Sh2-313 (Abell 35) has long been considered a large planetary nebula located in the constellation Hydra. Jacoby, (Astrophys. J., v 244, 902-911, March 15, 1981), studied the nebula and the bright central star (HIP 62905) and estimated the distance to be approximately 1200 light years from the Earth, however, recent research indicates the distance to the central binary star system is approximately 160 parsecs (520 light years). Jacoby also deduced the central star is a binary system with a very hot white dwarf (BD 22-3467) in a close orbit. This has been subsequently verified with the temperature of the companion white dwarf in excess of 150,000 K. As shown in the image above, there is a prominent bow shock feature to the right of the central star. This feature stands out strongly in OIII filtered images. It is also weakly visible in long exposure Ha images. This bow shock indicates the central binary star system is plowing through the interstellar medium at supersonic speeds. Hollis, et al (Astrophys. J. v 456, 644-650, January 10, 1996) supported these conclusions. This feature, and other recent studies, indicates the nebula is not a classic planetary but is a region of the interstellar medium that is being ionized by intense radiation from the hot white dwarf forming a Stromgren sphere.
The image above was assembled using hydrogen alpha (Ha) and doubly ionized oxygen (OIII) filtered images. The Ha image was mapped to the Red channel and the OIII was mapped to the Green and Blue channels. This method produces a near true color image that more closely portrays the natural color of the subject object. A full Hubble Palette version (SII-Ha-OIII) may be viewed here