Planetary Nebulae - Part 2

Some Fascinating Planetaries Inhabit 

the Constellations of the Winter Southern Skies 

Part II. Ring planetary nebulae

A winter starry night anywhere in the Southern Hemisphere shows a region in the sky, between Right Ascension 12 and 19hs, where some constellations like Crux, Centaurus, Norma, Ara, and Sagittarius host some interesting planetary nebulae. The four objects included in part I of this article are bipolar planetaries, one of the five types in the morphological classification by Schwars et al. (1992), i.e. Elliptical (e), Bipolar (b), Point symmetrical (p). Irregular (i), Stellar (st). Part II of "Intriguing Planetary Nebulae" is devoted to the so-called ring or annular planetaries. Below there are three good examples of this kind of planetary nebula. As an observer, you surely know the Vorontsov-Velyaminov classification which classifies ring-shaped objects as type 4.

If you have at least an 8-inch telescope, all of the planetaries mentioned in this article are visible. Of course, you will be able to find more features or analyze their morphologies in more detail if you observe them through bigger instruments. 

Shapley 1

The observing site in Pampa El Leoncito. The dome of the 2.15m telescope 
of CASLEO observatory is visible in the middle of the photo.
CASLEO  is situated about 14kms from this site in a straight line.

In the inconspicuous constellation Norma, in the southern sky, a fine and faint planetary nebula resides. I am talking about Shapley 1 which shows a nice annular shape at least in astrophotographies. Shapley 1 (α = 15h 51m41s , δ = −51◦ 31′ 23′′, J2000), discovered by H. Shapley (1936), was described as “nearly perfectly circular” in appearance by Bond & Livio (1990), see the paper "The morphology and kinematics of the Fine Ring Nebula, planetary nebula Sp 1, and the shaping influence of its binary central star⋆" by D. Jones et al. (2011). A hypothesis states that Shapley 1 is actually an axisymmetric nebula viewed almost pole-on. It is also classified as type 4 in the Vorontsov-Velyaminov classification scheme. Sources give a visual magnitude of 12.6 for this planetary that is also named PK 329+2.1.

I observed this planetary in two nights from a very dark site just a few kilometers north of CASLEO observatory in San Juan province, Argentina, in the majestic Andes mountain range. The first night was good enough to carry out deep-sky observation but the seeing was not as good as the second night. Even a cold wind bothered most of the night. 

At low galactic latitude, immerse in the band of the Milky Way, we can find this fine annular shape planetary nebula, a target for an 8-inch telescope if you know where to aim it in the sky.

Picture taken using the Photopic Sky Survey ©Nick Singer.

After aiming the telescope to the zone where Shapley 1 lies (see map above) I could found the 1-degree field of view surrounding the planetary. Shapley 1 is situated between the stars labeled with circles in Figure 1. The three stars linked with a red line helped for an accurate search of the position of the planetary as it forms a "rhombus" with them. 

Figure 1. 1-degree field of view with Shapley 1 at the center. North is up.

At 63x without a nebular filter, the planetary is not visible (at least that night under the aforementioned conditions). However, a UHC filter helps to detect the ghostly image of Shapley 1. It can be barely glimpsed using averted vision appearing like a very faint nebulosity rather smooth in brightness. The target was low in the sky for a more detailed observation that night (around 25°), so I decided to make another observation the following night. 

A new observation earlier in the second night (at the end of the local astronomical twilight) made possible to see the planetary higher in the southwest sky (37°). This higher altitude plus a more steady sky made possible a better detection and view of this Milky Way object. At 63x, Shapley 1 could be glimpsed even without a nebular filter, looking like a very faint, roundish, and hazy smooth patch. Applying a UHC filter the view definitively improves. Through this kind of filter, the object is undoubtedly visible. Averted vision makes possible to detect, for brief moments, the west edge appearing slightly brighter than the rest of the planetary (indicated with A in Figure 2).

At higher magnification (119x) Shapley 1 can be barely seen using averted vision. Now some faint stars can be identified in the zone, like those indicated with arrows on Figure 2 that is very elusive through an 8-inch telescope and you need averted vision to barely glimpse them. One of them is the central star of the planetary which has a visual magnitude of 14 according to several sources. That central star seems to be a close-binary central star system according to D. Jones's paper. At this magnification, it is visible with averted vision appearing very faint. Moreover, other faint stars are detected in the area. At this magnification, the detection of the planetary is faint through a UHC filter. However, this power makes it possible to detect the inner central part of Shapley 1 darker, a suggestion of its annular morphology. Again, the west rim of the planetary seems to appear slightly brighter when saw it with averted vision.

Figure 2. DSS image of Shapley 1 showing its annular shape.

North is up. East to the left.

At 162x the planetary appears very faint at the eyepiece. The faint stars aforementioned look a little easier, but they are still elusive. Because of their positions, that stars help to delineate the shape and angular size of Shapley 1. UHC filter helps but the view at this higher magnification is ghostly and challenging.                                                                  

Shapley 3

Position of the peculiar planetary nebula Shapley 3 in the winter southern sky.

Picture taken using the Photopic Sky Survey ©Nick Singer.

Figure 1. DSS image of Shapley 3. The gray circles on the image indicate areas of

brighter stars

Late August is a good moment to observe another remarkable planetary nebula, Shapley 3, which was first reported by Harlow Shapley in 1936. According to the book "Hartung`s Astronomical Objects for Southern Telescopes 2nd. Edition" by David Malin and David J. Frew, this planetary nebula is visible in an 8-inch telescope as a faint circular glow 30" wide. This object, also known as Hen2-341 and PK342-14.1, has a magnitude of 11.9 and a diameter of 36". To find this planetary is easy because it lies only 56 arc minutes to the south of the naked eye star Theta (θ) Arae (visual magnitude 3.7). If you can find NGC 6397 and enjoy the view of this nearby globular cluster, it can be useful to know that the planetary lies about 4.8 degrees northeast of it. 

Using the Optimum Magnification Methods and considering that a visual magnitude of 11.9 and an angular size of 0.6 arcmin are accurate values, the optimum magnification to detect this Milky way´s nebula is 100x for an 8-inch telescope working under a 6.2 limiting magnitude sky. So, let`s see what I could see at different magnifications from a site that offers not exceptionally dark skies but dark enough to carry out this kind of observation.

Figure 2. DSS image of Shapley 3 and its surrounding stars

At low magnification (42x) the field where Shapley 3 lies is rich in stars. Several of them are bright (around magnitude 9) for a telescope. In Figure 1 I indicate with circles the zones where the brighter stars lie. The zone between those mentioned areas shows not too many stars and all of them are faint. At this power, the stars linked with a blue line were used as a starting point to find the accurate position of Shapley 3. Without a doubt, this planetary is a challenging target for an 8-inch telescope. In spite of that, it can be barely glimpsed using averted vision as a quasi-stellar object, very small in apparent size. The very faint stars immediately surrounding this object can be also detected using averted vision, like the one indicated with the arrow in Figure 2 that was useful to find the accurate position of the planetary nebula. Using the Orion Ultrablock filter the planetary looks like a very small hazy spot. Averted vision is necessary to glimpse this faint object. With UHC the view is slightly improved. However, at this low magnification, the view is difficult appearing as a subtle and small hazy dot.

At 78x the faint neighboring stars are better viewed so it is easier to identify Shapley 3 in the field. Now the planetary nebula looks, even with direct vision, like a faint and small nebulosity surrounding a central star, which jumps to the view more easily when applying averted vision. With a UHC filter, the view is very different. It improves the view of the planetary a lot, appearing still faint but with a clear nebular nature, round, and rather smooth in brightness. For moments the central region appears brighter because of the presence of the central star. The view through the Orion Ultrablock filter is not so useful as that with the UHC filter. Shapley 3 does not appear so detached as with UHC. The central star is better viewed, always surrounded by subtle nebulosity.

Shapley 3. Image from Simbad database

Higher power, like 106x, allows you to see the central star which is better detected using averted vision. This 12 magnitude star is surrounded by a very subtle nebulosity. UHC filter improves the view of this planetary nebula appearing as a round hazy spot of smooth brightness that can be identified from the surrounding stars. Dark adaptation is a must for an optimum view of this faint object. Using the Orion Ultrablock filter at this magnification the view did not depart from that without a filter. It was not as useful as the UHC in order to get a better contrast.

I got similar results after observing Shapley 3 with a little higher power (148x). However, the planetary appears even fainter, especially through the UHC filter. Orion Ultrablock offers a not so detached view of this object when comparing with UHC.

IC 4642

IC 4642 is a faint planetary nebula situated in constellation Ara, not so far from NGC 6397, one of the nearest globular clusters.
Picture taken using the Photopic Sky Survey ©Nick Singer.

IC 4642 ©R. Corradi et al.

For those observers who want to look for more challenging planetary nebulae with an 8-inch telescope, this one is an interesting target situated at the core of constellation Ara. IC 4642 (also PK 334-9.1) was discovered by Williamina Fleming in 1901. Some planetarium software, like Skymap for example, classifies this object as type 4 (ring shape in the Vorontsov-Velyaminov scheme). This planetary nebula is listed in the paper "Precessing Jets and Point-Symmetric Nebulae" by J.A. Cliffe et. al (1995) as having a bipolar/point symmetric symmetry. 

This 15" size object is immersed in a field that shows several faint stars. Some brighter stars are visible in the east half of the field of a telescope working at low magnification. In a 1-degree field, the brightest star is the 8.3 magnitude HD 154970. The stars linked with a blue line in Figure 1 are useful to find the planetary which forms a sort of "question mark" shape along with four faint stars of 11 and 12 magnitude (linked with pale blue lines on Figure 2 below). The 12 visual magnitude variable star V788 Arae, a semi-regular pulsating star according to the Simbad Database, is indicated in Figure 2 with a blue arrow. To use that asterism makes the identification of IC 4642 very easy. At low magnification (63x) and without any nebular filter IC 4642 is visible, appearing like a very small hazy disc that can be glimpsed even with a direct vision.

Figure 1

Averted vision, on the other hand, allows seeing a more detached view. A UHC filter improves the contrast at this power.

Jumping to a higher magnification (118x) the view of IC 4642 is more obvious at the eyepiece, clearly looking like a small, round nebulosity among the surrounding stars. Through a UHC filter, the view is definitively beautiful with the planetary detached against the background sky, rather smooth in brightness, and circular in shape. An Orion Ultrablock filter also improves the view, which is pretty similar to that obtained with the UHC filter but somewhat brighter through an 8-inch telescope.

This power (160x) makes the planetary to appear round and smooth in brightness. With averted vision the core of this object appears to look a little darker for moments, suggesting (in a rough way of course) a ring-type structure. Seeing conditions at the moment of this analysis was not the best, so it would be good to make another observation under more optimum conditions to compare results. The observation using the Orion Ultrablock shows a round and smooth nebula. Averted vision makes possible to detect what appears to be a point-like feature, slightly brighter, on its west side (indicated with a white arrow in Figure 2). It is important to say that this feature was very difficult to confirm.

Figure 2

At 222x IC 4642, round in shape, seems to show a somewhat darker core. However, this view is very subtle. Nebular filters, as in other cases, helped to show a more detached view of this planetary.