Abell 35: Planetary Nebula? No! – Strömgren Sphere? Yes!
A.K.A: Sh2-313, PK303+40.1

OTA: CDK24
Camera: Moravian 6000M Pro (2x2 binning)
Observatory: Heaven's Mirror
Date of Capture: July 2023
Date of Processing: October 2023

Exposures:
R: 6 x 600 sec
G: 6 x 600
B: 5 x 600
H: 18 x 1200
O: 21 x 1200
Total Exposure time: 15.8 hours
Image Width: 31' 10"

Processing Tools:
1.    Commercial: PixInsight, Topaz, Photo, Luminar
2.    Pixinsight Addons: NoiseXTerminator, BlurXTerminator, StarXTerminator
3.    My Scripts: NB_Assistant, AC_Restar, Subframe Weighting Tool (Excel w/ J. Hunt)

Target Description:
     Why do planetary nebulae have a blue OIII inner region and a red HII outer ring? The answer involves the intense UV radiation emitted by the parent star, now a super hot, UV-emitting white dwarf. The energy of that UV radiation falls off with distance from the star. Because OIII requires more energy to be ionized than oxygen, the blue emissions occur close to the UV source, and HII emissions can arise farther from the source (1). When this occurs, we have a "Strömgren Sphere."
However, not all bright, somewhat circular blue-red stratified nebulae are planetary nebulae. The above mechanism works for any star with access to a gaseous cloud; the interstellar medium will also serve that requirement.
     In fact, such cases are known. The Rosette Nebula is an example of a Strömgren Sphere—it is not a planetary nebula. In contrast, planetary nebulae originate from the violent death of an approximately 1-8 solar-mass star when core helium burning ceases and ignites in the core's shell, explosively ejecting the overlying material at velocities up to 50km/s. The thermally radiating white dwarf remains, and thermal UV radiation turns the ejected materials into a Strömgren Shere.
     As for Abell 35, most modern interpretations suggest it is not (now) a planetary nebula—those gasses are long dissipated (3) if they ever existed. The dwarf star's low mass (0.48 solar masses) may have been insufficient to generate a planetary nebula (3). In either case, we now observe a Strömgren Sphere created from interstellar matter. (3) discuss this interpretation and the substantial evidence for it. Furthermore, the low luminosity of that small dwarf star is of a magnitude that its Strömgren Sphere "may well mimic a genuine PN" (3).
     My processed images support the Strömgren Sphere interpretation. Abell 66 (4) shows strong stringers oriented predominantly outward of HII and OIII. This orientation is consistent with radial material flow. However, Abell 35 shows almost no radial or abundant flow structures other than in the bow shock. A few outward-pointing broad, cone-shaped structures appear in the wake of the shock front. Some image features have bright edges from contrast enhancement that should not be mistaken for quills. (3) has shown that the star is moving through the interstellar medium from the lower right toward the upper left, a direction bisecting the shock front (as it should) at about 125km/s, which implies it has moved about one nebula diameter in the last 16,000 years.
Finally, the outer ring of gas present in Abell 66, the Helix Nebula, and other iconic planetary nebulae is absent in Abell 35. Again, it may have been present once, but since then, it has dissipated.

Processing Description:
     PixInsight supported all the preprocessing; subsequent processing utilized Topaz Studio2 (Clear, Dehaze, Precision Contrast, Precision Sharpen), Topaz Denoise, and Topaz Sharpen. The stars were removed immediately after stacking. The before and after star-removal images were compared to verify that no significant data were lost or altered by that process. The subtraction of the before and after images showed that no signal from the nebula was lost or distorted by this process. The Abell 35 processing steps follow those used for Abell 66 (4).
     However, the intentionally aggressive processing did cause high-contrast bright edges on some features in the nebula and background galaxies. These exaggerated contrasts do not affect any of the conclusions made in this article and, to some degree, assure that any radial quills were not overlooked.
     The two blinked images are at the same scale.

     Separate Ha and OIII images can be seen by clicking the images to the right. Also, an HORGB image is available there.


Statistics:
Distance: 1200 ly
Apparent Magnitude: 9.6
Average Surface Magnitude: 24
Pixel Span at Target: 3M km

(1)    https://arxiv.org/abs/1202.3334
(2)    https://en.wikipedia.org/wiki/Str%C3%B6mgren_sphere
(3)    https://www.aanda.org/articles/aa/pdf/2012/12/aa19536-12.pdf
(4)    https://astrob.in/jhlqbr/0/

Alex Woronow