RCW Nebula in Constellation Carina - Part 3

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

Part 3. RCW 54 Sections, RCW 55, RCW 56, RCW 57, and RCW 58

The map shows the positions in the sky of some RCW nebulae situated in the eastern part of the southern constellation Carina. The great Carina Nebula is highlighted in gray color. In addition, some stars (all brighter than magnitude 5.5) are indicated in the chart so the observer can use them as guides or starting points to find the different nebulae.

Some weeks after observing the faint nebulae close to NGC 3372 (Eta Carinae Nebula) I went to an observing site in Villavicencio and to Canota in July, 2009 to work on the last group of RCW nebulae, all of them situated in the eastern part of constellation Carina, thus finishing the project about these not very well known objects. 

The RCW 54 Complex and the Carina OB2 Association

A report on the biggest section of this complex (called by me “eastern b”) is included in part 2 of the RCW nebulae article. Here I have included reports about the other parts of this huge HII region. The map in the article “Identifying RCW Nebulae in Constellation Carina - Part 2” shows where the different sections lie in the sky, to the East of Eta Carinae nebula. 

RCW 54a (Gum 35)

Date:  July 18, 2009                Observing Site: Canota, Mendoza (-32.5°  -68.9W).

This is an HII region associated with the giant molecular cloud (GCB 88)13. This section of the RCW 54 complex lies about 1.8° southwest of the 4.6 visual magnitude star HD 97534. In the 1.2 degree eyepiece field given by my telescope (at 42x) the open cluster Cr 236 is seen detached from the surrounding stars. This 7.7 magnitude cluster (Trumpler classification III,2,p) shows a few stars with magnitudes around 9.5 and several more faint stars can be detected using averted vision.

According to the DSS image of this nebula (Figure 1) the brightest zone of this section lies about 6 arc minutes northwest of the 6.2 magnitude star HD 95324 (the brightest one in the field). However, no nebulosity is visible there without using a nebular filter. Using both the UHC and Orion Ultrablock filters and the same magnification a smooth and very faint nebula is visible with averted vision in the zone indicated by A. This faint hazy patch was very hard to see and it could be glimpsed only for few moments when I was observing it for several minutes. I had a similar view using a little higher magnification (53x) and the UHC filter. It is another target for owners of bigger mirrors, astrophotographers, or places with exceptionally dark skies.

RCW 54b

Date:  July 11, 2009         Observing Site: Villavicencio, Mendoza (-32.52°  -69W).

This is the westernmost of the two sections labeled as “b” (named here “western b”). It also appears in Gum Catalogue of Southern HII Regions as Gum 34a. If you check the eyepiece field picture (Figure 2) you will see that this nebula is situated just 25 arcminutes west-northwest of the nebula NGC 35031 which is visible in the same field of view of a telescope working at low magnification, and about half a degree west from the “eastern b” section described in part 2 of this project. Also, you can use the naked eye open cluster NGC 3532 to find the zone where this nebula lies, moving your telescope 1.5° to the southwest.

RCW 54b is situated very close to the bright star U Carinae (visual magnitude 6.3) which is obviously the brightest one in a field where several faint stars are visible. Toward the northwest, on the edge of the eyepiece field, the open cluster Trumpler 17 (Tr 17) is clearly visible like a hazy and round patch of smooth brightness. Its Trumpler classification of II,2,p matches well with the view at low magnification through my 8-inch telescope. Several faint stars populate it. Do not expect to see much more there with telescopes of this size. The section was not visible at all at 42x and 78x, even using the UHC and OIII filters. This is the last object I saw that night, the Moon rose around 11pm local time (UT – 3hours) so after 3 hours of observation (beginning at local astronomical twilight which occurred at 8:12pm local time) the sky got too bright to carry out more deep and detailed observations.

RCW 54c

Date:  July 11, 2009         Observing Site: Villavicencio, Mendoza (-32.52°  -69W).

This is another section of the huge complex RCW 54, also known as Gum 37. This section “c” lies close to a group of open clusters situated about 1.75° south of the naked eye cluster NGC 3532. You will be able to find this area easily using the 4.6 visual magnitude star HD 97534 (a member of Cr 240?).

At low magnification (42x) the field is very impressive showing open clusters with different Trumpler classifications and stars grouped in interesting shapes. “These open clusters form an extensive and almost continuous grouping of stars lying in a region where the Carina spiral feature is seen tangentially” (Clariá 1976). The naked eye star HD 97534 (indicated with an arrow in Figure 3) is the brightest one in the 1.2° eyepiece field of my telescope. A group of a few sparse stars is visible (lower left in Figure 3), the cluster Collinder 240 (Cr 240), a swarm of stars with a Trumpler classification of III,1,p,n and 25 arc minutes in size. Superimposed on Cr 240, toward the northwest, a smaller and more compact open cluster lies, NGC 3572 (Trumpler classification I,2,m,n), situated at a distance of about 2.8 kpc. The stars of this cluster show similar brightness. NGC 3572 probably belong to the association Carina OB2 and its massive stars are seen in projection close to the geometrical center of the cavity of the overall HI distribution toward Car OB2. 

Southeast of Cr 240, a stream of three open clusters is visible (Tr18, Hogg 12, and NGC 3590). Several of these open clusters (namely Cr 240, NGC 3572, NGC 3590, Hog 11, and Tr 18) may be physically related to the Car OB2 association (see the paper “A Study of the Interstellar Gas Surrounding Carina OB2” by J.R. Rizzo and E. M. Arnal).

Without a nebular filter no nebulosity was visible in the field. Using an UHC filter at the same magnification (which gave me a sharper image than the Orion Ultrablock filter) I could observe a faint and smooth nebulosity indicated by 1 in the DSS image here (Figure 3), involving also NGC 3572. If you want to see this section of RCW 54 you should keep your eye at the eyepiece for several minutes and explore the area carefully using averted vision as help. Seemingly, some of nebulosity is also visible embedding the line of three stars, including the 7.7 magnitude variable star V353 Carinae (indicated in the picture). I have also indicated with 2 a lane of faint nebulosity and with 3 a round and small hazy patch embedding a small group of a few stars.

Letter B indicates a dark zone visible in the eyepiece field where a very few faint stars are superimposed when you observe the zone without a filter. Observing through nebular filters this dark zone is clearly detected. Using averted vision I could see, for moments, a very faint extension of the nebula toward northwest (white ellipse) being a feature that was very hard to detect visually. If you see the 1 degree-wide image you can download from the STScI DSS image web page, this part of the nebula is clearly visible.

I got a good view of the nebula using 53x and the UHC filter. Even if the entire nebula is faint, the region indicated by the gray circle near the center of Figure 3 looks a little brighter and sharper at this magnification.

RCW 54d

Date:  July 11, 2009                Observing Site: Villavicencio, Mendoza (-32.52°  -69W).

This rather small nebula (Gum 36) lies 45 arc minutes to the East of NGC 3532 so the zone is easily found using this cluster as reference.
The small image to the left shows a nebulosity with an interesting shape. However, a very differente shape is visible when I observed it through my 8-inch telescope.

At 42x, the area shows a rich starry field. The eastern edge of NGC 3532 is visible in a wide field with the star ER Carinae being the brightest there (see Figure 4 below). The open cluster Stock 13 is also visible, containing few stars not very well detached from the surrounding field. After identifying the pattern of stars (indicated by A in Figure 4 and shown in detail in the right hand picture) no nebulosity was visible at this magnification and without using a nebular filter. In the section indicated by B a lot of very faint stars are visible when you use averted vision.

North is up.

Using an UHC filter, I first used the stars indicated by arrows to the right to search for nebulosity, but nothing was visible.
Through this filter, a very faint nebulosity seems to imbibe the open cluster Stock 13 and also the zones B and C.  Higher magnification is necessary to search for RCW 54d because it is a nebula with a small angular size.

Applying little higher magnification (78x) and using the UHC and Orion Ultrablock filters, I made a detailed observation of the zone between the stars indicated by arrows, where this nebula lies (see right panel in Figure 4), but I did not have fruitful results. 

At 148x without filter I could clearly see a group of faint stars, including one of the guide stars (left arrow in right panel on Figure 4). Working with the UHC and OIII filters, this nebula did not jump to the view. This section, which has a distance comparable to the distance quoted for the HI shell2 associated with Car OB2, is another example of a challenging RCW nebula.

RCW 55

Date:  July 18, 2009                Observing Site: Canota, Mendoza (-32.5°  -68.9W).

Half way between the variable star V371 and the naked eye open cluster IC 2602, known as "Southern Pleiades", lies this nebula (see map at the top of this article). 

At low magnification (53x), using a UHC filter and averted vision, some of faint nebulosity (in fact the brighter part of RCW 55) seems to surround a line of 3 stars which are clearly identified from the surrounding field and are also visible in the DSS image (1 in Figure 5).

A brief observation at 78x made possible a better view of the stellar stream. I also got a good view of the faint nebula and the stream at higher magnification (106x) using the UHC filter.


RCW 56. A Very Small Nebula

Date:  July 18, 2009                Observing Site: Canota, Mendoza (-32.5°  -68.9W).

The bright stars V371 and HD 96566 are visible in the same 1.2° eyepiece field of a telescope. Very close to the last one is situated RCW 56 (indicated by the arrow in Figure 6). The field surrounding the position of this nebula has few stars. The light from HD 96566 made the search of this small hazy patch hard. I scanned the zone with high magnification trying to avoid the presence of HD 96566 in the field. As you can see in the  picture, this nebula is very small in apparent size and very close to the star. 

Using 148x and the UHC filter, and then 296x with the same filter, nothing was visible through my 8-inch scope. Moreover, the altitude of the zone was not favorable for the observation (29° in the southwest sky).

RCW 57a & RCW 57b 

Date:  July 18, 2009                Observing Site: Canota, Mendoza (-32.5°  -68.9W).

These are nebulae that are easy to observe even without a nebular filter. They are situated about 20 arcminutes from each other so they are visible in the same field of view if you use low magnification. The observer can use the stars HD 96566 and V371 Carinae (visual magnitudes 4.6 and 5.2) to find them (see map at the top of this article). These HII regions are much brighter than others RCW nebulae in constellation Carina. In fact, they are two of the highest luminosity HII regions optically visible in our galaxy. They also have NGC numbers.

The zone shows several stars, with the brightest ones at the edge of a 1.2° eyepiece field. Two hazy patches are clearly visible without a filter. RCW 57a (Figure 7) is ionized by a large star cluster visible in infrared. It was discovered in 1834 by John Herschel who only saw the brightest parts of this nebula. For this reason there are six NGC numbers for this object which is usually known as NGC 3576.

At 42x, this HII region looks bigger than RCW 57b and elongated north-south (Figure 7, north is up). On the other hand, RCW 57b (NGC 3603), situated at 20,000 light years from us, contains some 10⁴ solar masses of ionized gas and may be (according to Eisenhauer et. al. 1998) the most massive HII region in the Milky Way aside from W94. It was viewed through my 8-inch telescope smaller and more compact than RCW 57a, showing a bright core that looks quasi stellar at 42x. Other parts in the surrounding field seems to have nebulosity.

Working with an UHC filter the view is much more interesting. Both nebulae are clearly detached from the surrounding field. As Figure 7 shows, NGC 3576 have some bright spots or regions. Observing carefully and using averted vision some of these regions are detected at this magnification. Two of them (NGC 3581 and NGC 3582) are situated very close each other and can be discerned using averted vision. They are the brightest part of the whole nebula. To the North I could see another region (NGC 3584) which is fainter than the other ones and is immersed in a wider and faint nebulosity. A thin dark lane seems to cross between NGC 3584 and the two brightest spots.

RCW 57b looks bigger through the UHC filter. A bright central spot is visible like a little defocused star. It is the central cluster of strongly UV-radiating stars of type "O" and "B" (see Figure 8).

Higher magnification is necessary in order to carry out a more detailed analysis of the nebula. It is elongated in shape and a few faint stars are superimposed. A lane of nebulosity is visible in the section indicated by 1.

At higher magnification (106x) and the same filter I could better identify, using averted vision, the different sections of NGC 3576. With this filter, the zone NGC 3584 looks elongated and the round patch of nebulosity known as NGC 3579 is visible. Also, the faint section NGC 3586 was glimpsed by moments but it was hard to see even with averted vision.

RCW 57b is clearly seen through the UHC filter at 106x, appearing as an east-west elongated smooth luminosity covering a section south of the central cluster (middle left in Figure 8). A group of researchers concluded that in terms of density and stellar population, NGC 3603 is a "galactic clone" of the cluster R136 in the 30 Doradus complex in the Large Magellanic Cloud (see paper "NGC 3576 & NGC 3603: Two Luminous Southern HII Regions Observed at High Resolution with the Australia Telescope Compact Array" by C. G. de Pree, Melissa C. Nysewander, and W. M. Goss).

I removed filters and applied even more magnification (296x). Then I aimed my 8-inch telescope to see the stellar cluster associated with the HII region NGC 3603, named HD 97950. I got a bad quality image but the small spot of light suggests a not smooth appearance. In Figure 8 I have indicated the position of an evolved blue supergiant star (B1.5 Ia) named Sher 25, located 20 arc seconds north of the central cluster. A group of researchers compare this star to the progenitor of SN 1987A in the Large Magellanic Cloud. 


RCW 58. An Annular Nebula

Date:  July 18, 2009                Observing Site: Canota, Mendoza (-32.5°  -68.9W).

The last nebula I observed in constellation Carina is RCW 58 which surrounds the 7.7 magnitude Wolf-Rayet star V385 (also known as WR40). I tried to observe this 7.0 x 7.0 arc minutes size nebula when it was about 28° high so the altitude was not the best. I first observed it using low magnification to find the accurate position of the nebula. The star is easily identified because it forms a pattern (triangle)  with other two stars with visual magnitudes 8.1 and 8.9 that are not visible in the picture here.

According to the image to the right, one of the brightest portions of the nebula lies in the middle of the way between the stars V385 and HD 96448 (visual magnitude 10). Other "bright" section is situated diametrically opposite so I focused my attention in that part of the field. The sections were not visible at all at low magnification and using the UHC and Orion Ultrablocks filters. 

A new attempt using higher magnification (78 and 106x) and the use of UHC and OIII filters did not give me a view of this nebula or any of its sections. Doubtless, it is a very faint nebula for an 8-inch mirror. However, it is labeled in Skymap Pro 6.0 planetarium software. I think it is good to carry out more observations under different (better) conditions and from different observing sites to have more information about the visilibity of deep-sky objects like this one.

Final remarks

For some months I have observed 12 of the 13 RCW nebulae situated in constellation Carina (except NGC 3372, the Carina Nebula, which appears in the RCW catalogue as RCW 53) including the five (5) sections of RCW 54 complex. Some of them were very hard to observe visually through an 8-inch telescope. Others, like RCW 58,  were invisible at all. The objects RCW 48, RCW 57A and RCW 57B were by far the brightest nebulae of the sample and they were visible even without using nebular filters. The use of these filters was necessary for a more deep and accurate analysis of the structures. Obviously these bright RCW nebulae have also a number in the very well known NGC catalogue (NGC 3199, NGC 3576, and NGC 3603 respectively).

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¹_  A brief description of this cluster with nebulosity in part 2 of “Identifying RCW Nebulae in Constellation Carina”, June 2009 Rosette Gazette

² _  Also, the HII regions RCW 54b and RCW 55 have similar distances and all of them are seen in projection on the same sky area where the HI expanding shell is observed.

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.)

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RCW Nebulae in Constellation Carina

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

 Part 1 - A Group of Nebulae to the West of Eta Carinae Nebula

The Eta Carinae Nebula

Early autumn in Mendoza is a good time to drive to Los Andes Mountains to enjoy the starry nights from there. Temperatures are nice and there are some places where you can find a dark enough sky to carry out detailed observations. From this latitude, the famous and conspicuous Eta Carinae Nebula (NGC 3372) is clearly visible to the naked eye among other bright objects in the same region when you observe from a dark sky site. In March, you can see it very high on the sky in the evening as a bright spot immersed in the bright lane of our Milky Way. Also, some open clusters like NGC 3532, NGC 3114, IC 2602 (nicknamed “The Southern Pleiades”), and NGC 2516 are easily visible to the naked eye. But, is Eta Carinae the only nebula you can observe there? 

Not really. As the map above indicates with green color, there exist some other nebulae “hidden” between those objects, situated to the east and to the west of the “great nebula”.

Working with detailed sky charts and data from SIMBAD and other information available in some specific web pages, I could make an observing program that contains up to 13 nebulae, all of them cataloged in the RCW (Rodger, Campbell, Whiteoak) catalog. This is an expansion of the Colin Gum´s Catalogue, the first major survey of HII regions in the southern sky published in the 1950s. All the objects in this observing project are situated in constellation Carina, surrounding the Eta Carinae Nebula. In fact, this is also an RCW object (RCW 53) but it is not included in my list.

In  the first part I make reports about four RCW nebulae situated to the west of Eta Carinae Nebula (i.e. RCW 46,47,48, and 49). The map above shows the distribution of RCW nebulae in the southern constellation Carina. This is a very rich part of the sky to observe even with binoculars. If you are a nebulae hunter you should keep in mind this area when you have a chance to visit the southern skies.

I observed the four mentioned RCW nebulae on Saturday, March 21 from a dark sky site where stars up to visual magnitude 6.2/6.3 were visible.

If you see the mentioned map, surely you will agree with me that QY Carinae is a good star to use as “starting point” to find at least three of the four nebulae. This star has a visual magnitude of about 5.8 and is a faint star, but visible with the unaided eye from a dark sky site.

The Faint Nebula RCW 46

Situated at about 0.8° to the northwest of the star QY Carinae a faint nebula lies. This object does not appear in some planetarium software like Skymap Pro 6 (C. Marriot) or Skychart by P. Chevalley, and in guides like SkyAtlas 2000.0 (W. Tirion). This is for sure an object to try to observe using big mirrors. However, I wanted to aim my telescope and have my own experience about the visibility of this nebula through an 8-inch telescope. 

Using 42x, I found the zone where this nebula is situated. I spent several minutes to find it. The eyepiece field is rich in stars and the brightest ones have a mean magnitude of 8.5. I could identify and I focused my attention on the zone surrounding the stars HD 88068 and TYC 8607-194-1 (indicated in the picture here) where images of RCW 46, specially the DSS image, show a more prominent nebulosity. The nebula was not visible with this magnification and without a filter. Even the use of the UHC filter did not help with the view. Higher magnification showed “nothing”. The H-Beta and OIII filters, used at different range of magnifications, did not help too. I hope to have a chance to aim a bigger telescope to this place. 

The RCW 47 Nebula 

Figure 1 (North is up).

This is the westernmost RCW nebula in this constellation and is another faint object to observe with an 8-inch telescope. A bigger instrument would help with an easier observation. This object was about 62° degrees of altitude at the moment of the observation, 1 hour before its transit through the local meridian, so it was at good altitude in order to get a good view. 

The nebula lies in a rich field where stars show interesting shapes. In the northern side of the 1-degree field of view (Figure 1) some stars form a shape that remind me a small version of constellation Scorpius (indicated with a red line). This helped to find the zone where the nebulosity should be.

Observing with low magnification (42x) and UHC filter an extremely faint nebulosity appears to be in the zone indicated with A, both in Figure 1 and in the DSS image (left). 

Observing with higher magnification (78x) and using again the same filter, this nebulosity seems to surround the line of stars situated at the south end of the indicated zone. 

I got a similar view using the H-Beta filter.

DSS image (North is up).

 

RCW 48, A Superb Nebula 

Figure 2. RCW 48

This is, by far, the brightest nebula of the observed group. It is also included in the very well known NGC catalogue as NGC 3199 (another name is Gum 28). This nebula, discovered in 1834 by John Herschel, lies about 0.76° east-northeast of the star QY Carinae. It is one of fifteen ring nebulae found to be closely associated with galactic Wolf-Rayet stars (Chu, 1981). You can read more about this nebula in the paper “The Nature of the Wolf-Rayet Nebula NGC 3199” by M. J. Whitehead, J. Meaburn and C.D. Goudis (1987).

I observed this nebula around 11:15pm local time (UT-3 hours) when it was roughly 64° degrees high in the sky. It is visible even without a filter as a smooth cloud in a starry field, showing an elongated shape suggesting its curved shape showed in the picture. For some observers it is known as the “Kidney-shaped Nebula”.

The view using low magnification (42x) and UHC filter shows a wonderful arc-shaped object. Its southeast end (indicated by 1 in Figure 2) looks brighter with a faint star seemingly embedded there. Actually, if you see this region with higher magnification you can see the star to the side of the nebula. Moving to the northwest (to the right on the picture), the nebula looks wider and a little fainter when you observe it using averted vision. 

To the northeast, a chain-shaped group of stars appears surrounded by nebulosity when it is observed at 78x. If you observe carefully the region labeled 2 (Figure 2) using high magnification (106x) and UHC filter, some small dark zones and filaments seems to be present here. 

The use of other nebular filters gave me different views. I got a bad view working with low magnification and H-Beta filter. This filter was useless, the nebula looked better without it. On the other hand, at the same magnification (42x) but this time using the Orion Ultrablock filter, the view of the nebula was excellent (maybe a little better than that obtained with the UHC filter!!). The stars are visible a little better and the image was a little brighter.

The star indicated with the red arrow in Figure 2 is HD 89358 (also WR 18), a Wolf-Rayet star exciting the nebula. It has a visual magnitude of about 10.6 so it should be visible even with smaller telescopes under good sky conditions.

The Star Forming Region RCW 49 and the Open Cluster Westerlund 2

RCW 49 is a luminous and massive HII region in our galaxy. Star formation is ongoing in this nebula (paper "A Glimpse of Star Formation in the Giant HII Region RCW 49", B.A. Whitney et. al.).

Figure 3. RCW 49 and the open cluster Westerlund 2. North is up.

It lies in the eastern part of constellation Carina (R.A. 10h 23m 54.0s   Dec. -57° 45' 00" J2000.0 - coordinates for the associated cluster). You can use the star QY Carinae to find it but even better is the 4.7 magnitude star HD 90772 (a circumpolar star from -32.9° latitude on the Earth). You must aim your telescope just 0.5° to the west of this star to find this object, associated with the open cluster Westerlund 2. 



The Open Cluster Westerlund 2

According to the paper “Early-type Stars in the Core of the Young Open Cluster Westerlund 2” by G. Rauw et. al. (Dec 2006), this is a young stellar cluster situated in a blowout region of the HII region RCW 49. The stellar winds and ionizing radiation of the early-type stars in Westerlund 2 have evacuated the dust in the central part of RCW 49 and filled the cavity with very hot low density gas. The Trumpler classification is I,3,p,n and the magnitude of the cluster 10.5. 

The cluster is an elusive target not because of its brightness but for its small angular size, about 1.5 arcminutes. Thus, you must observe the zone carefully if you try to find it using low magnification.

The field surrounding RCW 49 and Westerlund 2 is very interesting. At 42x the open cluster IC 2581 (also Cr 222) is visible to the east-northeast in the field (see left hand picture in Figure 3). The brightest star in the field is HD 90772, the star I used to find RCW 49. Another stellar cluster is visible in the field, NGC 3247 (classified as II,2,p,n). It looks like a not very well detached group of stars with similar brightness. Once I identified these two clusters I moved my eye to the region where the nebula and the small cluster lie. Two very small and faint hazy spots are visible there. 

The nebula, through a UHC filter at 42x, appears like a smooth fan-shaped cloud with a faint star just in its vertex. Higher magnification is necessary in order to have a better analysis of its structure. 

At 78x plus UHC filter the real shape of the nebula is better viewed (more similar to the shape showed in pictures). A couple stars are visible embedded in the nebula. The red arrow in the picture to the right shows a dark region that was clearly visible through my telescope. On the other side of this region, the cluster Westerlund 2 is situated, appearing like a defocused star surrounded by a very small nebulosity. The observation at the same magnification using the UltraBlock filter was very similar. However, it showed the two immersed stars a little better and a more contrasted dark lane.

Observing the cluster at 156x, I could see a very small aggregation of faint stars. A line of three stars is the more prominent feature (labeled with a red circle in the right-hand picture in Figure 3).