Planetary Nebula K1-22

This Ghostly Remnant of a Dying Star is a Good Target to Test Your Eye Acuity When is Observed With Small Instruments.

Figure 1. K1-22 planetary nebula seen close to the mountains. 

Photo by the author

Also known as ESO 378-1 and PK 283+25.1[1], this planetary nebula, nicknamed the "southern owl nebula" by some observers, is a challenging target for owners of 8-inch or similar size telescopes. April and May is a good season to observe it from the Southern Hemisphere because it is very high in the sky as soon as the sky gets dark, hidden among the stars in the long constellation Hydra (the water snake), specifically in R.A. 11h 26m 43.7s Dec -34° 22' 12".

I personally think that an 8-inch is a small telescope if we have in mind that some observers own much bigger instruments (e.g. 24, 30 or even 32-inch mirrors).

I observed this object from Potrerillos, a site you can reach driving for 1 hour to the West from Mendoza City (Argentina). It is a nearby site, so you will have some light pollution in the eastern sky if you go there. However, the sky is pretty dark at the zenith and toward the West where the high peak mountains of Los Andes are situated. I can say that the observation of this planetary requires a good dark adaptation (as with any other faint objects) and to improve as much as you can your environment to have a very dark site. I should say that covering your head with a black blanket is a must. I also suggest to slightly shake your telescope when using averted vision taking into account that, according to researchers, rods are better motion sensors than cones, thus getting the best of your night vision to catch this elusive planetary nebula.

It was a real surprise for me to be able to glimpse this member of our Milky Way through my 8-inch telescope even at very low magnification.

This video, by ESO Observatory in Chile, shows the intriguing planetary in Hydra.

At 42x, the 1° field surrounding the planetary, which has an angular size of  3.1'x2.9', is easily recognized by identifying the pairs of stars HD 99463 & HIP 55828 (visual magnitudes 8.5 and 8.7 respectively), and the fainter one composed by the stars CD --33 07733 & TYC 7223-280-1 (visual magnitudes 9.9 and 10.5). They are labeled in Figure 2. Once there, another two pairs of stars are useful as references to find the area where this elusive planetary nebula resides. Amazingly, even without any nebular filter, a very faint nebulosity seems to jump to the view. However, the view is very challenging so nebular filters (e.g. UHC, OIII, Orion Ultrablock) are necessary to enhance the view of this interesting object of our Milky Way. The use of different eyepieces (magnification) will make possible a comprehensive visual analysis of this deep sky object and its visibility through an 8-inch telescope.

Figure 2. The planetary nebula and the nearby brightest stars in the field.

If you have a UHC filter among your accessories do not hesitate to use it to have an improved view of K1-22[2]. At low magnification and through this filter the planetary looks round, smooth, and of course very faint. The ghostly image of K1-22 is a little better detected for moments when applying averted vision. Even a slight movement of the telescope seems to enhance a little the detection of the planetary. It shows a little more contrast but it is still very faint. The Orion Ultrablock filter offers an image of the planetary not so contrasted as that of UHC. However, the view improves a bit compared to that without a filter.

The Hubble Space Telescope (HST) images (see Figure 4 below) show two sources separated by 0".35 at the center of K 1-22; the blue northeast component is the CSPN (Central star planetary nebula) and the red southwest component is a cool companion (Ciardullo et al. 1999). The IR images show a source coincident with this close pair of stars, but cannot resolve them (paper "Spitzer 24μm Survey for Dust Disks Around Hot White Dwarfs" You-Hua Chu et al. (2011).

At 78x, K1-22 looks faint, so averted vision is necessary to improve the detection. UHC filter clearly enhances the view of the planetary. For a brief moment, a stellar feature seemed to show up inside the planetary. However, the stars "inside" or in projection on K1-22`s disk are very faint to be detected through an 8-inch telescope taking into account that they have a visual magnitude around 15.3. Sources state that at 80x an 8-inch telescope should show (under ideal conditions of course) stars up to visual magnitude around 14.4, so surely the view of that faint stars was just a false signal of my eye.  

Figure 3. The central star of this planetary nebula (CSPN K 1-22) 

is labeled on this picture.

Photo credit: ESO.

"The discoverer of this planetary nebula (Kohoutek 1971) noted that three stars appear near the center of the nebula and proposed that the brightest of these was the PNN, paper "New Planetary Nebulae". Smith & Gull (1975) confirmed that this star is very blue. Instead, Kaler & Feibelman suggested that a fainter object 4" east of Kohoutek´s candidate was the PN's true central star. Given the low stellar density in the field, the pair almost certainly form a bound system. Our derived distance of 1.3 kpc is reasonably consistent with distances based on statistical techniques (1.0 kpc, CKS ; 3.4 kpc, Zhang 1995)." (paper "A Hubble Space Telescope Survey for Resolved Companions of Planetary Nebula Nuclei" Robin Ciardullo et al. 1999. On the other hand, T. Rauch et al. claim, in their paper "Classification and spectral analysis of faint central stars of highly excited planetary nebulae" (1999), that absorption features is the spectrum of the central star of PN G283.6+25.3[3] (by the way another name for this planetary nebula) suggest the presence of a cool companion with the planetary being a circular object with two lobes and without a central hole.

The view of the planetary is very faint and difficult at 106x also. A good suggestion to better detect this elusive object is to cover your head with a black carpet. Once again, the UHC filter is useful to improve the contrast of K1-22. Averted vision is always necessary to glimpse this round and smooth disk. No stars were visible at this magnification through the mentioned filter. The performance of the Orion Ultrablock filter was, as with the lower magnifications, worse than that with the UHC filter.

Figure 4. Picture taken from the paper  "Spitzer 24 µm Survey for Dust 
Disks around Hot White Dwarfs"  by  You-Hua Chu et al. showing optical 
and IR images of CSPN K1-22.

166x did not show new features or something different beyond the smooth and faint patch "hidden" among the stars in this southern constellation.

I decided to go back and make a final observation using very low magnification (i.e. 32x). Knowing in advance where this planetary lies, something seems to be visible there, very elusive. However, you can use this low power to catch it if you have, of course, a detailed star chart and a good eyepiece field chart to know the precise position of K1-22.

I used an OIII filter with this eyepiece and I got a beautiful view, with K1-22 faint but visible with direct vision. Averted vision makes possible to improve the detection with this object appearing big, smooth in brightness, and round.

H-Beta was a totally useless filter for this planetary nebula.

K1-22 is a faint planetary nebula in the autumn southern sky to study visually and photographically. If you have seen the well-known "Owl nebula" Messier 97, try to observe its southern cousin, K1-22 if you have a chance to travel to this part of the planet and enjoy the starry skies.

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[1]   Perek and Kohoutek. Catalog of Galactic Planetary Nebulae.  Academia publishing house of the Czechoslovak Academy of Sciences,  Prague, 1967, 276 pages. Compilation of all Planetary Nebulae known in the Milky Way Galaxy in 1964. 

[2]   K is the acronym after Kohoutek.

[3] The nomenclature for the Strasbourg-ESO catalog of planetary nebulae.