Picture 1 - Objects in Table 3 

Original Simbad positional data are without any size of the object. Here 60' diameter is applied to these data, in order to compare the unusual positional shift with ease.

Colors of the depicted boundaries are identical with the designations of the related catalogs, which are lined up at the bottom of the image. The sources are given at the top of the ellipse, in abbreviated forms, easy to understand.

HII ionized regions, as well as two unrelated open clusters, are depicted with their identifications.

Here the H-alpha emission image of the object reveals all possible extent, which is larger than the visible core. Yet, with assumed perimeters, it is hard to accept that these can be in conformity with the actual boundaries of the object.

First of all, it is obvious that the strabismical presentation of Simbad cannot be regarded as a scientific reference. Although the circumference data is wanted, the proposed central coordinates of N2237 and N2238 are quite misleading. Presumably, positional data for N2244 is taken from RNGC, disregarding the separate N2239 of Sulentic.

Simbad does not recognize N2246, and confuses N2238 with LBN 948. Since N2246 is a bright patch in eastern part of the nebula, we may assume that it is believed to be annexed by some unknown entities! But, LBN 948 remains as an unresolved case.

In the "Catalogue of Bright Nebulae", LBN 948 and 949 are listed in page 180 (1965ApJS-12), with identical central positions. LBN 948 is marked as "NGC 2238" probably by mistake, instead of "NGC 2237", or perhaps for indicating that N2238 is included within. Yet, subsequently, all recent catalogues duplicate the same error without any consideration. On the other hand, the only electronic copy of the paper in circulation, does not reveal whether it is 2237 or 2238, for the last digit is dropped. But, it is confirmed in the original paper. Although LBN 948 and 949 bear the same position, in Lynds' paper, it is mentioned that the former occupies an area of 1.042 deg2 [d=69.1'] with 90'×60' dimensions, and the latter 0.060 deg2 [d=16.6'] with 18'×18' dimensions. Since N2238 is a small and faint patch of the nebula, and N2237 is the largest part of the same nebula, it seems silly to believe that Lynds could miss this explicit fact or might have confused the NGC designations, which are well-described since Dreyer. It simply does not seem logical to associate NGC 2238 with LBN 948, after declaring that it is 17 times larger than LBN 949, which is in the centre of the larger nebulosity.

After identifying bright nebula N2237 with OCl 511, and commenting that N2244 is included, what is proposed as the central position of N2237 by Simbad is becoming a mystery. OCl 511 in the ClA Catalogue (1970) is referred to N2237 with 06h30m24s +05°03'00" (J2000) and d=60', as an associated nebulosity. Also, OCl 515 in the same catalog is referred to N2244 with 06h32m24s +04°52'00" (J2000) and d=30', which is also a major reference by Simbad for N2244 with 06h32m18s +04°52'00", where a slight shift of 1.5' can be easily ignored. But, how come Simbad places N2237 around 06h33m45s +04°59'54" centre is not understandable. It is 50' apart from the only referred OCl 511. The above image for visual comparison shows this ambiguity clearly. Besides, Simbad seems to be the one and only source, which drifts N2237 into such a remote position. 

At first glance, NGC/IC Project seems like a tremendous initiative, which is likely to eradicate all previous mistakes. But, in this case, the project fails to meet the expectations. Publicized data via Internet show that not much of a scrutiny has been spared for this study:

Web page for NGC 2237 claims that no other cataloged designations meet their criteria, which are 06h30m54.6s +05°02'57", emission type bright nebula in the western half of the Rosette Nebula, with 90'×90' size. Mr. Corwin notes that the central position is based on his own measurements for very large diffuse nebulae, with a standard deviation about 0.1 degree {nmppos_2.txt, sources.txt). But, OCl 511 (=N2237) in the ClA Catalogue (1970) is only 7.6' apart from his centre for the same object. Only the sizes are not similar: Mr. Corwin spares a 1.767 deg2 area for the object, and the ClA Cat. only 0.785 deg2. The ratio is 2.25. Yet, after examining the perimeters on DSS images, it seems hard to tolerate such a large space for the western half of the main nebula, since there is not much left to other designations.

Web page for NGC 2238 of the same project places this object around 06h30m36.0s +05°01'00", names it as a bright nebula with Sharpless 3:3:3 classification, and defines it as HII region, part of the Rosette nebula. Although "NGC 2238 is a small patch of somewhat brighter nebulosity in the much larger Rosette Nebula", the proposed size of this small patch seems enormous with 80'×60' dimensions. Then, we find another central position for the same object in nmppos_2.txt file of Mr.Corwin, namely 06h30m40.4s +05°00'47". In sources.txt file, we learn that it is "his own measurements, offset from GSC stars unless noted, generally on the PSS prints, sometimes on the IIIa-J SSS films. Standard deviation around 2 arcsec if the GSC star/galaxy is within 2 arcmin of the object, around 4-5 arcsec for larger distances." This is perfect, but which data should we rely on?

NGC/IC project claims that N2238 is also cataloged as LBN 949, Sh2-274, GC 5361. I have already given my opinion about Lynds' note on LBN 949, and here it is a good reference, despite the positional difference. Yet, if the size of a "small patch" or "knot" is 80'×60', how can it be identical with LBN 949? Also, let us forget about Ced 76a, for a moment. But, Sh2-274 as a reference is becoming very unrealistic!

Current data for NGC 2239 is the same of NGC 2244 in NGC/IC Project. Here, simply SAO 114010 position is adopted, which is the brightest emission-line star HD 46150 associated with the cluster. In fact, the accumulated centre of the cluster is about 6' SE of this star. Also, GC 1420 is not identical with GC 1424, but it does not matter either. Yet, what about Lund 222 or OCl 644 as a reference? Does it mean Lund 229 and OCl 515, or is the web page confused with NGC 2243? Missing Ced 76b as a reference is tolerable, but not such grave mistakes!

Finally, the object, which is dismissed by Simbad, gets our attention: Web page for NGC 2246 of the NGC/IC Project presents another 80'x60' sized bright nebula with Sharpless classification 1:2:3 around 06h32m33.7s +05°07'42", which is described as "bright knot in eastern half of Rosette Nebula." In that case, why N2237 of this Project (as western half of the main nebula) already occupies more than half of this eastern half, is not known. Here, Sh2-275 and LBN 948 are reserved for the eastern half of Rosette. But, when we compare the proposed perimeters of three bright nebulae (N2237, N2238, and N2246), we find that all are jumbled together, and N2244=N2239 is placed in the conjoined part. Sixty years ago, Cederblad was claiming that the positional distribution of these three regions were wrong, and now we realize that nothing has changed since then.

Picture 2a - Parts of Rosette Nebula by NGC/IC Project

Left is the mosaic picture of NGC 2237-9, 2244, and 2246, based on annotated NGC/IC Project images. Their image archive is implemental, yet the dimensions are missing. Central positions of five NGC objects are marked by the project team. Since three pictures are merged to build this mosaic, N2239=N2244 seems twice marked, and the font size became larger in the left portion of the picture due to enlargement.

Proposed central positions for N2239 and N2238 may have some reasonable basis supporting their preferences. But the placement of N2237 and N2246 looks like somewhat arbitrary. Because, the purpose behind these choices are not well defined.

Picture 2b - Intensified Image of the same Region

Right is the image of the same region, which is given above. Here, the brightness of the H-alpha image is intensified on a logarithmic basis: Bright portions are overexposed, and dim areas are excessively darkened. Thus, it became easy to recognize which portion is what, at the core of Rosette Nebula.

Four ellipses in red color here, are to demonstrate the focal areas based on accumulated glow, if we want to follow the classical NGC designations. Large one at the left of the image is showing the core of the irregular, partially resolved, and relatively faint N2246, in which the focus is dispersed.

N2244 or N2239, whatever it is, supposed to be the open cluster in the heart of the nebula, which can be focused at by computing the weighted mean of the positions of the associated stars. Here, the circle at the bottom is only an approximation. Numerical values will follow hereafter. N2238 is described as a small patch, and the ellipse at the SW is related to the core of this portion. Finally, N2237, which supposed to be "pretty bright, and very very large", should be within the focal area, as it is seen above the core of the cluster and N2238.

My personal opinion is that dividing a large nebulosity into several areas is because of not being able to examine the whole, due to observational difficulties. Otherwise, it does not have any sense. What we observe here in general, is an extended HII ionized region, and a large open cluster situated in the middle of the visible part of this nebulosity.

Since all we get from the deep space is nothing but emitted or reflected light, either visible or detectible, categorizations should not be blurred because of difficulties brought by observation techniques. Human mind is prone to categorization, for it is not apt to comprehend the whole. Yet, further redundant classifications may sometimes obscure the whole, as it is seen in this case.

Picture 3a, 3b - Intensified Regions at the Centre of the Rosette

At the right of this column, two elaborated versions of H-alpha image of Rosette Nebula are placed, in order to get a clear concept about these alleged designations.

First one is created after appropriate channel calculations to distinguish luminous portions of the nebula. Largest of these areas is the one at the NW of the hollow-like centre of the nebula, which may be considered as the core of N2237. West of centre is where N2238 can be placed. It looks like a split flame above four less bright speckles. Open cluster is almost in the middle, and can be distinguished easily by bright stars associated with it, such as HD 46241 (12 Monocerotis) at SE of the cavity, or HD 46150 right in the middle of the cavity. Then remains the eastern part of the centre, which seems relatively loose, and appropriate in shape to be named as N2246.

Next image is created using the polarization technique. Here, the luminosity is stratified in a linear manner, so that the acmes can be more distinct. When compared with the above one, particularly the centre of the reserved area for N2237 is becoming more specific. It cannot be so close to the centre of N2238, as NGC/IC Project proposes. Also, it is becoming more apprehensible here, why focal centre of N2246 cannot be restricted to the point given by NGC/IC Project. The structural character of the eastern part of the nebula is not similar to the adjacent parts. Here, we have a recognizable sparseness that cannot be ignored. The focal point of the slanted oblong core should be calculated numerically, which is probably more close to the open cluster N2252, and certainly not 80'×60' in size.

In fact, despite the adopted idea in Dreyer's mind, the purpose of splitting the nebula under different designations does not seem very helpful.

Picture 4a - Dreyer's Positions

Right is a dimmed visible appearance of the nebula with gleamed dapples. Bicubic gamma extrapolation method is used in bleaching technique. Early spotters might have first detected these glimmering patches, and then noticed the reddish nebula.

But, central positions in Dreyer's original listing (p.70), as depicted on the left, do not fit exactly to this outline. According to Dreyer's definitions: N2244 is 21.4' E of N2239, N2238 is 5.7' NW of N2239, N2237 is 5.6' NW of N2238, N2246 is 15.1' N of N2244, and cluster N2252 is 38.9' NE of N2246.

Picture 4b - Bright Areas of Rosette

About 150 years ago, following his predecessors, Dreyer spotted some parts of the nebula, and defined them under separate designations. His placements are shown in Pic.4a, with 10' diameter red circles. Overlapped three circles at the western part are for N2237-39, in RA order. They seem to be lined up on a slightly bent diagonal line, with almost equal distances. The purpose of this stacking up is not clear. In 1946, Cederblad noted that N2237 and N2239 positions are erroneous, which is indeed right. But the same error has been carried into RNGC and NGC2000. Later, superfluous N2239 was ignored, yet by counting it similar to N2244, despite the original 22' distance between them. N2239 supposed to be an 8th mag. star in a large and bright cluster, according to Dreyer. This "bright cluster" is certainly not N2244, for none of the ~8th mag. associated stars can be found there. Then, we may assume that it was a misconception, and should not be misleadingly matched with N2244. Also, Dreyer's N2237 focal position is also a dilemma. What forced him to prefer a dimmer extent adjacent to N2238, is not known. Being aware of this, NGC/IC Project carried the point 9' eastward, yet without any explanation.

Chasing the Centroid

Focal point of an extended object is nothing but the centroid; the point representing the mean position of the matter in that object, centre of its total mass. When the object is a cluster of stars, it is not a problem to estimate the centre of gravity. For stellar objects possess compact masses easy to pinpoint, no matter how remote they are. But, an ionized region is like the residual of a puffed away smoke, or like randomly scattered hovering dust lumps. Since all we analyze is nothing but the radiation, we can estimate this point by comparing flux rates. If the subject is the density of detected ionization, then the centre of gravity becomes the mean centre of flux density, and can be computed by evaluating the image in different wavelengths.

Picture 5a, 5b, 5c, 5d - Extension of Rosette Nebula

Here we have three elaborated images of Rosette Nebula, showing the visible part and extended regions. Although the visible part seems compact and rounded, it is not difficult to conceive that the HII region extends in two distinct directions.

Pic. 5a shows the dense areas of this extension, in contrasting colors. One branch is extending to the North of the core, and the other is towards the E-NE of the core. Yet, the core seems to be enveloped by a well-shaped halo.

Pic. 5b is a more intensified version of the previous one. Here, we have a more distinct picture of the extension, in dark blue color. The core in silver can be detected easily as an isolated round shape, with some tiny detachments.

Pic. 5c shows the extended regions in a more explicit manner, colored in cyan. The quarter-to-twelve position of two extended arms are very characteristic, rare to see elsewhere. The other surrounding parts of the core seem more definite and distinct.

Finally, Pic. 5d (below right) gives the three color map of the nebula, where the core is in orange, surrounding  halo is in blue, and the main bulge is in red.

The junction point of two arms corresponds to the central cavity in the heart of the nebula, which can be assumed as the place of the focal point of the nebula.

Then, we may define a perimeter for the core, as:

R.A. = 06h32m17.27s ±03.99s    (J2000)

Decl. = +05°00'00.0" ±19.9"       (J2000)

Width = 100.30' ±9.97', Height = 92.58' ±3.96'

With these dimensions, an area of 2.0342 ±0.2878 deg2 will be occupied by the core of the nebula, as depicted below in two images:

Picture 6a & 6b - Core Perimeter of Rosette Nebula

Left is the proposed perimeter with H-alpha image of the nebula.

Right is the same perimeter with emphasized visible (yellowish) portions of the nebula.

My opinion is that further dissections may be done, by dividing the area into adjacent quadrants, sectors or segments.

But, NGC-type or similar designations do not seem to meet the need for a proper examination of the nebula, in details. Therefore, I believe that it would be more appropriate to desert old concepts, although they were quite useful in the past, in order to elevate our understanding up to a superior stage.

Picture 7a & 7b - Spatial Distribution of Possibly Related Stars

Before examining the cluster embedded in the nebula, it may be wise to overview the possibly related stars. Above are two star charts of the same region to frame the area, where the spatial accumulation of the stars is most distinct. Pic.7a on the left is showing a narrow angle (70') evaluation of the stars, while 7b on the right is a wide angle (210') one with less accuracy. Resolutions are equalized for the sake of comparability.

Spatial accumulation centre does not reveal the actual focal point of a cluster. But, most of the old photometric approximations are based on this method, namely, the arithmetic mean of the brightness levels by pixel.

Focal centre of the accumulation has been calculated as R.A. = 06h32m19s and Decl. = +04°50'36" based on narrow angle aperture, and R.A. = 06h32m20s and Decl. = +04°51'25" based on wide angle. Both results seem similar, with a slight deviation in declination. This preliminary focal point is very close to the brightest member of the cluster HD 46241 (12 Monocerotis), which is the proposed centre for the open cluster NGC 2244, mostly  by previous authorities.

The estimated preliminary size is about 64'×56', which is also akin to Cederblad's definition. Actually, this size is relevant to the visible core of the nebula, rather than the perimeter of the cluster. Because, star membership analysis reveals a smaller size, as it is given below.

Open Cluster in Rosette Nebula

Cluster of stars in the middle of the nebula, mostly known as NGC 2244, are formed about some four million years ago from the nebular material. The hot stellar wind of particles that streams away from these young stars is believed to hollow the centre of the nebula, and shape a cavity, which is insulated by a layer of dust and hot gas. Emitted ultraviolet light from the hot stars causes the surrounding nebula to glow. High energy light form the bright blue stars of the cluster knocks electrons away from hydrogen atoms, and thus ionizes the surrounding abundant hydrogen gas clouds. When the blown electrons fall back, they emit a red light, and cause the emission nebula to appear in a dominating beautiful reddish flowery glow.

A dedicated organization to the study of open clusters, WEBDA, maintains a good database for this purpose: 1458 stars are under investigation for this cluster. First 400 stars are based on Ogura's paper (1981), 773 stars are based on Massey's paper (1995), 577 stars are based on Park & Sung's paper (2002), and the rest are from historical studies of Hayford (1933), Zug (1933), Loden (1957), van Schewick (1958), Kirillova (1958), and Johnson (1962). Additionally, some 257 new stars with unknown whereabouts are based on Verschueren's PhD thesis (1991). Berghoeffer's paper on 138 X-ray stars (2002) is a useful supplementary work. Massey's, Park's and Berghoeffer's original data can be retrieved from the archives of WEBDA.

My present work is based on the following studies:

Essential studies:

2002AJ123-892 UBVI and H{alpha} photometry of the young open cluster NGC 2244 - Park & Sung

2002AA384-890 Optical properties of X-ray selected stars in NGC 2244 in the Rosette Nebula - Berghoefer & Christian

1995ApJ454-151 The initial mass function and massive star evolution in the OB ass.s of the North. MW - Massey, et al.

1981PASJ33-149 UBV photometry of NGC 2244 - Ogura & Ishida

Supplementary studies:

2004ChJAA4-153 Stellar X-ray sources in the Rosette Nebula - Chen, et al.

2002AJ123-2590 Strong emission-line stars identified toward the Rosette nebula - Li, et al.

2002RMxAASC14-33 Membership in the region of the open cluster NGC 2244 via the EM algorithm - Higuera, et al.

2001RMxAA37-105 Membership in the region of the open cluster NGC 2244 - Sabogal-Martinez, et al.

2000ESASP445-395 Age of and distance to the young stellar cluster NGC 2244 in North. Mon. - Hensberge, et al.

1998AA339-141 Chemically peculiar stars in the field of NGC 2244 - Hensberge, et al.

1990ApSS170-245 A detailed photometric study of the young stellar cluster NGC 2244 - Verschueren, et al.

1989PASP101-195 uvby-beta photometry of stars in the field of NGC 2244 and NGC 2264 - Perez, et al.

1987PASP99-1050 On the distances to the young open clusters NGC 2244 and NGC 2264 - Perez, et al.

1982AJ87-1497 Membership of the Rosette nebula cluster NGC 2244 - Marschall, et al.

1977AJ82-973 Uvby beta photometry of NGC 2244 - Heiser

1965ApJ142-974 Studies in spectral classification. III. The H-R diagrams of NGC 2244 and NGC 2264 - Morgan, et al.

1962ApJ136-1135 The galactic cluster NGC 2244 - Johnson

1959PASP71-32 Polarization in NGC 2244 - Hoag & Smith

1957ArA2-39 Measurements of polarization and colour of stars in the galactic clusters NGC 2244 & NGC 2287 - Loden

A text file tabulating the J2000 positions of 1375 membership candidate stars in RA order, derived from the works cited above, is available here. The list includes 708 "stars in cluster", 568 "stars in association", 62 "X-ray sources", and 37 stellar objects under nine different definitions, all from Simbad. The unweighted average of these points is 06h32m14.1s +04°55'02". A dozen irrelevant stars from Hoag & Smith are also maintained in the list.

Further study of Rosette Cluster is in preparation.

Copyright © 2004-2008 Haluk Akcam. All rights reserved.

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