Rosette Nebula & Cluster

Haluk Akcam

November 2004

Molecular Cloud Complex in constellation Monoceros, named "Rosette" due to the visual appearance of this nebula, has been examined intensively for centuries. Simbad data for this object are as follows:

Table 1 - Rosette Molecular Cloud Complex

In Dreyer's NGC (New General Catalogue of Nebulae and Clusters of Stars; 1888MmRAS-49-1. Add. & corr.; 1895MmRAS-51-185, and 1908MmRAS-59-part2-105.), the object was examined under five designations. Major complete revisions of this catalog are by Sulentic & Tifft (RNGC, Revised New General Catalogue of Nonstellar Astronomical Objects; 1973, CDS Cat.7001B), and by Sky Publishing Co. (NGC 2000.0, The Complete New General Catalogue and Index Catalogue of Nebulae and Star Clusters, ed. by Sinnott; 1988, CDS Cat.7118). Later, the NGC/IC Project by Corwin et alia have undertaken a new revision and correction of the catalogue, which is still proceeding.

Yet, although NGC defines this object under five designations, none of the above mentioned catalogs as well as other sources (Simbad, NASA/IPAC Extragalactic Database, Corwin's file nmppos_all.txt), as of Nov.2004, agree upon the precise positions (FK5 J2000/2000) of these portions:

Table 2 - Rosette in terms of NGC

Next intriguing case is about the type and size of each object:

Simbad does not recognize 2246, annuls 2239, defines 2244 as "open (galactic) cluster", 2237 as "bright nebula", and 2238 as "HII (ionized) region". RNCG defines 2237 and 2244 as "cluster associated with nebulosity", 2238 and 2246 as "diffuse nebula", and 2239 as "open cluster". NGC2000 proposes four of them (2237-9, 46) for "Rosette Nebula" in file names.dat, but defines 2237 and 2246 as "bright emission or reflection nebula", 2238 as "cluster associated with nebulosity", and additionally two separate objects 2239 and 2244 as "open star cluster", in the catalog.

For NED; 2237-8 and 2246 are "nebulae", and 2239 is a "cluster of stars", where 2244 should be identical with 2239. NGC/IC Project follows the same definitions, and gives some notes on each: 2237 (Western half of the Rosette Nebula: NGC 2237, 2238, and 2246 are all parts of the large annular HII region often called the Rosette. Embedded in the middle of the nebula is a bright cluster of young stars, NGC 2239 = NGC 2244). 2238 (HII Region. Part of the Rosette Nebula: NGC 2238 is a small patch of somewhat brighter nebulosity in the much larger Rosette Nebula). 2239/2244 (Sparse cluster in center of Rosette Nebula: NGC 2239 = NGC 2244, the bright young cluster in the center of the HII region called the Rosette). 2246 (Bright knot in eastern half of Rosette Nebula. NGC 2246 is a brighter patch of nebulosity in the eastern side of the Rosette).

Note: NGC/IC Project gives Lund 222 = OCl 644 as reference for NGC 2239=2244, which is indeed irrelevant! I hope that the project team will correct their related web page as soon as possible, for that object is related to N2243. Yet, it is plausible that the record number within the original table might have been confused with the record number on Lund disc, which is Lund 234 = OCl 500 (06h36m +07°40' J2000). But, in that case, the reference fits to N2254. Also, another mistake is with the N2238 reference of NGC/IC Project: Sharpless 274 region (Sh2-274) cannot be a wise reference for N2238, with 07h29m07s +13°15'43" J2000 and 8' diameter, if it is not an other typing error!

Historical Catalogs

Rosette Cluster was discovered by Flamsteed in the late 17th century, and later Rosette Nebula was first noticed by W. Herschel, J. Herschel, Marth, and Swift. Dreyer's description for several parts of nebula are: N2237 is pretty bright, very very large, and diffused (?=[GC]5361). N2238 is a small [=faint] star in nebulosity. N2239 is a star of mag 8 in large, poor, and bright cluster. N2244 is a beautiful cluster [of] scattered stars (12 Mon [= HD 46241]). N2246 is most extremely faint, large, irregular, partially resolved, some stars seen, round, and extremely difficult [to distinguish].

In 1946, Cederblad listed two objects under the same record number (76a & 76b) in his long article "Studies of bright diffuse galactic nebulae with special regard to their spatial distribution" (1946MeLu2-119-1, later CDS Cat.7231). In his notes, he defined #76a as "= NGC 2238 = GC 5361. Disc. Marth. (114, 578). R. This is a nebulous star of magnitude 14.5, which is probably to be considered as a part of the nebulous cluster listed as b". And for #76b he wrote "= The nebulous cluster NGC 2237, 2239, 2244. R. In 1857 Swift discovered a brighter patch of this nebulosity, which is catalogued as NGC 2237. The cluster was discovered by William Herschel in 1784. It is listed as NGC 2244 = GC 1424 = H VII 2. The designations : NGC 2239 = GC 1420 = h392 refer to the star 12 Monocerotis, which is a member of the cluster. The NGC positions of 2237 and 2239 are erroneous. The outer nebulosities were discovered by Barnard in 1883. WP 105. (...). HD stars involved or aligned : +4 1291 = HD 46056. +5 1279 = HD 46106. +5 1278 = HD 46107. +5 1281 = HD 259105. +5 1282 = HD 46149. +5 1283 = HD 46150. +5 1287 = HD 46201. + 5 1286 = HD 46202. +4 1302 = HD 46223. +4 1304 = HD 46241". J2000 positions derived from Cederblad's list are: Ced76a RA=06h30.6m and DE=05°03'N, Ced76b RA=06h32.3m and DE=04°56'N.  

In 1959, Sharpless listed the object under a single record number (Sh2-275) in his "Catalog of HII regions" (CDS Cat.7020), and the coordinates he gave correspond to RA=06h31m42.9s and DE=04°55'47"N (J2000/2000). The diameter of this region supposed to be 100 arcmin, according to him. In his notes, it is a "part of I Mon association. Contains cluster NGC 2244", and the region is defined as circular, well-structured, bright, and associated with nine stars. In his second table, with HD number, these stars are: 46056, 46149, 46150, 46202, 46223, 46485, 46573, 46966, 47129. First five of these stars correspond to those given by Cederblad (above).

In 1959, Hogg included N2239 and N2244 in his "Catalogue of Open and Globular Clusters" (Handbuch der Physik, Vol.53, p.129).[which I did not see]

In 1965, Lynds gave two record numbers (LBN 948 & 949)  in his "Catalogue of Bright Nebulae" (CDS Cat.7009), both with same coordinates (RA=06h33m and DE=04°58'N J2000/2000). LBN 948 supposed to be in the brightest category with a size of 80'×60', while the latter in the third of a six-grade scale, namely slightly above average with a size of 18'×18'. Area computations reveal an average diameter of 69.1' and 16.6', respectively. It is believed that Lynds referred N2238 to 948, but none for 949. Yet, he identified these two objects together with eight others: 929, 932, 936, 941, 943, 947, 948, 949, 951, 952.

Three of these NGC objects are also included in the "Catalogue of Star Clusters and Associations" edited by Alter, Balazs, and Ruprecht, 2nd ed. in 1970 (CDS Cat.7005A). OCl 515 referred to NGC 2244 (RA=06h32m24s and DE=04°52'N), and described with Trumpler code II3pn, which means "Detached cluster with little central concentration. Bright and faint stars are both present. Generally a few very bright stars and some moderately bright stars embedded in a body of fainter stars can be seen. A poor cluster. Associated nebulosity". Apparent diameter of the cluster is estimated 30 arcmin. OCl 511 referred to NGC 2237 (RA=06h30m24s and DE=05°03'N), and described as associated nebulosity with 60' diameter. OCl 512 referred to NGC 2239 (RA=06h31m00s and DE=04°57'N), and described as associated nebulosity with 16' diameter. In 1983, Ruprecht preserved the same designations (CDS Cat.7101A).

Positional Comparison of Sources

Table 3 - Comparison of object and star positions within the region, given by historical and recent authorities