The Monoceros R2 Molecular Cloud

The Monoceros R2 region was first recognized as a chain of reflection nebulae illuminated by Aand B-type stars. These nebulae are associated with a giant molecular cloud that is one of the closest massive star forming regions to the Sun. This chapter reviews the properties of the Mon R2 region, including the namesake reflection nebulae, the large scale molecular cloud, global star formation activity, and properties of prominent star forming regions in the cloud. 1. The Mon R2 Reflection Nebulae The Monoceros R2 region is distinguished by a chain of reflection nebulae that extend over 2 on the sky. The brightest of these nebulae were studied as far back as Seares & Hubble (1920), and were included in a larger study of nebulous objects by Hubble (1922), who demonstrated that the extended emission can be attributed to the associated stars. The nebulae are included in the Catalog of Bright Diffuse Galactic Nebulae constructed by Cederblad (1946). Various lists of nebulae identified from inspection of the Palomar Observatory Sky Survey appear in Dorschner & Gürtler (1963, 1966), van den Bergh (1966), and Herbst & Racine (1976). Gyulbudaghian, Glushkov, & Denisyuk (1978) identified seven additional nebulous objects as candidate Herbig-Haro objects, although they were later shown to be reflection nebulae (Cohen & Schwartz 1980). The nomenclature “Mon R2” originated with van den Bergh (1966) to indicate the second association of reflection nebulae in the constellation Monoceros1. Table 1 lists the nebulous sources in the Mon R2 region identified in the studies mentioned above. Coordinates were determined from the digitized Palomar Sky Survey using previously published finding charts (Downes et al. 1975; Herbst & Racine 1976; Carballo, Eiroa, & Mampaso 1988) and coordinate lists (Rodrı́guez et al. 1980; Carballo & Eiroa 1992). Figure 1 marks the location of the nebulae on the blue print from the Palomar Observatory Sky Survey. The sources GGD 12 through 15 are located within a small area of the sky and are often collectively referred to as GGD 12-15. The most extensively studied region is the active star forming site embedded in a dense molecular core (identified as the “core” in Fig. 1) that lies midway between the reflection nebulae vdB 67 and vdB 69 (see Fig. 2 for a more detailed optical color image of The Mon R1 reflection nebulae are part of the Mon OB1 association, which includes the Galactic cluster NGC 2264.