesting that it was launched some 90,000 yrs ago by HR 8119.
...
However, the alternate possibility that Ou4 is a bipolar planetary nebula, or the result of an eruptive event on a massive AGB or post--AGB star not yet identified, cannot be ruled out.

What the authors of the paper are saying is that everything they have been able to measure about Ou4 is consistent with the hypothesis that this nebula is inside the Sh2-129 region (the red emission nebula) and that the OIII nebula Ou4 was launched by HR 8119. They also say that they can't rule out the possibility that Ou4 may have nothing to do with HR 8119. To me, this doesn't mean that astronomers have no idea where Ou4 came from. To me it sounds as if all available evidence suggests that Ou4 was caused by some sort of outburst from HR 8119, and no available evidence suggests that it is a planetary nebula.

The authors wrote:
The improved determination of the distance to HR 8119 (composed of two B0 V and one B0.5 V stars) and Sh 2-129 is 712 pc.

I was surprised at the brightness of HR 8119. But there is now evidence that HR 8119 is only about half as far away as its (very uncertain) Hipparcos parallax suggests, about 2,300 light-years versus the previous estimate of about 4,300 light-years. Also HR 8119 is a triple star made up of three hot B0-type stars. Suddenly HR 8119 doesn't seem abnormally bright at all, although it must be pointed out that HR 8119 is a reddened star and therefore really brighter than than its reddened V magnitude suggests.

Ann

[Boomer12k]

Awesome, and fascinating discovery....

does not look "squid like" to me....more like..."The Flying, fickle, finger of fate"...

:---[===] *

[geckzilla]

For the squid-impaired.

[ta152h0]

scientist humor can be just as misterious

[Ann]

Great squid, Geck.

Click to view full size image

Photo: Marco Lorenzi

I feel the need to further defend my belief that Ou4 is an OIII structure created by hot bright triple star HR 8119. In the picture on the left, you can see the OIII halo around O-type star HD 148937. The OIII halo is faint, although not as faint as Ou4. But isn't it possible that still more hot bright stars may have have extremely faint OIII structures around themselves?

Ann

[geckzilla]

You wouldn't have to defend it if you didn't rely on belief. It's a little silly to be so firm on something that even the professionals who went to the trouble of taking some actual measurements are at least somewhat unsure themselves after their own efforts. I hope some more work is done on it so we can learn more about it. It's holding its secrets tightly, though.

[neufer]

To be fair, it is a pretty strange squid.

• A member of the class Cepheiopoda.

<<A squib is a brief satirical or witty piece of writing or speech, like a lampoon, or a short, sometimes humorous piece in a newspaper or magazine, used as a filler. It can be intended to ignite thinking and discourse by others on topics of theoretical importance.>>

[neufer]

Click to play embedded YouTube video.

<<The Homunculus Nebula is a reflection nebula surrounding the massive star system Eta Carinae. The nebula is embedded within the much larger Eta Carinae Nebula, an ionized hydrogen (H II) region. The Homunculus (from the Latin meaning Little Man) is believed to have been ejected in an enormous outburst from Eta Carinae. Light from this event reached Earth in 1841, creating a brightening event in the night sky which was visible from the Earth's surface at the time. During the event (as seen from Earth) Eta Carinae briefly became the second-brightest star in the sky, after Sirius; but the ejected gas and dust have since obscured much of its light. The massive — near supernova — explosion produced two polar lobes, and a large but thin equatorial disk, all moving outward at 670 km/s [~ 2.2 ly/millennium].>>

[Ann]

Geck wrote:
It doesn't have to be exactly centered on HR8119. It just has to be sort of near it. A dim star mixed up in a multiple star system could be difficult to find. This is a perfect example of human tendency to draw associations based on the appearance of a pattern.

I don't think the central star of a planetary nebula could be easily missed in this sort of environment. Bear in mind that if Ou4 is a planetary nebula, then it is a foreground object.

APOD Robot wrote:
Consistent with that scenario, the cosmic squid would represent a spectacular outflow of material driven by a triple system of hot, massive stars, cataloged as HR8119, seen near the center of the nebula. If so, this truly giant squid nebula would physically be nearly 50 light-years across.

So at the distance of Sharpless 129, Ou 4 would be nearly 50 light-years across. But according to Wikipedia, planetary nebulas are typically about one light-year across. So if Ou4 is a planetary nebula, it couldn't possibly be as big as 50 light-years, and it would have to be much closer to us than Sharpless 129 and HR 8119.

http://en.wikipedia.org/wiki/Planetary_ ... cteristics wrote:
A typical planetary nebula is roughly one light year across

Click to view full size image

The central star of a planetary nebula (here NGC 6563)
would look very blue in a field of stars. Photo: Adam Block

HR 8119, at about 2,300 light-years, is a reddened object, with a B-V index of 0.11, where we would have expected a B-V index at least as blue as -0.20 without the reddening. A foreground object would likely be less reddened, and moreover the central star of a planetary nebula would be intrinsically very hot and blue indeed, with a color index of no less than -0.30. Therefore, if Ou4 is a planetary nebula and a foreground object in front of Sh2-129, its central star would look strikingly blue compared with the other stars in the field, including HR 8119. It would also be very ultraviolet-bright. A careful search for a strikingly blue and ultraviolet-bright star somewhere in the field of Ou4 would certainly turn up any such star in the field, if it exists.

I think it would be a good idea to search for a possible hot white dwarf that might power Ou4. But until such a star is found, I think HR 8119 should be accepted as the most likely exciting star of Ou4.

Ann

[geckzilla]

I'm not saying it's not HR 8119 or that it can't be in SH2-129. I'm not sure why you seem to keep responding to me like I've said that. All I am saying is that the possibility is definitely there.

[Ann]

Geck wrote:
I'm not saying it's not HR 8119 or that it can't be in SH2-129. I'm not sure why you seem to keep responding to me like I've said that.

Well... the way I see it, you and I are having a miniature version of an astronomical "great debate". I vividly remember the "Jubilee Debate" in 1995 between Bodhan Pacynski and Donald Lamb about gamma-ray bursts. In that debate, Donald Lamb claimed that gamma-ray bursts were "repeat bursters" in the Milky Way's halo, and they couldn't be at cosmological distances, because that would make them too bright. Bodhan Pacynski argued that the isotropic distribution across the entire sky of gamma ray bursts meant that they had to be either extremely local phenomena a few hundred light-years from the Sun, where the nearby stars have an isotropic distribution, or else they had to be associated with incredibly distant galaxies, which are also isotropically distributed across the sky. And since, for various reasons, the bursters couldn't be very local, they had to be extremely distant and mindbogglingly powerful. I was incredibly impressed by Pacynski's reasoning, and I was also a little irritated at Don Lamb, who seemed to want to cut the gamma-ray bursters down to size.

So it seems to me that I'm defending the "Pacynski" position that location is extremely important, so that we should concentrate on the apparent position of Ou4 inside Sh2-129 and centered pretty much on the massive triple star HR 8119. I'm defending the position that the amazing juxtaposition of Sh2-129, Ou4 and HR 8119, plus the lack of an obvious white dwarf candidate to ionize and sculpt Ou4, strongly suggests that hot bright triple star HR 8119 is the ionizing star of Ou4. I'm also defending the the position that Ou4 most likely is not a planetary nebula but something much bigger.

I'm not saying that you are Donald Lamb, of course. I'm just saying that you seem less impressed with the location of Ou4 than I am. And it is interesting to have a debate. I'm trying to defend my belief that Ou4 was launched by HR 8119 by coming up with as many good arguments as I can.

Ann

[geckzilla]

Ok. I haven't taken any side at all and was unaware that there were sides. I am intrigued by the object but I just can't make a lot of sense of it yet. It's very tantalizing to think that it's this giant thing off in the bubble but it's so much flight of fancy until some more solid measurements have been taken. It will happen, but not right at this moment! At the end of the paper the authors mention that if the proper motion of the nebula can be measured then a much more accurate distance can be ascertained. At that point we can be a lot more sure about what we're looking at.

[starsurfer]

Ann I really admire your excellent research and your commitment to investigate and share your knowledge, it is always a pleasure to read your words!

I also agree that it isn't a separate planetary nebula but is simply a previously unknown part of Sh2-129, notice how the tips of the lobes seem to be more or less perfectly aligned with the edges of the emission arcs of the larger emission nebula.

However if Ou 4 is a planetary nebula, its distance must be closer than Sh2-129. Old planetary nebulae can have very large sizes ranging between 10-20 light years. Assume that Ou 4 is a definite planetary nebula, it hasn't been discovered until recently due to its material expanding for a very long time of many tens of thousands of years and being of incredibly low surface brightness.

Romano Corradi is one of my favourite planetary nebula researchers and Agnes Acker is simply legendary. A paper about another of Nicolas Outters' discoveries was also published by Romano Corradi earlier this year about Ou 5, see here: http://arxiv.org/abs/1406.4309

Also I would like to mention that this image is false colour. It was included in a presentation poster presented at the Asymmetrical Planetary Nebulae VI conference last year, see here: http://www.astroscu.unam.mx/apn6/PROCEE ... _Acker.pdf

Finally, below are some of my favourite images of Ou 4:

Stefan Binnewies and Rainer Sparenberg
http://www.capella-observatory.com/Imag ... ithOu4.htm

Nicola Montecchiari
http://www.skymonsters.net/personal/imm ... 29_OU4.jpg

Lee Buck
http://lbuckphotos.smugmug.com/Astropho ... BkDkDLz/X3

[starsurfer]

Makis Palaiologou, Stefan Binnewies and Josef Pöpsel
http://www.capella-observatory.com/Imag ... rnPart.htm