This photo has been in my drawer for a long time - since October 2023. Not because of any issues with processing. I encountered serious problems with preparing a proper description of the object- surprisingly little is known about it. So I consulted scientists from Leopold- Franzens - Universität Innsbruck and the University of Zielona Góra, Poland. And along the way I learned a lot more about planetary nebulae.

Ha:

OIII:

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Workflow

Ha and OIII:

DynamicCrop ,
BlurXterminator ,
StarXterminator,
DBE,
NoiseXterminator ,
NBColorMapper script to combine Ha and OIII and create RGB color composite image,
Stretching with STF+HistogramTransformation and GeneralizedHyperbolicStretch,
Topaz Denoise (with masks, no sharpening applied),
Colors, contrasts, retouching artifacts, etc. done entirely in PS.

Stars:

I prepared two versions in PIX using BlurXterminator - one with the "halo" parameter at 0.10, the other at maximum.
SPCC,
Stretched with GeneralizedHyperbolicStretch,
StarXterminator with the " unscreen " parameter on,
Both versions of stars uploaded to PS, corrected colors, saturation, artifacts, etc.,
Mixed both versions by clipping the version with max halo to the version without halo with the "lighten" function and adjusting the opacity.

Resize 66%, save as PNG.

A few words about processing.

Processing was difficult for a paradoxical reason. Poorly exposed HDW-2, after proper processing, is hazy and transparent, although much less of the structure is visible. However, when deeply exposed and stretched as much as the data allows, it becomes …lumpy, I guess. It's hard for me to describe it but processing it to avoid that unpleasant lumpy look was a big challenge. And I'm still not entirely satisfied.I wonder, if a properly deep image made with a big telescope, with high resolving power and long focal length would show that lumpiness to be interesting small-scale structures inside the bubble.

Stars - they’re definitely bigger here than in most of my images, but it’s on purpose, for aesthetic reasons. First of all, I included the beautiful open cluster Trumpler 3 in the frame, and it would not be appropriate to dim or shrink the stars - they should shine! Secondly, larger and brighter stars in this case do not interfere with viewing the main object and its halo. Therefore, I don’t publish the additional starless image.

I also strived to show a very particular feature of the Ha halo- stripes / striations that are largely parallel to the stripes in the PN itself. But the surrounding nebulosity is very dim, so getting any contrast there requires deep exposure and careful processing. There are few amateur photos that show this well. I gathered about half of Ha data under Bortle 3 skies on moonless night for that purpose.

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About the nebula and the help I got from two universities

Catalog numbers:

PNG 138.1+04.1
HDW-2
Sh2-200
Trumpler 3 is also known as Lund 101, Cr 36 and Harvard 1.

Coordinates:

It's is in Cassiopeia, near HFG-1 and Abell 6 Pne.
RA: 03h10m51s.727
DEC: +62°49′26″.22

Discovery and research

This nebula, discovered in 1959 by Stewart Sharpless, joined the catalog of Messrs. Herbert Hartl, Johann Dengel and Ronald Weinberger in 1983. In 1987, they discovered a halo surrounding the object. It was only in 2017 that spectroscopic measurements were carried out, which confirmed that it was a real planetary nebula.The waves/stripes visible in the halo surrounding the object were probably first described in by Corradi et al [MNRAS 340, 417 (2003) DOI: 10.1046/j.1365-8711.2003.06294.x] - Fig. 15 (publication provided to me by Prof. Kimeswenger).

Consultations

Since I found surprisingly little information about this nebula, especially regarding the two most striking features - the central "hole" as well as the stripes in the PN itself and in the extended halo, I decided to simply write an e-mail to Hartl and Weinberger, who worked at the University of Innsbruck.Unfortunately, I did not receive an answer, so... I wrote to the head of the Institute für Astro- und Teilchenphysik (Prof. Norbert Przybilla), who confirmed my suspicion - both gentlemen have been retired for many years.
However, he directed me to Prof. Stefan Kimeswenger. I got the answer after a few hours, but it took me many weeks to study it :-) Prof. Kimeswenger’s help was invaluable and I’m most grateful for his assistance.After reading a series of long and complicated publications on PNe, I came up with an idea that might explain why this nebula looks the way it does from our point of view. Dr. Michal Zejmo from the University of Zielona Gora managed to forward it to Prof. Wojciech Lewandowski, who effectively disproved my speculations :-) But more about that later.

Structure and lifecycle of the nebula.

Basic parameters.

Data from “Interacting planetary nebulae III: verification and galactic population based on the measurements of Gaia EDR3”, Mohery , M et al., DOI:10.48550/arXiv.2206.05638, (publication provided by Prof. Lewandowski).

Apparent size: 176 +/- 18 arcsec. Its real size is unknown since no distance measurements have been made yet.
The estimated expansion velocity is approximately 20 km/s.
Apparent motion of the nebula:
7.56 milliarcseconds (mas) in RA/yr (mu_alpha = 7.58 mas/yr)
-7.16 mas in DEC/yr (mu_delta = -7.16 mas/yr)

Central Star (CSPN)

Prof. Kimeswenger indicated that the CSPN is probably Gaia DR2 466746538582576640. This is the star indicated by an automated search according to the criteria contained in the publication "Searching for central stars of planetary nebulae in Gaia DR2” Chornay et al. [A&A 638, A103 (2020), DOI: 10.1051/0004-6361/202037554]. However, this has not been cross-checked yet. Certain characteristics of this star call its choice into question. It is intensely red, as evidenced by the spectroscopic parameter BP-RP = +0.95. On the other hand, it shines intensely in near ultraviolet (NUV), as observed by GALEX. Far UV (FUV) measurements haven’t been performed there yet. According to Prof. Kimeswenger, it may be a binary system, where the white dwarf is not directly visible, or a projection - where a distant star is obscured by a closer one.

Here it is on Aladin (DSS2 Red):

Morphology

My own observations and some conclusions follow:
HDW-2 shines in both Ha and OIII, with OIII dominating. Virtually the entire structure is visible in both emission lines, so the PN consists of both elements with similar distribution, except for the near OIII and far Ha halos.The "rings" surrounding the nebula are very clearly visible, but they are not rings, of course, because this is a three-dimensional object. This seems to be a spatially separate layer (or layers) surrounding the central bubble. There’s no information on whether this is a bow-shock-like structure, such as in HFG-1, or just the material of its central star unevenly distributed during the formation of the PN.
Stripes/striations/waves are very characteristic of this PN.Prof. Kimeswenger suggested that this may be the result of PN interaction with magnetic fields in the interstellar medium, as described here: "Interaction of Planetary Nebulae with a Magnetized ISM", Dgani & Soker [https://iopscience.iop.org/article/10.1086/304317, DOI: 10.1086/304317].
I haven't found anything about the "hole" in the nebula.

Diffuse Ha halo

Prof. Kimeswenger pointed out two possibilities - it could be the interstellar medium (ISM) interacting with the PN or a remnant of the CSPN stellar wind from the previous phase of its lifecycle, when it was a red giant on the asymptotic giant branch of the Hertzsprung -Russel diagram (AGB) and before it transformed into a white dwarf. At the end of the AGB period such stars shed mass in the form of intense stellar wind.
Whatever the case may be, according to Prof. Kimeswenger, over time dense material of the planetary nebula dissipates and becomes transparent to the intense UV radiation of the central white dwarf. This light ionizes not only the PN itself but also other surrounding gas.
My personal opinion is that this is why we see this Ha halo. It also suggests its spatial proximity to the PN. Also, I think that another argument for at least a spatial connection of the hydrogen halo with PN are the stripes:

Those visible in Ha halo are more or less parallel to the stripes in OIII:

Please note that they also occur in the OIII cloud directly above the PN bubble. This suggests that the PN and its extensive halo are influenced by the same magnetic fields, and therefore are very close to each other- this is my own supposition, neither Prof. Kimeswenger nor Prof. Lewandowski suggested it.

My speculations…

Last year I photographed HFG-1 PN, which moves supersonically in respect to the interstellar medium, generating very apparent bow-shock structure, composed of heated and compressed matter:

The direction of movement of HFG-1 is probably perpendicular to some degree to our line of sight.
I also stumbled upon a seemingly useful information in "Theory of the Interaction of Planetary Nebulae with the Interstellar Medium”, (suggested to me by Prof. Kimeswenger) where in chapter 3.2. “The adiabatic model” Dgani et al state:
"The main morphological features of the adiabatic flow are the ISM shock front, the decelerated PN shell and the RT instability. The last manifests itself as a “bump” and a hole in the nebula along the upstream symmetry axis.”

Since there are references to hydrodynamic simulations in many publications, I found them here:
"The interaction of planetary nebulae and their asymptotic giant branch progenitors with the interstellar medium", Wareing, Zijlstra, O'Brien.
They performed several simulations with different parameters and obtained the following results, which I thought applicable to HDW-2:

Therefore, I was wondering whether HDW-2 is moving along our line of sight- towards us, or in the opposite direction - away from us.

Hence, the "cavity" or “hole” near the center of the bubble may be a thinner region of the PN shell along its vector of motion (or even a perforation of the shell). Perhaps in this way the visibly separate part of the shell surrounding the entire inner bubble can also be explained. It could be a bow-shock-like structure surrounding the nebula, if it’s similar to HFG-1 and its vector of motion is mostly coincident with our line of sight.

… and their verification

I decided to ask dr. Michal Zejmo from the University of Zielona Góra to put me in touch with a pro. My speculations were reviewed by Prof. Wojciech Lewandowski. And that was the end of them :-)

Prof. Lewandowski pointed out that I had missed new data from Gaia on this nebula (“Interacting planetary nebulae III: verification and galactic population based on the measurements of Gaia EDR3", Mohery, M et al., DOI:10.48550/arXiv.2206.05638). It turns out we know its proper motion- over 7 miliseconds of arc / year in both Ra and Dec.
This means that we can basically rule out its movement exactly (or even approximately) towards us or away from us, along our line of sight, unless it is very close to us (not likely).

Prof. Lewandowski calculated the proper motion (mu) of the object based on this data:

mu_alpha = 7.58 mas/yr,
mu_delta = -7.16 mas/yr
Right Ascension- Alpha = 47.75,
Declination- Delta = 62.8.

Formula: mu= sqrt (mu_alpha^2 cos (delta) + mu_delta^2)
Result: approx. 8.8 mas/yr

There is no data on the distance to this nebula, so the tangential velocity cannot be calculated accurately. Estimating the distance at 1 kpc we get:v_T = 4.47 mu D

mu= 8.8 mas/yr = 0.0088 arcsec/year

D= 1 kpc = 1000 pc

v_T = 40 km/s

40 km/s is a typical velocity for objects in the galactic disk.

According to Prof. Lewandowski, if the PN was way less distant, let’s say 100 parsecs, it would mean that its tangential velocity would have to be scaled down accordingly, to 4 km/s. And with the typical speed of objects around the sun being around 50 km/s, this would mean that its radial velocity component must be correspondingly much greater.
However, the lack of observational data determining the radial velocity and/or distance makes it impossible to clearly prove whether my idea is true or false.

Thus concluded Prof. Lewandowski and put my idea to rest :-)

Proper motion, tangential and radial velocities

Here's a little graphical explainer - what are tangential and radial velocities and proper motion. It’s very easy to understand how radial velocity, tangential velocity and proper motion work and how changing one parameter has to affect the other two. Consider what will happen to the radial and tangential velocities if we move the object closer to the Sun- if the proper motion is to remain constant.

Conclusion

The fundamental problem with my idea is that if the object was very close, then its tangential velocity, appropriately reduced, could suggest that the nebula is moving primarily radially and not tangentially - i.e. more away from/towards us. And it’s improbable that this nebula is very close to the Sun.

I was wrong but it was a fun and rewarding exercise!

One more thing...

My friend, Slawek, (@diver on astropolis.pl) has identified two carbon stars in my picture. Here they are: