There are maybe 10 amateur images of this PN, and for good reason. This is by far the faintest object I’ve ever attempted. It’s hardly comparable even to things like HFG1. There are hints of structure on some images of ISWE1. I wanted to see if there’s anything more there than just some blobs of Ha and OIII. And that took a long time. I started in September 2023 and finished in November 2024

I’m happy to report that there is indeed more there, just a bit. So, my image consists of red and blue blobs, but with some brighter and darker patches, yay :-)

The data is far too shallow to allow any dramatic dynamic range or eye-catching contrasts, so I went for a more natural look (as far as that's possible) and colors. I consider this image to have more of a documentary value. It’s not particularly aesthetically pleasing.

WORKFLOW

Processing such objects is like doing plastic surgery with a hammer, but the result must look like makeup. It was a challenge- that’s why I’ve chosen this object. Most of the work was fishing out extremely faint signal from the sea of noise while avoiding artifacts.

1. DrizzleIntegration x2, drop shrink 0.9;
2. DynamicCrop;
3. BlurXterminator (just to correct some aberrations, there’s nothing to deconvolve here);
4. Resample to 50% resolution, Cubic B-spline, Smoothness: 1,5;
5. StarXterminator;
6. GradientCorrection;
7. NoiseXterminator;
8. StatisticalStretch, Median: 0,1;
9. Frequency-separated noise reduction with Neat Image, separately for Ha and OIII;
10. Loaded Ha and OIII to PS, corrected some artifacts and blemishes;
11. Synthetic luminance layer from Ha and OIII;
12. Ha and OIII colorized with Hue/Saturation adjustment layer;
13. Worked on colors, contrasts, did some more noise reduction with CameraRaw;
14. TopazDenoise – no „sharpening”, applied with masks;
15. Final touches- colors, contrasts, noise, etc.;
16. Added RGB stars.

OBJECT DESCRIPTION

Catalogue numbers / other names: PNG 149.7-03.3, PK 150-031, ISWE1, IW1.

Discovered in 1987 by…. you guessed it- Ishida and Weinberger!

APPARENT SIZE

We can safely assume that the nebula is a sphere (like cows) and its diameter is around 22 arcminutes, outer hydrogen structure included.

SURFACE BRIGHTNESS

OIII: 26.6 mag/arcsec2
Ha: 25.6 mag/arcsec2
For comparison, M42 has surface brightness of 21,3 mag/arcsec2 – ISWE1 is 76 times dimmer.

EXPANSION VELOCITY

OIII: 5km/s
Ha: 12 km/s

KINEMATIC AGE

Around 122 000 years, as given in „Spectroscopic investigation of old planetaries III”.

REAL DISTANCE

Around 1 385,34 light years (ly) or 424,74 parsecs (pc).

It can be calculated, because we know the parallax value of the planetary nebula central star (CSPN). It’s in GAIA DR3 data and has a value of 2,354 milliseconds of arc / year (mas/y).

Formula:

d (pc) = 1 / parallax value in arcseconds

px = 2,354 mas = 0,002354 as

d (pc) = 1 / 0,002354 = 424,74 parsecs

424,74 parsecs x 3,262 = 1 385,34 light years

REAL DIAMETER

Around 8,87 light years or 2,7183 parsecs.

This can also be calculated from known data with the following formula:

D = 2 x d x (theta/2), where:

D – real diameter in parsecs,
d – distance in parsecs,
theta – apparent diameter in radians

Thus:

theta (rad) = 22 arcmin x (pi x 10800) = 0,0064 radians

D = 2 x 424,74 x (0,0064 / 2) = 2,7183 parsecs

2,7183 parsecs x 3,262 = 8,87 light years

CENTRAL STAR (CSPN)

WD 0349+499

Gaia DR3 250358801943821952: https://vizier.cds.unistra.fr/viz-bin/VizieR-S?Gaia%20DR3%20250358801943821952

It’s very distinct on the image:

Surface temperature Teff: 90 000 K

Mag: 16,47

Type: PG1159

PG 1159 stars are quite peculiar. They aren’t fully degenerate stars – not yet. They’re called pre-white dwarfs- their next and last evolutionary stage is white dwarf. It’s believed that they’re the progenitors of hydrogen-deficient white dwarfs.
They’re located in the GW Vir zone of instability of the Herzsprung-Russel diagram. He, C and O are apparent in their spectra.
In the preceding evolutionary phase, after leaving the asymptotic giant branch of HR diagram they experience a born-again episode (very late thermal pulse – VLTP) or late thermal pulse.
LTP takes place when the star ceases to be a red giant, when nuclear fusion in its Carbon-Oxygen core stops but the Helium shell in the outer layers begins to fuse. Such star briefly (for a few decades or centuries) gets back on the AGB, increasing its size, luminosity and temperature.
VLTP is like LTP but shorter and more violent. It takes place later, when the star is in its cooling phase on the way to becoming a white dwarf. Helium flash is very short but energetic and it can last for only a few months.
GW Vir stars are often variables. Their outer layers aren’t completely stable. Gravity waves (in the fluid dynamics sense; not gravitational waves!) cause pulsations. These waves don’t cause the whole star to uniformly contract or expand, like it’s the case with Cepheids, Lyrae or Mira variables. They’re more asymmetrical in nature. This mechanism causes brightness variations with low amplitude (up to 0,15 mag) and short periods (300 – 6000 s).
However, WD 0349+499, CSPN of ISWE1 doesn’t seem to be a variable, although that hasn’t been ruled out yet.
According to current hypothesis, PG 1159, that have more Nitrogen abundance in their atmospheres, expressed as N/He ratio of around 1% are variables. Those with N/He ratio of 0,01% or less do not pulsate.

ISWE1 MORPHOLOGY

Unfortunately, there’s precious little written about this nebula, so I’ll take the liberty to speculate a bit.
This is a very old and large PN, that interacts extensively with the interstellar medium (ISM). Its original structure has mostly faded away.
Surrounding the central oxygen blob, which doubtlessly has its origin in the CSPN, is the extended cloud of ionized hydrogen. Some or most of it is old AGB-phase stellar wind, re-ionized by the CSPN when the PN shell became optically thin. Some of it is certainly the ISM.
Hydrogen-alpha image reveals a few denser, brighter seemingly elongated features. These may be the remnants of more intense interactions with the ISM that resulted in the formation of filamentary structures. I daresay that this phase is long over and what we see are just fading embers.
As a reference, below are images of ISWE1 taken by the Burell Schmidt telescope at the Kitt Peak observatory. These are relatively short exposures that have been published in „An atlas of ancient planetary nebulae and their interaction with the interstellar medium”.

Ha

NII

OIII

SOURCES:

1. AN ATLAS OF ANCIENT PLANETARY NEBULAE AND THEIR INTERACTION WITH THE INTERSTELLAR MEDIUM, Tweedy & Kwitter.
DOI: https://ui.adsabs.harvard.edu/link_gateway/1996ApJS..107..255T/doi:10.1086/192364
https://articles.adsabs.harvard.edu/pdf/1996ApJS..107..255T

2. The GW Vir Instability Strip in Light of New Observations of PG 1159 Stars: Discovery of Pulsations in the Central Star of A72 and Variability of RX J0122.9–7521, Sowicka et al.
DOI: https://doi.org/10.3847/1538-4365/acfbe4
https://iopscience.iop.org/article/10.3847/1538-4365/acfbe4/pdf

3. An investigation of planetary nebulae accompanying PG 1159 central stars, based on Gaia DR2 measurements, Ali & Alharbi
DOI 10.1088/1674-4527/21/6/151
https://iopscience.iop.org/article/10.1088/1674-4527/21/6/151/pdf

4. Late stages of expansion of planetary nebulae, Giesenking, Hippelein, Weinberger.
Astronomy and Astrophysics, Vol. 156, p. 101-105 (1986)
https://articles.adsabs.harvard.edu/pdf/1986A%26A...156..101G

5. Spectroscopic investigation of old planetaries. III. Spectral types, magnitudes, and distances, Napiwotzki, Schoenberber
Astronomy and Astrophysics, v.301, p.545
https://articles.adsabs.harvard.edu/pdf/1995A%26A...301..545N

I’d also like to thank @Sven Eklund for kindly allowing me to compare our raw data of ISWE1. I needed a reference to help me confirm that what I thought was there was really there.
Also, thank you to @zombi for pointing out a slight issue with star color.

EDIT:
I'd also like to post nearly unprocessed master files (crop, gradient correction, starXterminator), stretched with STF. No noise reduction or other kind of processing done. Native resolution: