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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 10solar 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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1A 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.