December 2021 - Picture of the Month
Planetary Nebula HDW 2 (Sh2-200) in Cassiopeia
I thought that it was about time for another planetary nebula. Not that you'd guess that from this object's designation: it was discovered and misclassified as an HII emission nebula by Stewart Sharpless, finding its way into the Sharpless catalog. In fact it wasn't until 2017 that a spectra was taken that definitively settled the nature of HDW 2.
HDW 2 is an old planetary nebula that is beginning to fade into the background. The bright shell in Don's image is about 6' in diameter, whilst Hartl and Weinberger (1987) felt that the Palomar Observatory Sky Survey (POSS) plates showed an outer halo of about 24' diameter as well. I've checked, and whilst it might be there, it could also be 'averted imagination'. They also estimated HDW 2 to be about 580pc (about 1900 light-years) from us, but don't say how.
Fortunately Corradi et al (2003) undertook narrow-band studies which show this outer halo much more clearly, and suggest it's 30' in diameter, which at 580pc is huge. What they're not sure about is whether this faint structure is part of the planetary nebula or an interaction with the Interstellar Medium (ISM).
As to the central star there seems to be no concensus. A hot white dwarf would be expected, and given that HDW 2 is relatively close to us it's a surprise that none has been conclusively identified.
The view of the Strasbourg-ESO Catalogue of Galactic Planetary Nebulae (Acker+, 1992) seems to be that the central star is of spectra type A and 12.6V magnitude, designation TYC 4053-643-1. Unfortunately they also add a note to say
It is not clear if the A-type star observed is the true central star of the nebula.
The spectroscopic survey of Weidmann and Gamen (2011) supports the A-type classification, but they also conclude that the central star is a binary. More recently Bear and Soker (2017) have concluded that the central star, whichever it might turn out to be, may be a triple system. Their work is based on the irregular morphology of the planetary nebula which they claim cannot be explained by a binary.
Professional debate aside, studying low surface brightness objects like these is what astroimaging does best, and even the inner shell of this wonderful object is full of structure revealed in O-III. There's also more than a hint of that extended halo in the lower-right corner of this image where it is brightest.
James Whinfrey - Website Administrator.