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
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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.
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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
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
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
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.
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.
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.
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.
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| North is up. |
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
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
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
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).
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
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
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
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.
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.