Star Formation in the Rosette Complex

The Rosette Complex in the constellation of Monoceros is a magnificent laboratory for the study of star formation. The region presents an interesting scenario, in which an expanding HII region generated by the large OB association NGC 2244 is interacting with a giant molecular cloud. Inside the cloud a number of stellar clusters have formed recently. In this chapter we present a review of past and present research on the region, and discuss investigations relevant to the physics of the nebula and the molecular cloud. We also review recent work on the younger embedded clusters and individual nebulous objects located across this important star forming region. 1. Historical Perspective The Rosette Complex (l = 207.0, b = −2.1) is located near the anti-center of the Galactic Disk in the constellation of Monoceros. The region is very popular, partly because of the staggering beauty of its main feature: a very extended emission nebula which hosts a large central HII region, evacuated by the winds of a central OB association (see Figure 1). The complex is part of a much larger structure known as the Northern Monoceros Region. This region comprises the Mon OB1 Cloud (host of NGC 2264 and the Cone Nebula), the Monoceros Loop (NGC 2252), and the Mon OB2 Cloud in which the Rosette is one of the most prominent features (see Figure 2). The catalog name for the Rosette can be somewhat confusing because it is not unique: The nebula itself is usually cataloged as NGC 2237 or NGC 2246 (especially by amateur observers), although NGC 2237 originally referred to the brightest patch at its western side and NGC 2246 originally pointed to a bright zone at the eastern side. In addition, while the central cluster is usually known as NGC 2244, it has also been cataloged as NGC 2239. However, this latter designation historically referred to the brightest star in the region, 12 Monocerotis. The cluster was first noticed by Flamsteed in the late 17th century and later reported by William Herschel — who did not notice the nebulosity — and John Herschel, who discovered several conspicuous nebulosity features and reported them in his general catalog (Herschel 1864, NGC 2239 = GC 1420). A contemporary report on nebulous emission was done by Albert Marth (Lassell 1867, NGC 2238 = GC 5361 = Marth 99), who described a “small, faint star in nebulosity.”