RCW Nebulae in Constellation Carina - Part 2

Published in the June 2009 issue of the Rosette Gazette Newsletter (Rose City Astronomers, Portland Oregon USA)

Part 2. RCW 50, RCW 51, RCW 52, and a section of the complex RCW 54

In part 1, “Identifying RCW Nebula in Constellation Carina”, published in the April 2009 Rosette Gazette, I included observing reports on four of the nebula (RCW 46,47,48, and 49) with RCW 48 being, by far, the brightest and easier nebula to observe.

The purpose of this project (the observation of non-common nebulae in Carina) is to determine which of them are visible through an 8-inch telescope, observing under a normal dark sky (6.2 or 6.3 limiting magnitude) and which nebulae are invisible through this kind of instrument,  needing bigger scopes to see them or even to know if they are invisible at all and reserved for astrophotography or observations in other wavelengths only.

On April 18, 2009, I went to the same observing site (Canota) with the idea of observing more nebulae of that catalog. This time the objects are mostly situated in the immediate area of the “Mother of the Nebulae” Eta Carinae (NGC 3372, also RCW 53 and Gum 33 ). One of the sections of the RCW 54 complex (section b) is situated a few degrees East of NGC 3372 (see map below).

RCW 50, RCW 51 & RCW 52. Three Very Faint Nebulae

I observed these nebulae under not excellent sky conditions even though the sky was good enough to carry out the observations. However, more observations under an even better sky would be necessary to compare results.

RCW 50 

Figure 1

This nebula is situated roughly half a degree north of the 4.7 magnitude star HD 90772, so the zone was easily found using this naked eye star as a guide (see map above).

At low magnification (42x) you can see a starry field where a small and round nebulosity is visible in the zone indicated with A on Figure 2. Using averted vision a group of faint stars is visible there. It is not labeled as an open cluster by the charts I checked so maybe it is just an asterism.

Focusing on the search of RCW 50, the use of nebular filters like UHC or Orion Ultrablock does not help to see the nebula clearly. To the west, the stars within the red circle appear to be embedded in a very faint nebulosity.

Figure 2. RCW 50 and surrounding field. 

North is up.

Observing carefully at higher magnification (78x) a faint hazy patch is visible in the zone that I have indicated with a gray ellipse at the center of Figure 2. Observing with averted vision, a group of very faint stars populates the region.

The use of even higher magnification (106x) and the UHC filter makes possible to see the nebulosity a little better. Using the same magnification and the Orion Ultrablock filter, a “stream” of faint stars in a hazy environment is visible in the zone indicated by B in Figure 2.

Darker skies and bigger telescopes are necessary in order to get a better view of this faint nebula in Carina.

RCW 51 and the Open Cluster NGC 3293

The beautiful cluster NGC 3293.

RCW 51 is an HII region surrounding the stellar cluster NGC 3293. I observed it half an hour before its transit, so the altitude was very good (64°).

At low magnification (42x), the zone surrounding the cluster shows a starry field with several stars of faint magnitude. Some faint hazy zones are also visible. NGC 3293 (left) is a young open cluster with a visual magnitude of 4.6 and with a Trumpler classification of I,3,r,n. It was discovered by Nicholas L. de Lacaille in 1751-52. Situated about 2° northwest to the Eta Carina Nebula, it is a showpiece for observers in the Southern Hemisphere. It is a good example of an open cluster even for owners of small telescopes. If you observe this cluster with low magnification in order to have a wide eyepiece field (1.2° for example) you will see, with direct vision and without a filter, a uniform nebulosity (1 in Figure 3).

The open cluster NGC 3324 lies there and also the yellowish star (the brightest in the field) V370, which is not visible in the DSS picture here. 

Doubtless, RCW 51 is a challenging target for visual observers with telescopes like mine. Observing with averted vision a very faint nebulosity surrounds NGC 3293 and a faint and short extension is also visible eastward (indicated with a circle in Figure 3).

Figure 3. North is up.

I observed the zone again with the same magnification (42x) but this time using a UHC filter. The nebula surrounding NGC 3324 (to the south of NGC 3293-RCW 51) is the most conspicuous feature in the field, appearing round and with a smooth brightness. Focusing on NGC 3293, the nebulosity surrounding the cluster is viewed a little better. With this filter, a broad lane of faint nebulosity is visible to the West of the cluster reaching the stars indicated by the rectangle.

At higher magnification (78x) and with both filters (UHC and Orion Ultrablock) I got a similar view but a little worse than that at low magnification. The H-beta filter was useless, offering a poor view.

The Elusive Nebula RCW 52  

  

Figure 4. North is up.

Situated about 1.2° North-northeast of the center of the Eta Carinae nebula a 7 arc minutes nebula lies, situated in a field that shows fewer stars than where NGC 3293 resides.

If you have an 8-inch telescope do not expect to see much when you aim your telescope to the zone where RCW 52 (also Gum 32) lies. At low magnification, this crescent-shaped nebula was not visible at all. Using a nebular filter (UHC) and the same magnification, RCW 52 was extremely difficult to glimpse. 

At 42x, the outer part of the big nebula NGC 3372 is visible in the south edge of the eyepiece field as shown in Figure 4. I could identify the pattern formed by the stars where RCW 52 lies (showed in the right hand picture in Figure 4), including two stars indicated by white arrows, the 9.9 visual magnitude TYC 8626-142-1 (the brighter one), situated at the heart of the nebula, and the 12 visual magnitude GSC-8626-0002, expecting to see some of nebulosity connecting them. 
An observation with a UHC filter at the same power shows the outskirts of NGC 3372 brighter and sharper.

At 78x with nebular filters it was not possible to see RCW 52 clearly.

It is an extremely faint nebula, at least for an 8-inch telescope. You will see it appears in some star atlases (e.g. SkyAtlas 2000 by W. Tirion, the software Skymap Pro 6.0, and Skycharts) as Gum 32. Even the observation through a 16-inch telescope does not show nebulosity so this is a nebula to observe with even bigger mirrors or maybe it is not at the reach of amateur telescopes and is a good target for advanced astrophotographers.

The RCW 54 Complex and the Carina OB2 Association

The sky in the eastern part of constellation Carina is home of RCW 54. This is an extended emission nebulae ionized by the stars of the Carina OB2 association. The nebulae complex spans some degrees in the sky and in the few web sites where you can find information about it they talk about different sections of the nebula, labeled a,b,c, and d.

The map below (north is up) shows where the different sections are situated in the sky. There are two sections labeled RCW 54b which have different numbers in the Gum catalog (Gum 34a and Gum 34b).

For an observer in the United States this part of the sky is only visible at low altitude (less than 10°) from a few places in south Florida.

Sections of the RCW 54 nebula complex.

RCW 54b

This section of the complex, the easternmost of the RCW 54b sections (I name it “eastern b”), also known as Gum 34b, is situated roughly at Right Ascension 11hs 4m and a Declination of –59.5° (J2000.0), immediately to the West of the optical center of the Car OB2 association. According to the DSS images of RCW 54, this part is seemingly the bigger one. To find it is easy because it lies about 1° south-southwest of the superb naked eye open cluster NGC 3532 (position angle of about 205°).

Figure 5. North is up.

At 42x and without a filter a zone with a lot of mostly faint stars is visible. The brightest stars are situated in the southeast region of the field (lower left in Figure 5). I used some of the bright stars to identify the zone where the nebulosity should be visible. No nebulosity is visible in the eyepiece field though. However, the faint stars in the northern part of the field (up in the pictures)  appear to be embedded in a very faint and smooth nebulosity.

Using the UHC filter nebulosity jumps to view. Two hazy lanes form a “tuning-fork” or “V”-shaped structure that crosses the eyepiece field. The “branches” join together in the northern part. I had a similar view using this filter and 53x but the upper zone in the eyepiece was observed a little better. The UHC was the best filter to observe this HII region. With the H-beta filter I had the worst view.

In Figure 5 I have indicated with B the zone where a dark patch is visible. Moreover, a little brighter patch of nebulosity is visible in C, embedding a line of 3 stars with visual magnitudes between 9.7 and 10.9 even if the entire nebula is faint. It would be good to make a new observation of this area from a darker sky or from a site at a higher altitude.

At low magnification and without a filter, the emission and reflection nebula NGC 3503 is visible in the same eyepiece field. At 42x it looks very small with a smooth appearance and elongated shape. A star is visible very close to this nebula (see DSS image to the left).

The use of higher magnification made possible a detailed observation. At 78x, three very faint and close together stars are visible when you use averted vision. They are separated about 8 arc seconds each other (I measured the distance on a picture of NGC 3503 using the software Aladin v2.0). A faint nebulosity surrounds the chain of three stars. This is better viewed when you observe at 156x and a UHC filter. Of course, averted vision is necessary to get this view and discern the stars.

The blue supergiant stars HD 96248 and 96261 (shown in Figure 5) are believed to belong to the Car OB2 association. Also, I have indicated with red color some of the earliest stars that are listed as members and possible members of Car OB2 in table 4 in the paper “A Study of the Interstellar Gas Surrounding Carina OB2”. The optical center of the association lies there (left edge in Figure 5) and is given here in galactic coordinates (l=290.1 deg. b= +0.6 deg.)

______________________________________________________