Published in the April 2010 issue of the Rosette Gazette newsletter (Rose City Astronomers, Portland Oregon USA)
The Observation of the Second Largest HII Region
in the Large Magellanic Cloud
The northwest corner of the Large Magellanic Cloud (LMC) is home of many different kinds of deep-sky objects including bright nebulae, open and globular clusters, and supernova remnants. Several of these objects are visible within the range of average size amateur telescopes.
But more than this, the northwest region hosts the second largest HII region of our satellite galaxy, the N11 complex. I began the observation of this interesting formation from an observing site at Uspallata Valley (Latitude -32.5° Longitude 69.3W 2,000 meters above sea level) at 01:25am (04:25am UT) on February 14, 2010, when the LMC was at 40° high in the sky. This is a circumpolar object at this latitude and it reaches about 53° of altitude when it transits. At the lowest altitude (inferior transit) it is visible at 13° above the South Horizon. Taking this into account, the best season to observe this galaxy is during the southern summer months (December, January, February, & March) when it is high in the sky.
The N11 Complex and Its OB Associations
N11, whose parts are known with the more familiar numbers in the NGC catalog, lies about 4.75° to the northwest of the 30 Doradus complex (Tarantula Nebula) and can be, according to Y. Nazé et. al. in their paper “XMM-Newton Observations of the Giant HII Region N11 in the LMC”, a more evolved version of this latter nebulae. Observing our nearby satellite galaxy through common and average size binoculars (e.g. 10x50) N11 is one of the most prominent features that are clearly visible among others, like the 30 Doradus Complex, the conspicuous off-center bar-type structure (see picture above), and the stellar arcs in the northeast part of the galaxy where Shapley´s Constellation III (one of the most enigmatic structures in the Local Universe: a coherent semicircular arc spanning several hundred parsecs, composed of thousands of bright young stars and tens of star clusters) in the LMC41 region lies.
I have indicated all these features in the picture above taken by Australian astrophotographers. If you come to the Southern Hemisphere to observe the sky have in mind that N11 is one of the bright extragalactic nebulae to be included in your observing program. The 4.8 magnitude star θ (Theta) Doradus, which is clearly visible to the naked eye from a dark sky site, can be a good starting point to find N11 (see the first picture in this article). This HII region lies about 1.8° west-northwest of this star.
A first view at low magnification (42x) shows a prominent nebulae complex about 15 arcminutes wide in PA2=234°. It is clearly visible even with direct vision and without a filter, containing some patches with different angular sizes and shapes. As mentioned in the paper above, it harbors some associations2 of massive stars: LH9, LH10, LH13, and LH14 (Lucke & Hodge 1970) and even a supernova remnant, N11L, which is included in another observing project I am carrying out with a bigger mirror (16 inches).
The other brightest nebula, NGC 1763, is also indicated in Figure 2. It is a structure with a brightness similar to that of NGC 1769 and elongated approximately East-West. A more detailed observation at the same magnification (42x) made possible the detection of a stream of faint stars within this hazy structure, the bigger one.
The zone labeled C, which is outlined with a blue line in Figure 1, is visible as several faint stars very close to each other and embedded in faint nebulosity. The brightest part is coincidentally in the area where nebulae NGC 1760 and NGC 1761 are situated in the eyepiece field (in the southwest part of the complex).
After the observation and identification of the whole complex, I decided to use the UHC nebular filter to observe each patch again using the same magnification. NGC 1769 and NGC 1763 appeared smooth in brightness through this filter and more contrasted against the background sky. On the other hand, NGC 1773 looked sharper and the nebulosity in the region labeled C more obvious, with a detached small spot in the south edge, surely NGC 1760.
Using an eyepiece that gives a little higher magnification (78x) two stars appear in NGC 1773 (also N11D) surrounded by a faint nebulosity (see Panel A). Embedded in the nebulosity lies LH 14, the least studied of the four OB associations3.
NGC 1769, at this magnification, shows the star-like core and the surrounding circular nebulosity with more detail. Mentioned above, there are some OB associations in the N11 complex. LH 13 lies in this bright component, also known as N11C. LH 13 contains two compact stellar clusters, Sk−66◦41 and HNT. The ages of the two clusters suggest there is no association between them (Heydari-Malayeri et. al. 2000). I think the stellar core I saw through my 8-inch telescope is actually Sk−66◦41, according to its position in NGC 1769 (see Panel B).
At 78x, the stream of stars in the elongated patch NGC 1763 (also N11B) is clearly visible on its south edge (Panel C). These stars belong to the OB Association LH 10 which is the youngest cluster of the N11 complex containing several O3 stars and, according to Nazé et. al. 2001, is still embedded in its natal cloud but its most massive components have already begun to blow bubbles around them.
Finally, the region indicated with C in Figure 1 looks very interesting at this higher magnification, with several stars easier to observe in hazy background. This group of stars I saw is the LH 9 association (indicated in Figure 1), whose action on its surroundings has triggered a burst of star formation in the periphery leading to the birth of the three other OB associations (Rosado et. al. 1996).
At 106x, using a UHC nebular filter, a very faint and smooth nebulosity with an irregular shape is visible in the zone where NGC 1760 lies (see Figure 1). The view I had of NGC 1760 was not as detailed as shown in the DSS image (Figure 3).
The Surrounding Field of N11
Other objects are situated close to the N11 complex. If we move about 25 arc minutes to the southwest we find the brightest star in the 1° field, HD 31754, a 6.4 visual magnitude reddish star. Very close to it lies the faint (magnitude 13.3) open cluster NGC 1733. I tried to observe it but it was not visible at 42x.
Three galaxies reside to the West of the N11 complex, the pair of PGC galaxies (16243 & 16244) were not visible at low magnification (42x). The same occurred with the galaxy LEDA 89996, not visible even at higher magnification (106x).
To the south of these galaxies lies NGC 1731 (see Figure 1). According to Wolfgang Steinicke's Revised NGC and IC Catalog and software Skymap Pro 6, this is a cluster with nebulosity. Through my 8-inch telescope, it looks like a faint luminosity engulfing the 10.7 magnitude star TYC 8889-619-1, indicated with letter D in Figure 1. The nebula extends toward the direction indicated by the blue arrow there.
Our nearby galaxy is home of several interesting nebulae complexes and group of stars. It is rich in objects to explore through telescopes, thus discovering the wonderful structure and content of this companion, along with the Small Magellanic Cloud, of our Milky Way.
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1_ Nebulae complex to the north of 30 Doradus and to the east of
Theta Doradus).
2_ PA= Position Angle
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.