This object is located 500 light years away in the constellation of Cassiopeia at a declination of +57 degrees. It spans 10 arc-minutes in our apparent view, which corresponds to a diameter of 1.5 light years.
Red diffuse circular-type DSOs like these can be either HII regions or faint ancient planetary nebulae. So which is it?
Reasons it could be a HII region:
- As a member of the SH2 Sharpless catalog of HII regions, one would expect this to be a HII region.
- Object has no OIII signal, which is unusual for a PN but not for a HII region.
- Surrounding region also has strong HII signal, more than would be possible from a single PN.
Reasons it could be a PN:
- Assuming the distance estimate is accurate, its size of 1.5 light years is more in line with a PN than a HII region.
- The brightest signal of the object outlines a circular shape.
- The existance of a magnitude 19 blue dwarf which is likely the central (progenitor) star.
- The bright long arc shapes are more representative of PN gas shock fronts than of a HII region.
- Several sources refer to a 1977 paper which concludes that this is a PN, but I could not find the original paper to confirm.
Ultimately, I believe this is a PN that lies in front of a broader HII region. My belief is based primarily on the bright long arc shapes. But I would like to examine the details of the paper to confirm that belief.
Thanks to Jerry for bringing this object to my attention through
his recent wonderful Astrobin image.
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