N79 & N83 Complexes in LMC

A Visual Observation of Interesting Structures in the Large Magellanic Cloud. Researchers Say one of Them Could be a Rival of 30 Doradus Nebula

Figure 1. The Complexes under observation

Early March, it is dark enough now (around astronomical twilight), so the Large Magellanic Cloud looks amazing, high in the southern sky. I am ready to aim the telescope (8-inch in diameter) to the west region of this galaxy, where the arm W is situated, to find and observe in detail other interesting complexes of our nearby galaxy (i.e. LHa120¹-N79 and N83). The extent of these regions are outlined in Fig. 1b in the paper "Ultraviolet and Optical Observations of OB Associations and Field Stars in the Southern Region of the Large Magellanic Cloud", Joel WM. Parker  et al. (2000). In the 1° field of view (Figure 1) the white-blue star HD 31518 (magnitude 7.2) is the brightest one. Near the center, LHa120-N79 is visible along with its companion LHa120-N83, both are small regions of nebulosity with N79 appearing more prominent and "wide". These complexes reside in a bigger structure named Shapley VII, one of the giant stellar and gaseous groupings of the LMC that has an angular dimension of 48x25 arc min (PA 90°). Shapley VII lies in an even bigger structure named SGS² 7 that has an angular dimension of 55x55 arc min. In the field of view, toward the East of these complexes, a region showing a higher concentration of very faint stars embedded in subtle nebulosity, elongated East-West, can be detected. We are talking about LHa120-N94 which includes the OB association³ LH⁴8, which encloses, in turn, the cluster of stars NGC 1767, discovered by John Herschel. A few brighter stars are superimposed in that region. This region seems to be connected to the two main nebulosities by very subtle nebulosity.

N79 is an irregular HII region, roughly 17x14 arc min in size, that contain the OB associations LH1 (also NGC 1712) and LH2. Its coordinates are R.A. 04h 52m 00s Dec. -69° 22` 30" (J2000.0). This region has a star formation efficiency exceeding that of 30 Doradus by a factor of ∼ 2 as measured over the past 0.5 Myr. The first observation of this object was made at low magnification (42x). The animation below showing the N79 complex (Figure 2) was made using a DSS image to show what I think it could be nicknamed the "Magellanic Horsehead Nebula" because it resembles, in my opinion, the profile of a horse. That section of the complex is called LHa120-N79E and is the most prominent one (see Figure 4), appearing as a rather elongated nebulosity E-W. Using averted vision the nature (as a nebula) of this object is more obvious, appearing somewhat wider and more contrasted. Some of very subtle nebulosity seems to lie in a small area southwest of LHa120-N79E. Higher magnification and nebular filters will surely help for a more detailed description of this area.

Figure 2. Zoom in  into LHa120-N79, a complex in LMC.

Using the same magnification we move to the complex N83, situated about 15 arc min northeast of N79.  Henize (1956) identified four ionized gas regions lying in that direction (N83A, B, C, D). Lucke & Hodge (1970) detected an OB association (LH 5) spanning over 16 square minutes (3600 square pc) around N83. It contains 26 blue stars (Lucke 1974) the brightest of which is Sk-69◦30 (Sanduleak 1970), a G5 Ia with a visual magnitude of 10.18 so it is the easiest star to see of three that are visible there (see paper "HST study of the LMC compact star-forming region N83B" by M. Heydari-Malayeri et al.). Another one of the three stars looks like a small roundish nebulosity where a star is visible within it when using averted vision. Actually, that star, Sk -69 25, is the main exciting object of the mentioned nebulosity which is known as N83A, also NGC 1743 (seeFigure 6). 

At the same magnification a nebular filter, like UHC, improves the view of both complexes. N79E is visible, with averted vision, as a rather elongated nebulosity where a couple of stars are glimpsed for moments. With a similar configuration of that in N83 complex, three stars can be detected in the N79 area, to the south of N79E. One of them looks actually like a star embedded in a small and round nebulosity, this object is MCELS⁶-L25 (see Figure 4), while the other ones are the 10.7 magnitude blue supergiant HD 268718 and the 8.5 magnitude HD 31722. The whole area seems to be engulfed by very subtle nebulosity. 

Back in N83, the component N83A looks like a small nebulosity while the entire star-forming region seems to be engulfed by some nebulosity also.

Figure 3. The object H72.97-69.39.

Picture courtesy of Bram B. Ochsendorf.

Taken from the paper "The Star Forming

Complex N79 as a Future Rival of 30 Doradus" 

written by him and his team and included here

with his permission. 

The so-called high excitation blob⁵ N83B (also NGC 1748) which lies about 2.5 arc minutes north-east of N83A (Figure 6), and is around 25 light years in diameter, probably represents the most recent massive starburst in the giant HII complex N83 and the OB association LH 5. Heydari-Malayeri et al. (1990) discovered a compact HII region toward N83B. This object, which they named N83B-1, turned out to be a member of the so-called high-excitation blobs (HEBs) in the Magellanic Clouds. The brightest blob is the compact HII region N83B-1.

The zone of LH8 shows, through a UHC filter, smooth nebulosity, being more obvious toward the edge of the eyepiece field and less visible (maybe absent at all) in the zone between it and the complexes under study.

Now, a view of MCELS-L25, situated close to the 11.8 magnitude star RMC 54F (according to SIMBAD Database), with higher magnification (78x), shows a very small and detached hazy patch that looks brighter toward its center. N79E looks like an elongated structure where some stars, members of the OB association LH2 (NGC 1727), are embedded and associated with the nebulosity. Averted vision improves the view of this section.

Through this eyepiece, giving 78x, only N83A appears like a roundish object, compact, and with a hazy appearance. Some of nebulosity is hard to detect in the area where the object MCELS-L55 lies. 

Applying a UHC filter, the view of N79 and, in particular, N79E is very interesting. N79E is, by far, the most prominent feature there, where faint stars member of LH2 can be glimpsed using averted vision. For moments, and of course using averted vision again, the component N79D can be barely seen, very faint and round in shape. The star cluster KMHK 187 is associated with this nebulosity according to SIMBAD Database. MCELS-L25, through this magnification and filter, looks like a small hazy spot with a star-like brightness at the center.


H72.97-69.39, a precursor to the R136 cluster in 30 Doradus? 

At the heart of the large-scale complex N79 lies an extremely luminous object which immediately draws parallels to the central cluster of 30 Dor, R136 (Nayak et al.) (See Figure 3). Bram B. Ochsendorf and his team refer to it as ‘H72.97-69.39’ (I indicate it with red letters in Figure 4). This object is more luminous than any MYSO⁷ or compact H II region discovered with large-scale IR surveys of the LMC and Milky Way (read more in the paper "The star-forming complex LMC-N79 as a future rival to 30 Doradus" by Bram B. Ochsendorf et al. 2017). So, if you observe this region of the Large Magellanic Cloud with your telescope, remember that a peculiar and intriguing object resides there.

Figure 4. LHa120-N79 components

At this magnification (78x) and UHC filter, N83A is clearly visible being the most conspicuous feature (in brightness) of the whole complexes. It looks, as with lower magnification, like a round and small nebulosity with its star at the center. A very challenging object, difficult to see, is MCELS-L55, appearing like a ghostly round patch using averted vision.

There is a good view of the nebulosity in N79E using the Orion Ultrablock filter. Through this filter the stars associated look a little more clear. N79D is an elusive object and it could not be clearly visible at this magnification and this filter. On the other hand, MCELS-L25 appears like a small patch of nebulosity close to the 11.8 magnitude star RMC 54F.

In the complex N83, N83A is visible with not difficulty appearing more bright and nebulous than its counterpart in N79 complex. N83A reminds me a kind of small and bright planetary nebula. MCELS L-55 is elusive and a challenging object through this filter also. It seems to show up very briefly but I can not assure it is visible. It is, without a doubt, a good test for rods in your eye.

Using higher magnification...

Figure 5. N83B detailed image
Credit: M Heydari-Malareri & NASA/ESA.

106x is a good magnification to see the whole structure in the field containing the two complexes under study. Now the association LH2 and the associated nebulosity is visible in more detail. The star CTIO85 263 (visual magnitude 12.4 according to SIMBAD Database), is the brightest one visible in that association through an 8-inch telescope (indicated with an arrow in Figure 4). N79D is barely detected using averted vision, appearing seemingly round in shape. At this power, N83A looks now like a small and compact stellar cluster.

Through a UHC filter N79E looks faint, hazy, and without stars. The nearby structure N79D can be detected using averted vision but still elusive, faint, and challenging. It appears like a round structure separate from N79E by no nebulosity.

N83A looks bright, nebulous, and rather round in shape.

Figure 5 is a composite image of the region N83B taken by HST/WFPC2. The brightest blob is the compact HII region N83B-1 and the fainter one below it (∼100 in size) is N83B-2. The small arc-nebula further south, centered on a relatively bright star, is N83B-3.

Figure 6. LHa120-N83 components

A final observation of the complexes using 166x allowed me to see even more details. The component of N79, MCELS L-25, seems to be composed by two "star-like" objects, like two slightly defocused stars. N79E is visible with more detail, showing several stars embedded in faint elongated nebulosity. N79D is again visible, round, faint, and displaying smooth brightness. Averted vision is necessary.

N83A is clearly visible showing some faint nebulosity in its outer part. The section named MCELS L-55 is hard to see, challenging and faint. A few stars seem to be glimpsed embedded in extremely faint nebulosity.

Orion Ultrablock filter and averted vision make it possible to see two objects associated with nebulosity in MCELS L25. The view of N79D through this filter seems to get worse with respect to that obtained without any filter.

Another stunning formation in our satellite galaxy awaits to be seen by observers with telescopes during a clear night in the southern hemisphere summer.

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1_  LHa-120 N- This is the full name of an entry in the Henize catalogue of LMC emission nebulae. "L" refers to the Lamont-Hussey Observatory of the University of Michigan; "Ha" means the Hydrogen-alpha emission line, the key signature line used in the survey; "120" refers to the plate number (objective prism plate) for the LMC; "N" labels the object as a nebula, as distinct from a star (label "S").

2_ SGS are the supergiant shells, the largest of complex filamentary structures in irregular and spiral galaxies, indicative of a violent ISM, with diameters approaching 1 kpc (Goudis & Meaburn 1978). SGSs are thought to be formed by the fast stellar winds and supernova explosions of multiple OB associations.

3_ OB Association: The concept of a stellar association was originally introduced in 1949 by V. A. Ambartsumian, who later separated them into OB and T associations (Ambartsumian 1968). Morgan, Sharpless, & Osterbrock (1952) considered as a stellar association any loose group of stars within an area where bright OB stars exist and with evidence of a common origin.

A recent definition of a stellar association (Kontizas et al. 1999) refers to it as a single, unbound concentration of early-type luminous stars, embedded in a very young star forming region.

4_  LH is a catalogue of OB associations in the Large Magellanic Cloud compiled by Lucke & Hodge.

5_ The compact HII regions called High-Excitation Blobs (HEB) constitute a rare class of ionized nebulae in the Magellanic Clouds. They are characterized by high excitation, small size, high density, and large extinction compared to typical Magellanic Cloud HII regions. These objects are tightly linked to the early stages of massive star formation.

6_ MCELS means "Magellanic Cloud Emission Line Survey". This is a joint project of Cerro Tololo Inter-American Observatory (Chile) and the University of Michigan using the CTIO Curtis/Schmidt Telescope.

7_ MYSOs stands for Massive (>8 Msolar ) young stellar objects.