With the beginning of this year 2013 I have started to observe protoplanetary nebulae from the list by Reiner Vogel: http://www.reinervogel.net/index.html
I studied the brighter ones like Frosty Leo, Minkowski 1-92, Minkowski 2-9 and the Egg Nebula earlier this year, and have now turned to the fainter and smaller objects. After ruling out those nebulae that are too faint or lower than Dec. -25 (I observe from 48 N), I ended up with four objects: Henize 3-1475 (Garden Sprinkler) in Ophiuchus, MWC 922 (Red Square Nebula) in Serpens (not to be confused with the Red Rectangle in Monoceros), IRAS 19024+0044 in Aquila, and IRAS 19234+1627 in Sagitta. These are experimental observations. I was just curious to find out what is possible regarding these very small objects for us amateurs.
1. Henize 3-1475 (Garden Sprinkler)
17 45 15
-17 56 46
Theory:
The Garden Sprinkler is the brightest, largest and best known of these three objects. There is a spectacular Hubble photo that also explains its nickname:
GardenSprpotw1241a.jpg
[img]GardenSprpotw1241a.jpg[/img]
http://www.spacetelescope.org/images/potw1241a/
The Garden Sprinkler is listed with a blue magnitude of 13.8. Thus it is well within the range of medium sized telescopes. Its overall length is 10’’ and its maximum width is 2.5’’. There is a very nice comparison of Minkowski 1-92 (Minkowski’s Footprint) and Henize 3-1475 in an article by M. Manteiga et al., published in the Astronomical Journal 141 (2011). Please follow this link, scroll down to Figure 5 and click for “High Resolution Image”:
http://iopscience.iop.org/1538-3881/141/3/80/fulltext/
As you see here, both objects have the same bi-lobal structure and are equal in length. But the fainter half of Minkowski’s Footprint is much brighter than that of the Garden Sprinkler. Also the brighter half of the Garden Sprinkler is much narrower (ca. 2.5’’) than that of Minkowski 1-92, but not much fainter. Its length is ca. 4’’ measured from the southeastern rim of the bright blob to the first bright knot in the Northwest. It is not implausible to count the first knot in, considering that the photo in the article was taken through Hubble’s Filter F555W which has its FWHM between 480 and 600 nm.
At the eyepiece:
The Garden Sprinkler is readily visible as a tiny, but clearly elongated object. The elongation (NW-SE) makes it distinguishable from several other 14m stars in the area. I used powers of 500x, 720x and 1000x to see more details. Filters were of no avail. I assume that not only the central part of the brighter lobe contributes to the image, but also the NW knot #1. Otherwise the image would not be elongated but just round. The main problem is the smallness of the object and the limited seeing at its low position in the sky. The size of the bright part (2’’-3’’) equals the range of my normal seeing conditions, which makes it very hard to discern any structure within the moving speckles. There are three features that I suspected. First, in rare moments the innermost part of the fainter lobe seemed to become visible as a very small nebulosity. Second, the bright part repeatedly separated in two points of light aligned NW- SE. If this was not just dividing speckles but a real detail, it could be the central part of the bright lobe plus the NW knot #1. Third, at times the SE point of light seemed to be somewhat extended and roundish towards its SE end, where the dust torus is located.
Although these phenomena appeared repeatedly, I cannot exclude that they were merely caused by poor seeing. Therefore, I would be very interested to hear about other observations, possibly from more southern localities.
2. MWC 922 Red Square Nebula
18 21 15.9
-13 01 27
MWC922Keck.jpg
Image courtesy W.M.Keck Observatory
Theory:
With ca. 14 mag. this object is bright enough for observation even in medium size telescopes.
There is a nice presentation of MWC 922 by P. Tuthill, which also explains the striking rectangular shape:
http://www.physics.usyd.edu.au/~gekko/redsquare.html
The sides of the bright inner square measure 3’’. This would be large enough for visual observation. However, the problem is that this photo was taken in near infrared at 1.6 micron. In this Hubble photo, taken in the visual range of light, the object has a side length of even 20”.
At the eyepiece:
MWC 922 is a little fainter than Henize 3-1475, but still easily visible as a starlike object. Does it appear as a square? Even after three nights of observation, I am not sure. I used powers of 500x, 720x and 1000x, but the side length of 3’’ is still close to limiting seeing conditions, so there was much instability in the image. At moments the object did appear more like a truly two-dimensional plane than like a smeared out point source. At these moments it also seemed to have straight sides instead of a circular shape like the stars next to it. But these impressions were too vague to support strong claims.
Are there other observations supporting or falsifying my impressions?
3. IRAS 19024+0044 in Aquila
19 05 02
+00 48 51
There is a Hubble photo taken in orange light and in infrared:
Adlerpotw1136a.jpg
Due to its 5 lobes this objects is sometimes called “starfish”, but to me it looks more like a bluish moth flying in the night sky. I did see the moth visually, but only as a very faint star shining up occasionally in averted vision. It was way too faint to search for any structures.
4. IRAS 19234+1627
19 25 41
+16 33 04
Here is a Hubble photo of this small ringlike object:
RingIRAS19234+1627.jpg
This object is not visible on the blue POSS II plates, but it clearly shows up on the red ones. So I hoped to see it through a UHC-S filter. The story of my observations is quickly told. Although I could precisely identify the position, I could not see anything at this place, neither with nor without filter. Its light is probably too red.
Greetings
Johannes
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