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RA: 00h 59m 14.8s
Dec: −72° 11′ 01″
Ch: MSA:501, U2:441, SA:24
Type: bright nebula (HII region)
Mag: B=?, V=?
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"A very young component in the pre-eminent starburst region of the Small Magellanic Cloud". A&A 517, A39.
"LHA115-N66, or in short N66 (Henize 1956), is the largest and the most luminous HII region in the Small Magellanic Cloud (SMC). It is also known as DEM S 103 and NGC346, the latter referring to the bright OB association located at its center. … N66 is considered to be the scaled-down counterpart of the Large Magellanic Cloud starburst 30 Doradus. It indeed hosts the largest sample of young, massive stars in the whole SMC with 33 O-type stars among which 11 are of type O6.5 or earlier. It contains at least one W-R star in the massive binary or maybe triple system HD5980. An age of ∼3 Myr has been estimated for NGC346 from evolutionary models in the H-R diagram.HD5980 lies behind a SNR which has no known optical counterpart. Compared to the Orion Nebula, N66 has an H-alpha luminosity almost 60 times higher. This radiant flux is also reminiscent of those of giant HII regions in distant metal-poor galaxies, such as regions A1 and A2 in IC 4662 lying 2.44 Mpc away. Therefore, N66 offers a valuable template for studying these kinds of distant galaxies with high resolution.
"Apart from its recently formed massive star population mentioned above, NGC346 also has a large population of low-mass, pre-main-sequence stars covering a mass range down to the subsolar regime. The PMS population is found to be mainly concentrated in a number of subclusters away from the massive star association. The typical ages of the PMS population derived from models appear to suggest that low-mass star formation events occurred at two different epochs about 4 and 10 Myr ago.
"The present study is concerned with massive star formation in the N66 complex. Clustered mainly in NGC346, as mentioned above, massive stars dominate the central part of the whole HII region with their strong UV radiation field. Twenty two of the above-mentioned 33 O stars are contained in the central cluster. The hottest star, W3, is reclassified as O2 III(f*). The most massive star,W1, of the central cluster, classified O4 III(n)(f) , has multiple components and the mass of the brightest component is at most 85 M_solar. The cluster has disrupted the bulk of the natal molecular cloud, and therefore not much CO emission is detected towards N66, except for two positions which are mapped in the (1–0) and (2–1) transitions.
"One of these CO peaks is associated with a remarkable feature of the whole landscape, a compact HII region, called N66A, according to Henize (1956). The HII region apparently lies at the south-eastern end of an absorption lane that runs over some 60 pc from north-west to south-east below the NGC 346 cluster. This paper is mainly devoted to this compact HII region. Despite extensive research on various components of the N66/NGC346 complex, few studies have so far dealt with this HII region. We attempt to demonstrate that this region represents the youngest episode of massive star formation in N66.
"A word of caution seems necessary about the name of this object. From their observations of H2 emission line and the ISOCAM LW2 band, Contursi et al. and Rubio et al. detected several embedded sources towards N66, which they alphabetically designated from "A" to "I". The IR source A should not be confused with the Henize N66A HII component, which corresponds to the IR source "H". In a similar way, NGC346 corresponds to "C"."
Their Figure 1 presents a "composite three-color image of the SMC HII region N66. The star cluster above the curling absorption lane is the OB association NGC 346. N66A is the brightest compact HII region lying at the eastern end of the dark lane. Note the wind-driven bubble centered on the brightest star HD 5980. The other bright star lying towards the field center is Sk 80. The image, taken with the ESO NTT/SuSI2, results from the coaddition of narrow-band filters H-alpha (red), [OIII] (green), and H-beta (blue). The field size is 336″ x 350″ corresponding to 100 x 103 pc. North is up and east to the left. … The NGC 346 cluster appears to be at the center of an HII bowl, the southern border of which is delineated by a compressed ionized gas front and an absorption lane running over some 60pc. In particular, N66A stads out as the most compact HII nebula of the whole region, with coordinates (J2000.0) RA = 00:59:14.8, decl = -72:11:01. The compact HII region is apparently associated with the compressed gas front and the absorption lane.
"The field of view of the NTT image is larger than that of HST ACS. It also displays a turbulent environment in the eastern side of N66 with many indications of shocked gas. In particular, the wind-driven bubble centered on HD 5980 is quite impressive. A narrow ridge can also be discerned towards the southern outer boundary of the complex. This feature is also affectedby stellar shock winds, as indicated by its remarkable [SII] emission."
Figure 2: "A composite three-color image of SMC N66A created using the HST ACS images in H-alpha (red), filter I, F814W (green), and filter V, F555W (blue). Field size 512 x 512 pixels, or 26″ x 26″ (approx 7.5 x 7.5 px). North is up and east to the left. … Figure 2 presents a high-resolution composite image of the N66A HII region … The compact HII region is about 10arcsec in diameter, corresponding to about 3 pc. It contains a strong absorption lane. Interestingly, two bright stars, labelled #1 and #2, are located towards the central part of the region, above the dust lane (see also Fig. 3.). Separated by 0.7 arcsec (about 0.2 pc) they are the main exciting stars of the HII region."
"N66A is clearly the most compact HII region of the N66 complex in the optical. Its relative compactness, brightness, and location suggest that it is probably a relatively younger generation in the N66 complex. It should belong to a distinct and rare class of HII regions in the Magellanic Clouds (MCs) called High-Excitation "Blobs", or HEBs. In contrast to the typical HII regions of the MCs, which are extended structures with sizes of several arc minutes corresponding to physical scales of more than 50pc and powered by a large number of exciting stars, HEBs are relatively dense and small regions of about 5 arcsec to 10 arcsec in diameter in the optical, corresponding to about 1.5 to 3.0 pc and excited by a much smaller number of massive stars. … These compact HII regions are also heavily affected by local dust compared to other ionized features of the complex in which they are hosted. This is also the case for N66A, which is marked by a prominent absorption lane of local dust crossing the whole nebula. The two other known examples of HEBs in the SMC are N88A and N81, which were also observed with HST.
"HEBs are usually located adjacent to ordinary giant HII regions or seen lying across them. This implies that they form as a consequence of triggering by a previous generation of massive stars in the complex. Simple reasoning suggests that HEBs and their small exciting clusters are formed from the material remaining after a preceding massive-star foramtion event. More specifically, the apparent association of N66A with the compressed ionized front and the absorption lane, both centered on the NGC 346 cluster, suggests that N66A is a secondary, younger generation of stars."
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