We tackled again a difficult target with regard to the brightness of our tube (f 2450 to Fd 8) Despite the 49 hours of exposure in narrow band and the 10h30 in RGB, the result is satisfactory without unfortunately reaching what a CMOS camera would have allowed in less time...
SH2-313 or Abell 35 is located in the constellation Hydra. This planetary nebula is very large, the full moon at about 31 arc minutes in diameter, and SH2-313 10 arc minutes in diameter. Abell first identified this planetary nebula in 1966. It is perhaps the largest known PR at 1.6 pc in diameter, and is about 360 pc apart (Jacoby 1981).
It is also the oldest known PR (Bohuski 1972). SH2-313 or Abelle 35 (PN A66 35 for Simbad) has long been considered a planetary nebula (NP) and still is according to SIMBAD, which refers to the publication of Frew et al. dating from 2013.
However, as early as 1981, reservations were expressed about the nature of Abell 35 and the latest publications revealed that Abell 35 was not a planetary nebula: "[...], the particular nebula of type PN Abell 35 (Jacoby 1981; Hollis et al.1996) appears to be an arc nebula inside a photo-ionized Strömgren sphere in the ISM" thus a Strömgren sphere ionized by bd's warm white dwarf companion DAO - 22º 3467, the star surrounded by the central parabolic arc that recently evolved from the planetary nebula phase. The system produces a strong wind, likely coming from the rapidly rotating subgiant, which interacts with the inner area of the emission nebula to produce this shock arc.
So it's really an unusual planetary nebula. It has a reasonable target size of about 10 arc-min in diameter. This compares very closely in size to the inner disk of the famous Helix nebula which extends over 8X19 arc-min. However, SH2-313 is much older and more distant. As planetary nebulae age, they tend to lose their beautiful, defined shape. Sharpless 2-313 has an extremely low surface brightness of only 26.2 mag/arc-sec2.
Basically, what appears to be a very bright star is a binary star system responsible for creating the surrounding nebula. Spectroscopic measurements reveal that the pair rotates very quickly, almost enough to tear. One of the stars is a white dwarf. The incredible forces generated by the rotation of the white dwarf caused it to lose its outer layers thousands of years ago. This produced the surrounding planetary nebula that we can see today. The glow is caused by UV radiation from the white dwarf ionizing the expanding shell of the material as it expands into space.The shock of the arc is due to the movement of the nebula and its central star moving through the gas between the stars. It is quite rare to look at stellar objects and see features that suggest movement. This can be represented by wave-like structures, localized lightening of gaseous regions or distortions; this phenomenon is found also in the Skull Nebula.
Abell 35, Lotr 1 and Lotr 5 (Abell 35 type objects) are the only three PNe with binary nuclei known to contain a very hot and active chromospheric UV primer. Mass transfer between their partners or common envelope interactions explain the morphological properties of some planetary nebulae. As for how these binary systems form is unclear and presents a challenge for theories of the evolution of binary stars. (A. A. GattiJ, E. DrewS, LumsdenT, MarshC, MoranP, Stetson 1997).
It should be noted that this is a very dark diffuse nebula, and the galaxies can be clearly seen behind Abell 35 and they show easily even after passing through a nebula.