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).